Military surplus Troubleshooting_and_repair_of_radio_equipment_TM11-4000

DEPARTMENT

/ /

TROUBLESHOOTING

OF

THE

ARMY

TECHNICAL

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HEADQUARTERS, DEPARTMENT '

;2

AGO

1001GA

APRIL

- Feb

1958

OF

THE

ARMY

WARNING!

EXTREMELY

EXIST

IN

DANGEROUS

RECEIVERS

DON'T

TAKE

VOLT

AND

TRANSMITTERS.

CHANCES!

AGES

AGO 10016A

*TM

11-4000

I

TECHNICAL

No. 11-4000

CHAPTER

Section I.

CHAPTER

Section I.

1.

2.

II.

III.

3.

II.

III.

IV.

v.

VI.

VII.

VIII. Signal generators

IX.

X.

XI.

XII.

XIII.

MANUAL

}

DEP

TROUBLESHOOTING

INTRODUCTION--- -----------CAUTIONS

General _ High-voltage hazards

Other hazards_____________________________ TEST

Importance of Multimeters

Using multimeter

Circuit loading________________________________________________________________ 29-

Isolating multimeter fr

Extending range of voltmeter Output

Cathode-ray oscilloscopes____________________ ____________

Tubetesters Frequency-measuring meters________ _

Field-strength meters Substitution of

__

_________

EQUIPMENT

test

equipment__________________

------

-----------

----

and

db

meters

___________

-----------------------

test

equipment_

__________________________ ______________

------------om

-------------------------

------------------------

AND

REPAIR

- ------ ----

_________________

-------------------------------------

---------------------

circuit_ _____________________________________

---------------------------------------------------_________________________________________

-

-------------------------------

____

________

_____________________________ _________

OF

----

-----------

_________

_____________

__________

-

-------------

______

___________________________

-- -

------------

HEADQUARTERS,

ARTMENT OF

WASHINGTON

RADIO

--------------------

______

_____

_______________________ 5-11 5

___

_____________

-

---

________________

25,

EQUIPMENT

________________

____

________________

_____

--------------

-------------------· 41,

-----------------

THE

ARMY

D.

C.

, 2

April1958

__

_____

____

__

-------------20-23

- 24-28

____

_____

_ ,

__________ 43-48

_____

__ 49-

-----------

- 55-58 34

______ 59-

--

___

_ 70-72

Para

graphs Page

1,

2 3

3,4

12-15

16-19

35

36,

37

38-40

42 54

67

68,

69

5

13

16 17

19

21 24

25 25

27

30

36 39

40

l

CHAPTER

Section

CHAPTER

~

4.

GENERAL TROUBLESHOOTING

I.

Introduction Checking tubes and component

II.

Signal substitution and signal

III.

IV.

General troubleshooting procedures Isolation of troubles in individual stages________

v.

TROUBLESHOOTING VEHICULAR INSTALLATIONS _________________________

5.

6. TROUBLESHOOTING RECEIVERS

Section I General receiver troubleshooting techniques

II.

Troubleshooting dead am receiver-----

III.

Troubleshooting weak am receiver

IV. Troubleshooting distorted am receiver

V.

Troubleshooting

VI. Troubleshooting receiver

VII. Troubleshooting noisy am receiver---

VIII. Troubleshooting am receiver

IX. Troubleshooting calibration-oscillator section

X. Troubleshooting fm receivers-------- --------

anual

supersedes TM 11-4000,

------------

intermittent

20

-- -

for

April 1945.

----------------------parts

tracing

am receiver _______________

hum _______ _

that

6A

_________________________

______________

------------------

______

------

------------

-------

squeals or motorboats _________

----

-- -

-----

----

____________

-----------------------_____

-----------------------------

_____

________

-----

_______________

_____________

----------------------

-------------

____

-------------------

--------------

-------------

------------------

____

______

_____

_____________

____

_____

_______________

___

_____________

____

_____________

_____

____

_____

-------------------________

_____

____

----------

______ 75-

--

----

_________

- -

---------

------

-----· 150-152

___________ 157-

________

____________

___________

------- 181-183

73,74

83

_ . 84,

85

---

86-

93

__

94-107

108-111

__ 112-115

- 116-149

--- - 153-156 159

__ 160-164

165-170

171-175 176-180

42 42

50

52 54

62

66 68 76 78

80 82 85

87 88 89

CHAPTER

Section

7.

TROUBLESHOOTING

I.

General

transmitter

II. Troubleshooting dead am

AM

TRANSMITTERS

troubleshooting _________

transmitter _____

III. Troubleshooting for weak output_

______

________

___

________

_____________________________ 184-186

____

_______

____

______________________________ 202-209

__________________

Paragraphs

187-201

Page

92 93

98

IV. Troubleshooting for lack of modulation ___________________________________________ 210-214 100

V.

Troubleshooting for distorted

VI.

Troubleshooting special

outpuL

circuits--

------

------------------

---------------------

------------------------------------------------

215-217 101 218-220 103

CHAPTER

8.

TROUBLESHOOTING FM TRANSMITTERS -

9.

RECEIVER ALINEMENT

Section I. Basic concepts

II. Equipment needed

III

. Am receiver

Fm

IV.

CHAPTER

10. REPAIRS AND ADJUSTMENTS

Section

I. Repairs-

II. Field

III.

Adjustments

CHAPTER

Section

11.

FINAL

I. Transmitters

II. Receivers

CHAPTER

12. RADIAC PROCEDURES

Section

I. Introduction to radioactivity-

II. Radiation

III. Radiation detectors

IV. General repair procedures

V.

Repair of typical radiac set_ _____________________________________________________ 316- 319 156

CHAPTER

13.

TROUBLESHOOTING TRANSISTORIZED

14. REPAIRING

INDEX

__________

___

-------

---

-------------

-------------------------------------------------

-------------------------------------------------------------

alinement_ ________________________________________________________________ 242-249

alinement------------

---------------------

-------------------------------------------------------------

expedients

----

---

------------

----

----------------------------------------

-------------------------------------------------------------

-----------

-----

--------------------

----

---------

-

---

--

---

221-223

224-225 109 226-228 110 229-241 110

-----

250-261 118 262

---

268

- 267 123

- 274 126

CHECKUP

----------

---------------------------------------------------------

hazards

PRINTED

____

_____________________________________________________________________

-----------------

---

---------------------------

------------

--

---------------------------------------------

-

----------------------------------

------------------------------------------------

-------------------------------------------------------

EQUIPMENT-----------

WIRING ASSEMBLIES ________________________________ 326-331 168

---

-------------------

------------

-------

-----

--------

275-282 130 283

- 292 133

293-297 137 298- 302 140

-----303-306 143 307- 315 148

320-325 160

-------

104

113

170

2

AGO 10016A

CHAPTER

1

INTRODUCTION

1.

Purpose

a.

This manual

an~

Scope

is

a practical reference guide to the troubleshooting and repair of military radio receivers and transmitters. Figure 1 shows the troubleshooting of a receiver-transmitter. The methods and procedures in this manual are

broadly general, and

are

based upon successful experience. Theory is used for purposes of clarification only.

b. This manual is not a substitute for the

technical manuals issued for particular radio equipments. Instructions for troubleshooting and repairing a specific receiver or transmitter are given in the technical manual for

that

equip-

ment, and those instructions are to be followed

an

carefully. This manual takes

overall view of the subject of radio equipment troubleshooting and repair in general. Therefore, ments the equipment technical manuals.

it

supple-

It

supplies a background of general information which the equipment technical manuals assume and take for granted.

c.

This manual is designed as a refresher for

It

the experienced Army technician. be of use

to

others, well-grounded in radio theory,

may also

as an introduction to troubleshooting and repair methods.

d.

This manual assumes familiar with ordinary tools. fore, only those special tools

that

the reader is

It

describes, there-

that

are occasionally

required but are not normally used in radio repair work.

2.

Role

of

Repairman

a.

World War I was the first

war

in which

radio equipment was used. The limited amount

it

of equipment and the uses to which plied required very few repairmen to keep

was ap-

it

in working order.

b.

Radio uses in the Army today have reached

a point where there is scarcely a vehicle, includ-

that

ing tanks and many types of trucks, not have a radio set as

part

of its original equip-

does

ment.

c. The many applications

and the number of sets now in

of

radio equipment

use

mean

that

a greater number of repairmen are needed. The complexity of the equipment calls for highly trained skilled technicians to maintain it.

d.

The success or failure of a mission depends

on

the effectiveness of the communications.

It

is possible for a military unit in combat to be-

come

lost or completely separately from other units because of the failure of radio communications equipment.

e.

The radio repairman is as important as the man who fires a because all

parts The radio repairman of

that

team.

gun. The Army wins battles

of

it

work together as a team.

is

a very important

part

AGO 10016A

3

Figure 1. T1·oubleshooting an A

1·my

receiver-transmitter.

AGO 10016A

CHAPTER

CAUTIONS

2

Section

3.

The

Safe

Way

The electronic technician must learn and fol-

low

safety rules. The first rule

car-

eful! Nev

you

know

matic may

be

er

touch a point in a circuit unless

that

it

is not alive. Refer to the sche-

di

agram to

find

out how much voltage

present, then connect a voltmeter into

the circuit and measure the voltage.

of

safety

If

is-B

you

do not know whether a certain equipment is dangerous,

do

not find out the hard way

fir

equipment

on

ing

4.

5.

high-powered equipment.

Safety

a.

Safety procedures must

High-Voltage

a.

Special safety precautions must

st. Have an assistant when work-

Procedures

Power

Supply

-use

be

developed by

Section

Dangers

be

your test

II.

HIGH-VOLTAGE

observed

when troubleshooting and adjusting equipment

100

in which voltages higher than

appear. Troubleshooting in transmitters and power supplies is especially hazardous because these equipments contain circuits capable of delivering

at

large amounts of current

voltage equipment

that

power line and contains

high voltages. High-

operates from a

60-

or 120-cycle recti-

60-cycle

fiers is especially dangerous. The filter capacitors

in

such equipment are relatively large and can

store a heavy charge

that

can remain for long periods after the equipment has been turned off.

Contact with a charged 4-microfarad capaci-

tor

of

the type used in transmitter power sup-

be

just

plies can

as deadly as touching a live

circuit.

b.

The most qangerous circuits, high-voltage

that

or low-voltage, are those

can deliver high

currents. The danger is even greater when

I.

GENERAL

so

that

habit

when the technician is rushed with

repair work, he automatically takes precautions.

who

For example, the technician

e

hand when working

on

high-voltage equipment

uses only

may get only a jolt if he accidentally contacts a high-voltage point, but if he carelessly puts the sweaty fingers of one hand into the equipment and casually rests the other hand

he

metal cabinet,

may suffer a severe or fatal

injury.

b.

Bleeder resistors may to discharge capacitors been turned

off.

To

be

open

and thus fail

after

the equipment has

sure

that

the capacitors are discharged, short circuit them with a shorting stick (par. 8).

HAZARDS

dampness

is

present or when the hands are perspiring. Under such conditions, the body resistance is

and a high value

of

current can

low pass through the body if accidental contact is made with a live circuit. A current of only milliamperes (rna) through the body can cause death. Even

les

s current can

be

fatal if the

victim is fatigued or if his general health is poor.

of

Because

one

keep material

c.

juries also can

such dangers,

hand in your pocket and to stand

that

is a

In

addition to shock and burns, other in-

be

voltage equipment.

good

caused by working

it

is

good

insulator.

It

is common for the body muscles to contract violently when the body comes in contact with high voltage. This violent contraction of the muscles can hurl the victim into near-by objects with enough force to cause severe cuts; bruises, or even broken bones.

d.

Sometimes, contact with high voltage

causes the muscles to contract

so

one

on

the

practice to

on

high-

tightly

that

80

on

AGO

10016A

5

the victim freezes to the equipment. happens, has been turned

6. High-Voltage Danger

The technician must know

do

rwt

touch the victim until the power

off.

Points

what

If

this

and where

the danger points are in high-voltage circuits,

that

he

so

can avoid them when working equipment with the power turned ity

of

danger points is indicated below.

a.

In a transmitter (fig.

chassis has terminals

that

2),

the power supply

are

on.

The ident-

at

a very high

on

potential.

b.

The secondary terminals of the plate transformer and the plate caps of the rectifier tubes are

at

high

ac

potentials.

c.

Beneath the chassis, filament terminals

the

transformer,

the

filament terminals of the

on

rectifier tubes, and the components of the power supply, which includes the bleeder resistor, filter choke, and filter capacitors,

are

at

a high

de potential. A high-voltage power supply with the filter capacitor terminals exposed is shown in figure

d.

voltage points exposed; the danger points

shown in figure

e. In figure with radio frequency danger points. Both mally appear

3.

The modulator section also has high

2.

4,

a different

at

the power amplifier tube plate

(RF)

de

and

transmitter

as well as the

RF

voltages nor-

is shown

de

are

de

caps and the tuning coil and tuning capacitor.

TM4000·31

Figu1

·e

2.

Tmnsn

~itte1·

,

showing high-voltage

6

dang_e1·

points.

AGO 10016A

HV

RECTIFIER TUBE PLATE CAPS

HV

FlLTER CAPACITOR TERMINALS

TM4000-32

Figure

POWER AMPLIFIER TUBE

PLATE TERMINALS

3.

Transmitte1· powe

?'

supply, showing high-voltage dange

TU

NIN

G C

APA

CITOR

TUNING

COI

?'

points.

L

TM

4000

- 33

AGO 10016A

Figu1·e

4.

Transmitte

?'

RF

section, showing high-voltage dange

?'1Joints

.

7

High voltage is present

at

the terminals of the

transmitter meters. The zero adjusting screw

of any indicating meter may also

de

potential. Sometimes meters with high volt-

be

at

a high

ages present are mounted behind a glass or

Be

plastic recessed panel. is turned

off

when adjusting the meter pointer

sure the equipment

to zero.

f.

Another danger point is the fuse panel.

Note in figure 5

that

the high-voltage fuses

are

several inches long to prevent the high voltage

from arcing over when a fuse blows. There may also

be

a high voltage between the fuses and

equipment panel or cabinet.

g. In many equipments,

and meters

that

give a visual indication

there

are

glow lamps

that various circuits are energized. These components may become defective and cannot be relied upon completely.

h.

Be especially careful of equipment relays tinue to be applied

to

apply power because power will con-

if

the relay contacts stick,

that

uses

Figu1·e

8

5. Tmnsmitter fuse panel, showing high-voltage fuses.

AGO

lOOlGA

even though the relay a result, even with the relay energized and all warning lights out, the

coil

is not energized.

coil

circuit

As

de-

con-

tacts can still apply power to the equipment.

i.

Do

leasing

is

off.

not depend

as

a positive indication

Sometimes the armature

on

the sound

of

a relay re-

that

the power

of

a relay can

be

heard releasing while the contacts remain fused together, in which case the equipment is still energized.

7.

Interlock Switches

a.

High-voltage equipment usually includes a system of interlock switches for the protection of

personnel from accidental contact with live

In

circuits. switch is located

some transmitters, an interlock

at

every removable panel and

access door. Usually, all the switches are wired

off

in series to automatically shut

the power

the equipment whenever a door or panel

to

is

opened. Interlock switches are safety devices

be

and must not

tampered with.

necessary to disable the interlock switch

power can

troubleshooting purposes,

be

applied to the equipment for

be

If

su1

·e

to restore the

it

becomes

so

that

switch to its original position before the repaired

equipment is returned to service.

b.

There are several commonly

One

type

(fig.

6) is

that

has a relatively

low

ity. This type usually

an

ordinary push switch

current-carrying capac-

is

used

interlocks.

connected to a relay

coil which controls the power to the equipment.

If

an interlock switch becomes short circuited internally, and if a relay controlled by an interlock

is defective, the power will remain

on

even

though there is no visible indication.

c.

Another type of interlock

control the current

flow

through a relay but is

(fig.

7)

does

not

wired directly into the power circuit. When a

of

panel door is opened, the disk

the interlock

comes out of the receptacle and the power is in-

turn

off

terrupted. Always

all power switches and discharge all filter capacitors with a shorting stick before working

8. Shorting

a.

One

Stick

of the simplest and most important

on

the equipment.

aids in troubleshooting high-voltage equipment

It

is

used

so

to dis-

that

they

safely is the shorting stick. charge all high-voltage capacitors will cease to

be

a source of danger during tests.

FiguTe

b.

gested in figure

A

6. Examp

good

le

shorting stick can

8.

Be

of

a push-switch intm·loclc.

be

made as sug-

sure

that

the

wood

is thor-

oughly dry. For the ground lead, use about 2

feet

of

heavy bare flexible wire with a piece of

plastic tubing slipped over it. Bare wire and

so

that

clear tubing are used

a break can be

seen and repaired. An unseen break can lead

to injury or death, because a shorting stick with

a defective ground lead will fail to discharge the capacitors. In addition, charged capacitors will appear to

discharged

-

becau~e

of the lack of

be

sparking when they are grounded. Use a heavy

be

solder lug and connects to the lug is a

sure the joint where the wire

good

solder joint. When

the stick is used, first connect the clip to the

hook

chassis; then touch the

be

point to figure 8

discharged. The stick indicated in

is

for large transmitters; use the di-

to the high-voltage

mensions as a guide for constructing smaller

sticks.

AGO 10016A

9

---- -,

Figw·e

c.

Before using the shorting stick,

7. Equi1Ymen

CONNECT

GROUND

BEFORE

REMOVE

SHORTING

BEFORE

TURNING

Fig

tt?'e

t access cove

CLAMP

TO

USING

PROBE

STICK

ON

POWER

8.

Const?·uction

be

sure

.

that the handle is clean and dry ; this prevents leaks from developing and from allowing high voltage to cause arcs to jump across the stick to ground.

off

Shut

the power and

c1ip

the ground clamp of the stick to the chassis of the equipment. Hold the stick near its end and as

far

from the hook

?·,

showing two-piece ·interlock

FLATTEN

APPROXIMATELY 3 INCHES

FOR

DRILLING

AND

MOUNTING

~"''"'~""'

-GROUND

TRANSPARENl SLEEVE

of

a shorting sticlc.

as poss

shown in figure

ible. Touch the hook to the terminal, as

switc

h.

'"""J

HEAVY

COPPER

HOOK

SHORTING

HIGH

VOLTAGE

WHILE

WORKING EQUIPMENT PRECAUTION SHOULD THE POWER ACCIDENTALLY.

9.

AS

BE

TURNED

CLAMP

TM4000-30

Make contact for several

BAR

ON

TERMINAL

ON

AN

ADDED

ON

TO

seconds to make sure each capacitor is discharged.

d.

A charged capacitor may become un-

grounded. As a safety precaution, s

that

terminals of

capacitor together as shown

hort

the two

10

AGO 10016A

in figure 10. Leave to one of the high-voltage terminals

the

shorting stick hooked

on

the

capacitor while working on the equipment; if

on,

the power is turned

off

cut

the power. Be sure to remove

ing stick before turning

on

soon as work

9.

Precautions Before

a.

A basic law of safety is to

the equipment is completed.

power before making resistance checks.

a fuse will blow and

the

on

the power and as

Making

Resistance Checks

turn

short-

off

'J'hi

the

warning is many times repeated and is too many

times forgotten.

b.

Be sure that the power is off, not only to

avoid damage to the ohmmeter, but for your safety and the safety of others. ment

ha

s a main switch, turn

If

it

off, and remove

the fuses. Keep the fuses in your pocket.

circuit breakers are used in the equipment,

them

to

the off positions and lock the panel

Keep the key in your pocket while working.

.s

c.

Post a large sign such as.CAUTION! MEN

the equip-

If

tu1"'n

doo:r.

AGO 10016A

Figw·e

9.

Using

GROUND

sho1·ting

CLAMP

st-ick

to

discha1·ge

ca1Jacitor

to

g1·ound.

TM4000-38

11

CAPACITOR TERMINALS

GROUND

CLAMP

TM4000-41

Figure

WORKING

ON

POWER! in a conspicuous place near the

ON

EQUIPMENT;

main power switch.

and plug,

charge all a shorting stick

pull

the plug from the outlet. Dis-

high-voltage

(par.

10.

Using sho1·ting

DO

NOT TURN

If

the set uses a line cord

filter

capacito1

8).

This precaution ap-

·s by using

sticlc

to

plies whenever high-voltage equipment is being worked

charging a capacitor through

on.

d.

An ohmmeter is damaged as readily

it

as by connect-

by

dis-

ing the ohmmeter to a live circuit. Always

12

discha1·ge

ca1Jacito1·

te1·minals

di1·ectly

.

discharge the filter capacitors with a shorting stick before making resistance measurements.

10.

Troubleshooting

a.

Troubleshooting defective equipment

High-Voltage

Circuits

measuring voltages in high-voltage circuits with an

external voltmeter (some equipments have

built-in

ll

a

vo

ltmeters) is not recommended unless

other methods have been tried and have failed.

Always troubleshoot high-voltage circuits first

by resistance and continuity measurements

AGO

by

after

lOOlGA

analyzing the symptoms. Sometimes

it

is nec-

essary to troubleshoot or adjust high-voltage

on.

If

equipment with the power turned voltage precautions must

b.

is

Make

100

volts or more, all special safety

be

observed.

sure the test leads and prods are

the

properly insulated before making high-voltage measurements.

floor

the or damp

is absolutely dry. Working

floor

not work

is inviting trouble.

alone.

Make sure

on

a wet

Most

equip-

Do

ment is grounded to earth and accidental contact by

the

body

with a high-voltage point can cause

current to pass from a grounded point through

Use a

good

the feet to the heart or brain. mat

on

the

floor.

Beware of black mats; they may contain conductive carbon compounds. no

mat

is

available, use

some

dry boards, or sev-

rubber

If

eral layers of corrugated cardboard. Take these

you

precautions even if

Do

not take chances!

c.

The safest method for making voltage

think the

floor

is dry.

measurements in high-voltage circuits is to make all meter connections with the power turned

d.

If

the high-voltage equipment

no

one

tested with lock

is

jumpered out, be sure to hang a caution

standing

by,

and the inter-

is

off.

being .

sign nearby. The sign would caution anyone from accidentally touching exposed parts in the

equipment and receiving a dangerous shock. A better way to prevent such accidents is to avoid leaving exposed circuits unattended.

11. Low-Voltage Dangers

a.

Death can

voltage

as safety precautions circuits are to low-voltage circuits. a current that

low

voltage will

b.

High-voltage sets often are clearly marked to indicate the potential dangers present, but voltage sets may or may not

be

caused just as quickly

it can by high voltage. The

that

apply to high-voltage

be

observed when working with

If

a

low

voltage can cause

of

80

rna to

be

flow

through the body,

fatal.

be

marked. High-

by

low

sam~

low-

voltage equipment usually contains interlock switches. Low-voltage equipment may not have

interlock switches. This is dangerous for two

is

no

reasons : First, there

protection to prevent accidental contact with a B-plus point if a panel is removed which leaves such points exposed. Second, sion

it

gives the technician the false impres-

that

the equipment is harmles

s,

or

it

would have interlock switches. Low-voltage equipment is therefore treated too casually, with the

that

result

the technician tends to become care-

less.

Section Ill.

12.

RF

Burns

a.

RF

voltages behave somewhat differently

de

from

or low-frequency ac voltages. Accidental contact with circuits which contain relatively high

RF

voltages can produce a painful burn.

Th~se

times reaching the an odor very painful not only .

burns penetrate the skin deeply, some-

bone.

of

burning flesh. An

At times there may

RF

burn can

at

the moment of contact

be be

but afterwards, especially if an infection sets in. The danger of accidental contacts resulting

RF

in mitting equipment, especially

burns is present in, and around, trans-

in

the final am-

plifier tank circuit.

b.

High-frequency burns

equipment

that

operates in the burn may be experienced from equipment operates in the upper audio range and sonic frequencies.

For

?-re

not confined to

RF

range. A

that

super-:.

example, speech equip-

OTHER

HAZARDS

ment may develop a parasitic oscillation in the

supersonic range. Accidental contact with a

RF

terminal of a result in a use

a flyback-type high-voltage power supply

operating

at

inflict a painful

that

has

RF

voltage

at

its terminals,

power output tube could easily

RF

burn.

about

Some

television receivers

16

kilocycles (kc), which can

RF

burn. A circuit component

present may also have a high

so

that

contact with

can result in double danger.

c.

RF

and other voltages tend to reach out in search of a short path similar to the action of a corona (blue arc) discharge. Any high-voltage circuit component or solder joint

that points is a potential danger spot. This is especially true in the output section of a transmitter.

-points or edges produce a corona

Sharp burn component

d.

All high voltages which are not confined

parts

as well personnel.

properly can cause damage to equipment. Equip-

de

it

has sharp

that

can

AGO 10016A

13

ment operated in damp surroundings may be

to

subject may not

corona discharges. In daylight, corona

be

seen but

at

times can

be

heard as a hissing sound. Darkening the surrounding area may help locate the point of corona discharge; locating this point quickly will lessen the time needed ponents are subject

13.

Heat

a.

to

correct the fault and the time the

to

damage.

Burns

Almost

all

technicians have suffered from

com-

a heat burn either while learning electronics in

school

or while working

on

defective equipment

afterwards. Heat burns are painful and can serious if not given proper medical care. Working with hot solder, soldering irons, and equipment with hot tubes requires a little extra care on

the

part

of the technician.

b.

The technician is surrounded with many possible sources of heat burn s. Large tubes resistors in high-powered equipment are ponents

that

normally run hot. There are many

parts, such as power transformers, chokes,

resistors,

c.

that

run hot when an overload occurs.

Soldering is a daily operation for the tech-

anu

com

and

.nician. Usually, a soldering iron, kept hot during

the entire work day, is available. Keep the

so dering iron in a place where personnel cannot be

burned accidentally, and where the danger of fire to benches, furniture, and the building is

at

a minimum.

d.

Severe burns can result from touching hot tubes, even after the power is turned some tubes stay hot for long periods of time. move

receiving or other small hot tubes with a

tube puller. By using a tube puller, the

off,

because

Re-

tech-

nician is not only protected from possible burns,

it

but tubes without bending the pins. is

this is usually the coolest

14. Handling Cathode-Ray

tial danger ray tubes.

is a convenient method for extracting

If

a tube puller

not available, grasp the tube

part

a.

Many technicians

that

To

help prevent injury, wear a pair

do

not realize the poten-

exists when handling cathode-

by

of the tube.

Tubes

the ba

se-

of protective goggles and heavy gloves. Work slowly and carefully.

b.

The tube contains a high vacuum and,

pending

on its s

ize,

will have a force of hun-

de-

dreds of pounds, or even tons, of pressure act-

ing

on

its outside surface.

If

the

tube is broken, an implosion will result, and glass will all directions with tremendous force.

c.

Some cathode-ray tubes have an inner and

outer conductive coating

that

acts as a capacitor, with the glass between them serving as the dielectric. This capacitor can store the full power supply

voltage which may be thousands

of volts.

d.

Do

not touch the high-voltage terminal the side of the tube; contact with this point and the outer grounded coating can result in a terrific shock. The shock may not cause

be

arm

but

it

h

I •

the tube.

can cause

If

the

tube is resting

the

any

technician to drop

on

a bench, or

in the equipment, a shock will cause the tech-

nician to draw his hands away very rapidly, and probably touch another source of high voltage,

or cause bodily injury to a fellow worker.

e.

The

fact

th::

vt

-

for some time

the tube has not been used

doe~

not mean

that handle. A charge has been known to remain between the conductive coatings for weeks the tube was removed from the equipment. Be-

fore handling any cathode-ray tube, discharge it, by shorting the high-voltage terminal to the

l-

outer coating or ground. Even then

a

lw

ays safe, because a residual charge may be

For

present. regard a

f.

Do

neck, or l

this reason,

ll

cathode-ray tubes as dangerous.

not pick up a cathode-

et

it

fall, even a distance of 1 or 2

inches. When carrying at

all times. Grasp the widest with both hands, and rest body.

Be

very careful not to bump into any-

it

is good practice

ray

the

tube, use two hands

part

it

lightly against

thing or anyone.

g.

Do

not leave a cathode-ray tube in the open

it

when not in use. Keep

If

the tube i removed from the equipment temporarily, and tube face down

no

carton is available, place

on

in a carton, face down.

a cloth to prevent

from being scratched. Surround the tube

boxes or equipment bumped or

15. Selenium Rectifier Dangers

a.

be

Equipment using selenium rectifiers pre-

so

it

will not be accidentally

in the way of working personnel.

sents a hazard in the form of poisonous fumes and deposits. These poisonous gases

fly

on

physical

it

is safe to

after

it

is not

to

tube by the

of

the

bell

the

face

with

are

pro-

in

1s

14

AGO

1001GA

duced whenever a selenium rectifier burns out or arcs over.

b.

A burnout may result from a short circuit

which causes excess current to

flow

through the rectifier. An arc-over may occur because the voltage applied to the rectifier is too high, or because the rectifier

is

failing intermittently.

Either be

the immediate area

damaged rectifier until

type of failure will cause

the

poisons to

released.

c.

When this type of rectifier fails, ventilate

at

once.

Do

not handle the

it

has cooled, and then

avoid direct skin contact with it. Use pliers to

handle the

part

while

it

is

being replaced.

AGO 10016A

15

CHAPTER

3

TEST

Se

ction

I.

IMPORTANCE

16.

General

a.

This chapter contains information

many different types of test equipment are available for the technician's

use

out the kind of job each is intended to

test

though the

sets mentioned here may suitable for use with specific equipment, the ones listed in the technical manual should be

so

that

used

the readings obtained can be pared directly with the values listed in the manuals.

It

is not intended

that

the ment mentioned in this chapter replace test equipment authorized for use with specific radio

sets. Reference is made to specific ment merely to aid in the selection of suitable equipment when none is specified.

b.

Also included are special instructions which

se

deal with the u

ment.

Such items include the effects of circuit

of some types

of

loading, isolating requirements, interpretation of

Lissajous figures, and other necessary infor-

mation.

17.

Troubleshooting Without

a.

It

is possible for a

Test

Equipment

good

technician to diagnose many types of troubles in electronic equipment without using any

test

equ

ipment. A

automobile mechanic often can detect the fault

in an engine it

makes. In both cases, the senses and a knowl-

of

edge

b.

A stage can be alined roughly without a

by

merely listening to the sounds

the equipment are used in the diagnosis.

signal generator by tuning in a weak signal and adjusting trimmers for maximum output. This method, however, is very crude and inaccurate.

It

is possible frequency fore, the output is would

quency

that

the stage is being peaked

that

is not of the proper value; there-

of

the stage is not as

be

if

it

were alined

by

using a signal generator.

at

the correct fre-

on

the

which

and points

do.

Al-

com-

test

equip-

test

equip-

test equip-

good

at

great

as

EQUIPMENT

OF

TEST

EQUIPMENT

c. The technician is assumed to have a basic

knowledge of the Therefore, the theory of test equipment will not

be

dealt with here. Mention is made, however, of the use, the precautions to be observed, and the advantages and limitations of each type

be

te

st

equipment.

18 .

General

a.

The test equipment available to the tronics technician is varied, and that

the technician know the use for which each type is intended, kept to a minimum.

b.

Because of the different conditions under which troubleshooting type of

test

equipment select vary. Test sets used for work in the field must of necessity be more rugged and more compact than those used in depots and other fixed installations. Generally, field equipment is

rate

than

depot equipment. weight, because certain essary for making field measurements are

omitted.

19

. Troubleshooting With Test Equipment

a.

A technician will

place new equipment into operation, perform routine preventive maintenance, or repair plex equipment. In most cases, the job cannot be performed without cases, the job can be done more quickly, accu-

rately, and efficiently if the used properly.

a

b.

The technician will find a large variety of uses for tube testers, multimeter multimeters, signal tracers, oscilloscopes, and other devices which will be of

Types

so

test

sets he normally

of

Test Equipment

that

troubleshooting can

may

be performed, the

ed

It

parts

that

at

times be called

test

equipment; in all

test

great

it

is important

for a job may

is lighter in

are not nec-

equipment

s,

help to him.

uses.

elec-

be

le

ss accu-

on

to

com-

is

electronic

of

16

AGO

lOOlGA

c.

Many circuits require potentials

close

a certain value within

tolerances.

sure this without test equipment

to

is

impossible.

be

To

of

in-

Other circuits will have an output in which the

be

wave form must voltage and resistance measurements may very

close

to that required, yet the circuit may

of a certain shape. Here the

be

not function properly. device

is

the

only

An

oscilloscope or similar

instrument that can

be

used to display a visual indication of voltage wave forms. Another instance is in determining the field

strength of a transmitter

If

point. be

obtained, a

the proper

field

strength meter must

field

strength pattern

at

a specified

be

is

to

used.

20.

General

a.

Probably the most important single piece test equipment the technician has is

the multimeter.

used

item

in troubleshooting.

are useful for making ac and

It

is

the basic and most useful

Most

de

Section

at

his disposal

multimeters

measurements

II.

of

and resistance checks. Voltage and continuity

used

tests, the most useful functions, are

in the

simplest troubleshooting assignments.

b.

Multimeters are available in a variety types,

some

of which are very accurate, deli-

of

cate, and large in size. Others are small, rugged,

so

and not

accurate. Each type has its advan-

tages and disadvantages.

21.

Pocket-Type

Multimeter

(fig. 11)

a.

The pocket-type multimeter for relatively simple troubleshooting where curacy is secondary. The sensitivity

1,000

ohms per volt, which in itself prevents the

is

used mostly

is

usually

ac-

indications from being accurate under certain

conditions.

It

is

used especially for making

continuity and point-to-point resistance measurements. Because of its small size and portability,

it

is

very convenient for checking equipment in

field.

It

the

can also

be

used

for relative output

meter readings.

b.

The ranges on most small meters are

limited to those needed for doing simple jobs.

is

The pocket-type multimeter

suitable for

checking fuses, battery voltages, measuring the

of

output plies, and checking cables for cuits. ments

dynamotors or vibrator power sup-

open

and short cir-

It

is

not suitable for making measure-

in

high-resistance circuits because the

low meter resistance will load the circuits and give false readings. paragraphs

Circuit loading is expla\ned in

29

through

35.

The pocket-type voltmeter, properly ,used, can give very informative voltage measurements.

MULTIMETERS

Figut·e 11. Pocket-type multimeter.

22.

Portable

a.

A portable multimeter may be more accurate than the pocket-type. probably the most useful troubleshooting.

b.

The portable multimeter differs from the pocket type in measurements.

(db) scales; others give the technician a of two sensitivities for 000

ohms per volt and 1,000 ohms per volt. By

Multimeter

that

it

Some

multimeters include decibel

It

one

has a wider range of

de

measurements-20,-

0000-

5

is

larger and is

for all general

choice

AGO 10016A

17

using the proper meter,

accurately the voltages present and those should a sensitivity of

adequate for

be

present. Usually, the

1,000

ac portable multimeter with a sensitivity ohms

per volt

is

accurate enough for most volt-

age measurements

it

is possible to compare

ac

ranges have

ohms

per volt as this is

(not

RF)

measurements. A

that

are normally required in

of

that

20,000

electronic troubleshooting.

c.

The only serious limitation is

that

sometimes a 20,000 ohms-per-volt portable meter is not sensitive enough to indicate accurately the voltages in high-impedance circuits.

For

these

special applications, a vacuum-tube voltmeter

(vtvm) is required. A portable multimeter with 20,000 ohms-per-volt

ohms-per-volt

ac

sensitivity is shown in figure

de

sensitivity and 1,000

12.

23. Electronic Multimeter

(fig.

13)

a.

The most sensitive type of multimeter available uses a vacuum-tube circuit to increase the sensitivity. The indicating meter usually indicates plate current

that

varies in proportion

to the voltage under test. This instrument the electronic multimeter; measuring

ac

and

de

voltages and resistance.

it

is capable

of

The electronic multimeter is useful in troubleshooting high-impedance circuits where loading effects caused by nonelectronic types could erroneous indications or, in s

ome

cases, render the circuit inoperative. The input impedance about

11

megohms. This type of meter often

give

is

referred to as a vacuum-tube voltmeter, or

vtvm.

b.

A well-designed probe is useful for measur-

RF

ing

voltages with frequencies

megacycles (me).

RF

voltages are measured

up

to

1,000

by using a diode as a rectifier, then applying the rectified be in the meter case proper,

de

to the multimeter. The diode may

but

usually

is

located in the end of a probe to reduce the

capacitive loading

is because of the short lead between

on

the circuit under test. This

the

test point and the diode. Almost the entire length from the test point to potential, RF loss

es

rather are

than RF.

also reduced.

the

meter is

It

can

be

at a de

seen

that

is

18

FigU?·

e 12. Portabl e multimeter.

TM

4000

·86

AGO 10016A

TM

4000·

7

MOLOED POLYSTYRENE HEAO

Figu1·e 13. Elect1·onic

Figu1·e

14.

RF

multim

WATERPROOF

SEALING NUT

probe.

eteT.

.

PROBE . CABLE

™

4000-9

Section

Ill.

USING

24. Precautions

a.

Certain precautions must

be

observed when using a multimeter to prevent damage from mishandling, jarring, or dropping. Carelessness in making resistance measurements also may result in damage to the meter.

b.

Be careful when measuring voltages to set the voltage selector switch to the highest range before connecting the meter into the circuit. the meter is connected to a voltage source

AGO J0016A

If

that

MULTIMETER

is too high for the selected range,

be

probably will

c.

If

the multimeter is set to read current

ruined.

the

meter

or

resistance and a voltage is applied across the test

it

prods, damage will result. Therefore,

that

portant

all switches

on

the meter be

is im-

set

at the proper positions before making any measurements.

25. Measuring

a.

With the aid of a schematic diagram,

De

and

Ac

Voltages

de-

19

termine whether the voltage to ac

or

de,

and set the meter switch accordingly.

If

the meter switch

is

set for

ments and is connected to a source

be

measured is

de

voltage measure-

of

ac,

the meter needle will not be deflected and damage to the meter may result. For measuring voltages that are positive with respect to the chassis, the negative test lead (black) usually is connected to the chassis or other reference point,

be

and the positive lead to the point to

If

the opposite polarity is to

be

measured.

measured, the

red and black leads can be reversed.

b.

Sometimes a 60-cycle ac line voltage will cause the needle to quiver meter. This can violent motion

be

of

the needle

on

destructi

de

settings of the

ve

because the

could

shake the

meter movement out of balance.

c.

Unless the multimeter being used is specifi-

only

cally designed to read peak voltages,

sinewave voltages will give accurate indications. The ac scales brated to read

s

in

e-wave voltage. The meter indication multiplied peak voltages can loscope. This will

26.

a.

on

the conventional meter are cali-

by

1.41

is the peak voltage.

Measuring

Occasionally,

.707

times the peak value of the

If

be

measured with an oscil-

be

covered in another chapter.

RF

Voltages

it

is necessary

to

measure

necessary,

RF

voltages. This is usually done with an electronic

multimeter and an tions must ings.

be

To

minimize the effects of stray

RF

probe. Certain precau-

.observed to prevent false read-

fields

near the point in the circuit being tested, the probe tip must with a piece of wire for conveni

extension could result in erroneous readings

be

left

as

it

is;

it

should

not

en

be

ce.

extended

Such an

at

higher frequencies, because the extension will

pick up stray

good

practice not to grasp the probe

or to bring the hand too

radiat~d

RF

close

energy.

to the probe tip.

It

too

is also

firmly

Body capacity effects may introduce additional errors.

b.

Another

using a probe

ground leads

cau13e

of

erroneous readings when

is

a faulty ground connection or

that

are too

long.

A sign

of

a poor ground connection to the probe is the inability to repeat a voltage reading when the ground

con-

nection is shifted to another point.

27.

Measuring Current

Current readings are se

the circuit must

into the circuit.

it

ranges,

cause if

should be used

it

be

opened to connect the meter

If

the meter in use has current

is set to a

ldom

made because

with

special care,

low

current scale and is

be-

connected into a circuit of higher current, the meter will be ruined. The important thing to

that

remember is meter must cuit. The current range

sufficient to pass the

for current measurements the

be

connected in series with the cir-

se

lected must

current

present in the

also

be

circuit.

28.

Measuring Resistance

a.

One

of the most important tasks

of

trouble-

shooting is the measuring of resistance. Under

no

ordinary circumstances, there is

that

of damaging a meter

is being used

possibility

to

measure resistance. However, when making re-

on

sistance measurements that

has been in use

a piece of equipment

just

prior

to

troubleshoot-

ing, caution is necessary.

b.

Circuits which include high values capacitance can capacitors should else is done.

be a so

be

If

this is not done, the voltage re-

urce of trouble. The

discharged before anything

maining in the capacitors may

be

sufficient

of

to

throw the meter needle against the stop pin.

This will ruin the meter or make the readings

become inaccurate.

c.

When measuring resistance with a multi-

it

meter, his fingers

is important

off readings may which will

be

the test prods. This is because the be

in parallel

that

the technici'an keep

affected by his body resistance,

with

the

part

being

measured.

d.

When a multimeter is stored for future

use,

the selector switch should

be

set to the

off position or to the highest voltage range. This will protect the meter from

a technician uses

it

in a high-voltage circuit

ssible damage

if

po

without first checking the setting. The battery should also

e.

The resistance

be

removed from the multimeter.

of

individual parts and circuits is often difficult to check because of the shunting effect of other components in the equip-

It

ment. one terminal of the

will then be necessary to disconnect

part

in question from the

rest of the circuit to obtain accurate readings.

20

AGO 10016A

29. Definition

a.

When a voltmeter points added

in

a circuit, the resistance

to

the circuit. This may change the cir-

cuit voltages.

of

Circuit Loading

is

connected between two

Such a change

Section IV.

of

the meter is

would

be

CIRCUIT

due to

circuit loading by the meter.

b. The amount of circuit loading produced

a voltmeter depends meter.

Circuit loading results in a meter read-

on

the resistance

of

by

the

ing that is lower than the actual circuit voltage

without the meter. The technician must aware

of

this and must learn to make allowance

be

for it.

c.

The ideal voltmeter should measure the voltage in a circuit without upsetting the circuit conditions. However, every meter draws

power from the circuit under test. is

of

the low-sensitivity type and is connected

If

some

the meter

across a high-resistance circuit, the meter in parallel with the circuit under test forms a

low-

resistance circuit. The total current increases;

flows

this increased current

through other resis-

tors in the circuit and produces an increase in

the voltage drops across them. The result is

that there is less voltage available to operate the meter and a lower reading is indicated.

30. Example

a.

To understand the effects

ing, refer to the circuit shown in

This is a simple series circuit consisting

of

Circuit Loading

of

circuit load-

A,

figure

of

15.

two identical resistors and a battery. For the values shown, the voltage between point P and ground

be

should

half the battery voltage;

volts. Suppose the technician

uses

that is,

a 1,000 ohms-

6

per-volt meter to measure this voltage. The volt-

that

age

is expected to

therefore, the meter scale would probably

be

indicated is 6 volts;

be

set

to the 10-volt scale. This means that the meter

now

resistance is

10,000 ohms. When the meter

is connected from point P to ground, the circuit

no

longer like

is in

B.

Notice meter, is in parallel with 10,000

ohms. ohms; this is the effective resistance series with ground is

that

in

A,

but is like

that

R~t.

the 10,000 ohms in the

R~,

The resultant resistance is

that

shown

which is also

5,000

that

is in

Rt. The voltage from point P to

no

longer half the battery voltage,

but is now only 4 volts.

LOADING

b.

For the example used, the difference is quite large. volts instead the difference loading error

tolerance

-=-12V

Figu1'e 15. How a low-sen

31. Reducing

a. When

Since the meter indicates only 4

of

the true value, which is 6 volts,

is

33% percent. Notice

far

outweighs the 5 percent

of

the meter.

Rl P

IOK

it

GV

1,000 OHMS-PERVOLT METER

10

-VO LT RANGE

4V

8

TM4000-I

Effects

sitivity

ci1·cuit.

of

voltntete1·loads a

Circuit Loading

is necessary to measure voltages

that

the

A

in a high-resistance circuit with a low-sensitivity meter, the effects duced by using a higher range than required. This makes the meter accurately, because the needle is flected

only

a small amount. However, the higher

of

circuit loading can

it

more difficult

that

be

re-

actually

to

read

de-

range increases the meter resistance and re-

duces the loading effect.

b.

In

A,

figure tween point P and ground with a per-volt meter, but the 50-volt scale is used. shown in

is

now

B,

50,000

Rt results in an effective resistance of

ohms. The voltage read which means

figure

ohms.

that

16,

the voltage is measured be-

1,000 ohms-

16,

the meter resistance,

This value combined with

8,333

on

the meter is

5.1

volts,

the difference caused by the

As

RM,

AGO 10016A

21

Rl

IOK

6V

p

1,000 OHMS-PER-

METER

VOLT

50-VOLT

RANGE

Rl

IOK

6V

p

20,000

OHMSPER-VOLT 10-VOL

METER

T RANGE

-

Rl

IOK

-=-12V

Figu1·e 16. R edtteing

5.1V

p

cinuit

voltage

loading

1·ang

e.

8

TM4000-2

by

using higher

A

meter is now only 15.7 percent. While this

difference

ing to a higher range can reduce the effect

is still

too

high,

it

shows

that

switch-

of circuit loading. The result should be compared with

that

obtained in figure

32.

Using More Sensitive Voltmeter

a.

Because the error is the result

relatively

low

resistance

15.

of

the voltmeter, a

of

the

higher resistance voltmeter should be used to

low

keep this difference as sensitive meter requires

as possible. A more

le

ss current for its

operation and therefore uses a higher value of

multiplier resistance for the same voltage range.

Thus, the more sensitive the meter, the higher is

the ohms-per-volt rating and the lower is the

on

loading effect

b.

ACJ.

example of-using a more sensitive meter

is

shown in figure between point per-volt meter. In

be

200,000 ohms. This resistance in parallel

,

with R

which is 10,000 ohms, -results in an

2

effective resistance current is about

c.

The meter indicates

half the applied voltage,

now

is

only 5 percent, which is acceptable

the circuit under test.

17.

The voltage is measured

P and ground with a 20,000 ohms-

B,

.6

figure

of

rna.

17,

RM

is shown to

9,524 ohms, and the

only

slightly less than

5.8

volts. The difference

fo:r

-

Rl

IOK

Figu1·e 17. How a se

ff

ects

e

5.

8V

p

nsitiv

e voltrnete1·minimizes the

of

ci1·cuit loading.

8

TM4000-3

A

most work. Thus, by using a more sensitive

meter, the accuracy of voltage readings obtained

in high-resistance circuits is improved.

33.

Advantage

a.

The advantage of using

meter is

that megohms means

that

of

Electronic Voltmeter

an

electronic volt-

its input resistance is about

on

any of the meter ranges. This

there will be practically

no

10

circuit

loading.

b.

The electronic multimeter is connected from point between these two points becomes The current is volts. The difference is only

34.

Practical Example

a.

P to ground (fig. 16). The resistance

.6

Figure

rna and

18

shows a practical example of

the

voltage is

.17

percent.

of

Circuit Loading

9,990 ohms.

5.99

how circuit loading can give a voltage reading that

is

far

removed from socket voltages of this audio amplifier stage measured with a voltmeter of

20,000 ohms per volt. This sensitivity usually is adequate for most measurements. In 18,

the cathode voltage is developed across

a relatively low resistance.

the

true value. The

that

has a sensitivity

If

the voltmeter is

A,

are

figure

Rr<,

set to the 10-volt range, the total meter resist-

RM,

ance,

is 200,000 ohms. With

this

value

of

22

AGO 1001GA

resistance shunted across

sistance across

RK

RK,

the change in re-

is negligible and the meter

indicates the true value of 8 volts.

b.

Suppose the technician desires to read the bias voltage from grid to cathode. meter is the

now

connected from grid to cathode,

500K

grid resistor is in series with

If

the same

one

meter

lead and the voltage source. The voltage source

in this case is the drop across the cathode bias

resistor.

c.

The circuit in

version of

that 8-volt drop across in series with the divider is formed by

RM.

ance,

The result is the meter indicates voltage drop is across

at

B,

figure

A.

The battery represents the

RK.

500K

Ra and the meter resist-

only

18,

is a simplified

This voltage

is

resistor. A voltage

that

instead of 8 volts

2 volts. Most

Ra.

If

a meter with a

applied

of

the

lower sensitivity were used, the voltage reading

RL

+B

A

would

even less. The voltage as read

on

any

be nonelectronic multimeter, even a very sensitive one, must decision

be

carefully interpreted before any

is

made as to whether there is a defect

in the circuit.

35. Summary

a.

Circuit loading is a problem in

as well as in

meter resistance or impedance should

of

Circuit Loading

ac

de

circuits. In either case, the

be

circuits

many

times the resistance or impedance between the

meter and the point being measured. When the

no

circuit has little or power supply or battery, no

problem. Here the shunting resistance, a bleeder for example, may left out. However, the meter drain is

compared to the available current

series resistance, as in a

circuit loading is

be

very high, or even

so

small

that

loading

is unimportant.

b.

To minimize the effects

the following points should

of

circuit loading,

be

kept

in

mind and

followed:

(1)

Use

a high-resistance voltmeter.

For

(2)

negligible loading when making voltage readings, the meter resistance should than the resistance

be

at

least

20

times greater

of

the circuit being

measured.

(3) Use the highest voltage range practica-

ble which still allows an accurate reading to range

be

obtained. The higher the

used,

the higher the meter resist-

ance and the less the circuit loading.

Fig

AGO 10016A

K

'------...:;+

Me

18. Meas'u?·ing bi amplifie

2V

BV BIAS

111---------.

as

?·

stage.

voltage

in

an audio

B

TM4000-4

( 4) Use an electronic voltmeter if possible. (5)

To

determine whether the meter is loading the circuit, measure the voltage

on

two or more ranges.

voltage is not read

on

If

the same

all ranges, the

meter is loading the circuit.

c.

The chart below shows the relative circuit loading effect of a nonelectronic voltmeter as compared with voltmeter. Note

that

of

one

type of electronic

that-on the highest range

on

the nonelectric meter, the input resistance is greater th

an

that

of the electronic voltmeter.

23

Range (volt

Input

resistance

(meg)

s)

Vt

vm

Nonelectronic•

Circuit

londin

g effect

5 10 50

100 500

1,000

•

Nonelectronic

36. General

a.

Another type of loading which

voltmeter

10 10 10 10 10 10

(20,000

Section

ohms

per

volt)

V.

. 1

. 2 1 2

10

20

.

ISOLATING

is

similar in effect to meter loading occurs when certain types of test equipment are used. This is capacitive loading of a circuit and

it

is caused

by

the test leads of the instrument being used to measure de

voltage when

RF

is present. An example of this type of loading occurs when the technician attempts to measure grid bias voltage in the local oscillator of a superheterodyne receiver. Circuit loading is noticeable mostly when the test

leads are effectively connected across a

tuned circuit. The capacitance between the meter

test leads can

be

large enough to cause detuning of the circuit, resulting in the oscillator operating

at

the wrong frequency.

b.

To

minimize this type of loading, isolate the circuit from the test equipment as much as possible. This is accomplished most conveniently in the vtvm

by

including a large series resistor

directly in the probe, thus effectively separating

the

test

lead capacity from the circuit. The

series resistor in the probe combines with the

test lead capacitance to form a low-pass filter

RF

which confines the

to the circuit under test,

ISOLATION

Nonelectronic voltmeter

Nonelectronic voltmeter Nonelectronic voltmeter Nonelectronic voltmeter 5 times Non electronic voltmeter same Nonelectronic voltmeter

MULTIMETER

FROM CIRCUIT

yet allows the though the

de

100 times 50 10 times

half

de

voltage to be measured.

times

as

that

of vtvm.

vtvm.

of vtvm.

of vtvm .

of

vtvm.

voltage impressed on the meter is reduced, this is of no consequence because the vtvm has been designed and calibrated with the resistance in place.

37. Using External Isolation Resistor

By

the

use of a low-sensitivity multimeter which is not suitable for making measurements of grid bias in low-powered

RF

circuits,

possible through adding a high-value resistor

(usually

usable indication

10,000 to 100,000 ohms) to obtain a

that

the circuit is functioning

(fig. 19). The resistor may be clipped tempor-

arily to the

around the dicated

on

point or

test

prod. Although

the meter will

it

may be wrapped

the

voltage in-

be

influenced by the

test

divider action of the isolation resistor and meter resistance, at

the grid of a low-powered oscillator is suffi-

cient evidence

the

presence of bias voltage

that

the circuit is oscillating. Another indication of oscillation is meter needle will show a quick prod is touched to the

grid;

lciclc

then

as the

it

V.:m

to zero.

that

return

Al-

it

the

test

ia

24

Figu1·e

VOLTMETER

1.9.

Method

{o1·

measu1·ing

oscillato1·

bias voltage

by

using

I

TM4000-24

iso

lating 1·esistor.

AGO l0016A

Section

VI.

EXTENDING

RANGE

OF

VOLTMETER

38.

General

Sometimes a voltage

the range of the multimeter available.

to

be

measured is beyond

Some multimeters have a high-voltage probe especially designed to extend the range

If

a high-voltage probe is not a

equipment, two methods can

One

the voltage.

requires the addition of sev-

of

the instrument.

part

be

used to measure

of

the

eral series-connected resistors across the voltage

so

that

source to form a voltage divider

be

measurement can

made across

one

of

a

them and the total voltage computed. The other method requires the addition tiplier resistor in series with the meter lead

of

an extra mul-

to

increase the range.

39.

Voltage-Divider

a.

The simplest method

resistors in series across the voltage source.

Method

is

to connect

10

equal

To avoid loading the circuit, these resistors should be

quite large, but small compared with the

meter resistance. Then the voltage measured

across

resistor is multiplied

by

10

to

give the

one

total voltage present.

b.

As

an example, the technician knows

that

the total voltage should be about 10,000 volts.

on

The highest scale meter available is resistance of the meter

of

used. Each

the

the 1,000-ohms-per-volt

1,000 volts. Therefore, the

is

1 megohm

10

equal (approx. 250,000

on

the scale

ohms) resistors in series has 1,000 volts across it.

c.

As

a safety precaution, the measurement should cuit to the first resistor in the voltage divider that

be

made from the ground side

of

the cir-

so

the lowest potential is applied to the meter.

As an additional precaution, the connections

be

should off.

made only when the power is turned

After the connections have been made,

apply power and read the meter.

40.

Added

a.

When using this method,

Multiplier

Method

it

is

necessary

to know the meter sensitivity for nonelectronic types, or the input resistance of an electronic type. For example, to check a 7,500-volt circuit with a meter having a sensitivity per-volt and a top range of convenient since the meter will

to

increase the range

now

indicate 10,000 volts full scale, all readings must to

find

the voltage present.

b.

The first step is to determine the total resistance required for measuring scale reading. This total resistance is found multiplying the sensitivity age reading; plied by the

1,000-volt range already includes a

that

is, 5,000 ohms-per-volt multi-

10,000 volts, or

by

50

of

5,000 ohms-

1,000 volts,

10

be

multiplied

it

is

times, and

by

10

10,000 volts full-

by

the full-scale volt-

megohms. However,

5-

megohm multiplier resistor. Therefore, an addi-

45

tional

series with the meter lead

indicate

c.

20

megohms and a maximum voltage range of 1,000 volts, resistance to form a In

this case, the added resistor must be 9 times as great as the meter resistance, or megohms. In practice, a resistor of may not

megohms in resistance is required in

to

cause the meter to

10,000 volts full-scale deflection.

In a vtvm which has an input resistance

it

is

necessary to add sufficient series

10

to 1 voltage multiplier.

180

megohms

be

available.

It

can be formed by

of

180

con-

necting several lower value resistors in series.

Section

41.

Use

of

Multimeter

a.

The power output of a receiver or audio

as

Output

VII.

Meter

OUTPUT

amplifier must often be measured to determine whether the equipment is operating properly and has the required sensitivity.

One

procedure

for making power measurements involves the

use of an output meter. This kind usually has a selection

of

loads which can be connected

AGO 10016A

of

ac

voltmeter

standard resistance

to

the equipment

AND

DB

METERS

to be tested. Output meters are usually calibrated in decibels.

b.

An

ac

voltmeter and a resistive load can be

used in place

of

an output meter. The normal

load for the equipment may be a headset or loud-

speaker, but these are not suitable for accurate

power measurements because they

constant impedance to all frequencies.

do

not have

More uniform results can be obtained by using a resistive load. The resistive load must have the

25

(!)

(i

w

OJ

fill;

~

@

Q

Rf

SIGNAL ANTENNA

GENERATOR

Figur

·e

20. Checking

OUMMY

r·ec

eiver· se

0

nsitivity

0

0 0

RECEIVER

by

measur·ing

OUTPUT

8 -

LOAO

output

0HM

pow

e1·

.

same value as the impedance of the receiver output and must have a wattage

rating

as great as the power expected from the equip-

ment under test.

c.

Since the meter can indicate only the

voltage across the resistive load, the technician

do

some

must

simple computations

power. However, the technician should know

how to

himself. Figure

20

shows a typical

do

it setup for checking the sensitivity of a receiver with an ac voltmeter used as an output meter.

test

This

(AF)

value when a

requires

output power

sta

applied to the antenna terminals.

that

the audio frequency

be

at

least of a certain

ndard modulated

RF

If

resistance is 8 ohms and the minimum power output required is must indicate

be

can

P

watt

found by using the formula E =

d.

Another formula

E

'~-

=R

Note. In both formulas, P is in watts,

.

s is written as

100 milliwatts, the voltmeter

.89

volt ac or more. This value

that

can

be

.and

.1

watt.

AC

VOLTMETER

TM4000-8

at

to

find the

signal is

the

VJ!R.

used is

100

least

ac

loa<l

milli-

26

Figure 21.

Ac

electronic rnultirnetm·.

TM4000·JO

42.

a.

Fi

Db

Meters

Ac

voltmeters intended primarily for use

VOLTS

RMS

1.5

gur

e 22. Met

er·

fac e, showing nonline

ar

TM4000-17

db scale.

AGO 10016A

with audio frequencies usually have a in addition to the voltage scales. This makes

db

scale

it

convenient to make power output measurements

db.

An

ac

when the power level is given in

.db

meter having a

scale is shown in figure

The scales are shown more clearly in figure

volt-

21.

22.

The voltmeter uses a nonlinear scale for voltage

measurements and a nonlinear scale for

db measurements. The range of the meter can be changed in 20-db steps by using the selector

on

switch

b.

to read directly in

the panel.

The meter shown in figure

db

when the meter switch is

21

is

calibrated

set to the 0-db range and is connected across a

600

load resistance of ence

for

db

measurements is 1 milliwatt (mw)

ohms. A

across a load resistance of

0

db.

taken as .775

volt. When using the

This corresponds to a val

common

600

ohms,

db

ranges, a correc-

refer-

and is

ue

of

tion factor must the reading to obtain the true ing

that

the load resistance correction factor is marked opposite the setting of the selector switch each range. A reading with the selector switch indicates a figures are added algebraically. is calibrated for 1 used

to make measurements across a load

600

not

ohms, the

level

be

of

added

mw

to

or subtracted from

db

level

is

600

ohms). The

on

the meter panel

on

the

db

scale of

on

the

-20-db

-15

db.

Note

If

the

across

db

readings require inter-

600

ohms, and is

(assum-

that

db

+5

db

range

these

meter

that

on

is

pretation or correction. A correction factor to be

added

ing can

to

or subtracted from the meter read-

be

calculated from the formula

below,

where Z is the impedance of the circuit under test.

db

to

be

added=

600

10Jo~:

z

Section

VIII.

SIGNAl

43. General

A signal generator is a piece

that

ment

generates an ac signal. An

of

test equip-

RF

signal

generator is, in effect, a small radio transmitter.

be

The generated signal may modulated, and can

be

modulated or un-

used for alinement of tuned circuits, dynamic troubleshooting (signal tracing), sensitivity measurements, field-intensity and stage-gain measurements, and signal

substitution.

44.

RF

Signal Generators

(fig. 23)

a.

The

RF

signal generator is used for locat-

ing troubles that are difficult or impossible to find by making voltage and resistance measurements. Trouble may

be

localized to a specific stage by injecting a signal from a signal generator into the suspected stage or section and

observing the results.

b.

RF

signal generators are constructed for

different frequency ranges and accuracy. The

an

RF

output of

signal generator may

lated, unmodulated, or audio. Any

three types by

the repairman for troubleshooting receiving

of

output may

be

selected and used

be

one

modu-

of the

equipment.

c.

Some generators have both am and fm out-

GENERATORS

puts. Portable units are of necessity smaller

and lighter and are limited in their accuracy.

may

be

necessary to check the frequency calibra-

tion with an accurate frequency meter.

It

Some

types have a meter for accurately measuring the

RF

output level.

d.

RF

signal generators used in depots

usually have accurate frequency calibration

be

and the modulation percentage can In

addition, the

accurate control

RF

output level is metered for

of

the output. This makes these

adjusted.

generators very suitable for work where ex-

low

tremely

signal levels of high accuracy are required. This is important for certain alinements and for measuring signal-to-noise ratio

low

that

levels

and signal-plus-noise to noise. Generators are not accurately calibrated to very in output voltage can give misleading results.

45.

Sweep Generators

a.

A sweep generator is an fm signal genera-

tor

that

sweeps across a range of frequencies

and repeats the sweeping

at a fixed example is a signal sweeping between 32

me

at

a rate of

60

cycles. A sweep generator

rate. An

28

and

is especially useful for certain types of aline-

ment where a special shape of response curve is

required.

b.

A sweep generator may have a marker gen-

AGO 10016A

27

F

igu1·e

28.

Amplitude

modulation

signal

generato1·.

erator built into it. A marker generator is a

signal· generator

that

is capable of producing an accurately calibrated signal. Its purpose is indicate the frequency

at

various points

on

the

response curve by superimposing a marker sig-

on

nal an oscilloscope.

the curve. The curve must

If

a marker generator is not built into the sweep gener marker generator can be used in

be

viewed

at

or, a separate

parall

el with

on

it.

46. Audio Frequency Signal

(fig. 24)

a.

An audio frequency signal generator, sometimes called an audio oscillator, is capable producing signals of frequencies ranging from 20

to

20,000 cycles.

It

is used mainl y in troubleshooting audio frequency amplifiers, but is al useful in helping the technician determine the harmonic content of signals in audio circuits.

Refer

to

paragraphs

b.

When the generator is used for simple sig-

121

and 156.

nal substitution, a signal of any audible fre-

be use

d.

quency can

to

be

checked for harmonic content, the shape

When an audio amplifier is

Generators

so

of the wave is very

AF

square-wave generator can be us

variations in

to

important. In

the

output wave shape can

determined and compared.

47.

Matching Impedances

a.

The output impedance of

tor

should

the circu

be

matched to

it

under test. This is important, espe-

the

signal genera-

the

input impedance of

cially when alining a receiver and when measur-

ing the sensitivity of a receiver

after The signal generator usually can be terminated properly by using one of p

li

ed

as

part

of

termination eliminates ing

waves

b.

If

a signal generator with an output im-

pedance

of

of the

on

the

test

50

ohms is used, and is terminated with a 50-ohm load, the correct the

receiver input terminals can be measured.

If

the proper load were not us readings would res nected to

the

ult. With

cable and

receiver with an impedance of

test

the

cable.

the

resistive pads sup-

equipment. Proper

possibility of stand-

RF voltage

ed,

a 50-ohm load con-

cable connected to a

300 ohms, there

this case, an

ed,

so

that

be

alinemen

t.

at

inaccurat~

28

AGO 10016A

TM4000·I2

FigU?·e 24.

would be an impedance mismatch which would

cause inaccurate readings.

c.

An impedance-matching network can

be used to match the signal generator to the receiver input. An example is shown in figure The coaxial cable ohm

resistor, and the receiver input

sees

the

50

ohms

in the

sees

25.

50-

the

total resistance of the three resistors in series.

290

ohms,

The total resistance is close

match for

48.

Using

a.

Some general rules to keep in mind when

TO

SIGNAL )

GENERATOR

Fi

gu1·e

300

ohms.

Signal Generator

~on

COAXIAL

CABLE

l

25.

Imp

t"

edance-matching ne

which

120

:---:,~:!::·

120

is

TM4000-222

twoTlG.

a very

Audio

oscillato1·.

using

RF

signal generators are given below. For

best results, read and follow the instructions

given in the manual for the equipment. Although

most generators have control markings which

are self-explanatory, special instructions must

be

often

b.

followed to avoid erroneous results.

Whenever possible, the signal generator specified for the equipment in the technical manual or Repaired Equipment Standard should be

used, otherwise inaccurate results may occur

of

because

incorrect impedance matching. When

alining a receiver, best results are obtained if a

If

no

dummy antenna is used.

200

UUF

I(

c~

400

(

FROM

SIGNAL

GENERATOR

l

0

20UH

UUF

dummy antenna

~

Figu1

·e 26. Dummy

ant

enna, schematic

diagmm.

0

I

TO

RECEIVER INPUT TERMINALS

j

0

TM4000·61

AGO 10016A

29

is specified, a dummy antenna can

be

constructed (fig. 26). This is important because the correct dummy antenna simulates the im-

that

pedance

would

is connected. The unit should

be

present when the antenna

be

inclosed in a

grounded metal container and used with a sig-

that

nal generator

exceeding

c.

capacitance

50

This dummy antenna looks like a

at

has an output impedance not

ohms.

low

frequencies, is a complex im-

200-~Lf

me,

pedance around 1 resistance

d.

It

is import

at

frequencies from 2 to

capacitor in series le

ad. This is done to prevent accidental contact

with high-voltage

and looks like a 400-ohm

30

ant

to connect a

with

the signal generator hot

de

from damaging the gen-

de

me.

blocking

erator. In some equipments, this capacitor is a part

of the unit, but unless a blocking capacitor

is known to be present, an external capacitor

ld

shou

be used.

49.

Section

General

IX.

CATHODE-RAY

The cathode-ray oscilloscope is an electronic instrument of

one

that

provides a visual representation

electrical voltage as a function of another on the screen of a cathode-ray tube. The main feature of the oscilloscope is its ability to por-

tray

graphically and instantaneously the

tuating circuit conditions. The electr

on

fluc-

beam has negligible inertia; therefore, the cathode-ray tube responds to much higher frequencies than any other electrical indicating device. A typical general-purpose oscilloscope is shown in figure

27.

50. Advantages of Oscilloscope

a.

The oscilloscope is useful for certain types of troubleshooting where multimeters and signal generators are not suitable. Ordinarily, the

multimeter is the ideal instrument for indicating

common troubles such as incorrect or

no

voltage, short or open circuits, changes in value of resistors, and defective capacitors. However, when troubles occur which

not show

up

as incorrect

do voltage or resistance readings, using the oscilloscope is often the only sure way to locate the

For

trouble. distorted signals in the output amplifier can be more quickly isolated

example, such troubles as hum or

of

a receiver or

if

an oscilloscope is connected to various points in the circuit. In the same manner, an open bypass

be

capacitor can signal voltage appears where there shou none. An example of this is

located by noting whether the

ld

be

at

the screen grid of an amplifier tube; here the capacitor bypasses the applied signal to cathode or ground. oscilloscope indicates a signal voltage present

the screen grid,

it

means there is

no

If

the

at

bypass ac-

OSCILLOSCOPES

tion

Figu1·e 27.

at

the signal frequency and the bypass

General-pu?'JJose

oscilloscope.

capacitor is therefore open.

b.

Other examples of troubles where ible wave form is helpful the

power supply filter

that

to see

the ripple is reduced to a negligible

amount; checking the

are:

for

gain

of a stage by noting

when checking

open capacitors and

the

vis-

the increase in signal amplitude from the grid

to

the

plate circuit; checking

supplies to see

that

the output wave form is cor-

vibrator

power

rect, and checking circuits where nonsine-wave

are

voltages

involved, such as in multivibrators

and similar circuits.

c.

In

addition,

the

oscilloscope is useful for

determining frequency ratios by means of Lis-

30

AGO 10016A

sajous figures, or checking frequency response or 'phase shift in audio amplifiers, and checking the percentage of modulation in transmitters.

d.

The oscilloscope is also useful for certain

types of alinement. which requires

that

the response curve have a special shape. Examples of this

adjusting the

IF

response on re-

are:

ceivers which use stagger tuning for a broad

pass band where a fiat top characteristic is re-

quired; alinement of the discriminator in fm receivers which use automatic frequency con-

IF

trol, or the response of the

amplifiers in a

television receiver in which the response is not

but

symmetrical

which must have a special wave shape for proper operation of the receiver. In

order to use the oscilloscope to examine the response curves mentioned above, a sweep generator

output is required

at

the input to the

receiver. This requires special techniques for

synchronizing the sweep generator with the oscilloscope

so

that

the proper stationery image

is produced.

51.

Care

of

Oscilloscope

a.

Since

the

oscilloscope is comparatively

complex, the technician should become familiar

with

it

before using it. He will not only use it

it

safety but will learn to use

do

this

best way to manual procured with

b.

When using the oscilloscope, observe these

is to read the technical

the

properly. The

oscilloscope.

precautions to avoid damaging it.

(1) Never leave the brightness turned

when sent; should never be

there that

is no sweep voltage pre-

is, a single spot of light

left

on the screen.

up

If

a spot of light is allowed to remain on the

screen, a hole will be burned in the

screen material and ruin the tube.

Usually,

the

oscilloscope is used with

the internal horizontal sweep in opera-

tion ; therefore there is less danger of

damage.

Some tests require

(2)

turned

off

and external voltages ap-

that

the

sweep

be

plied to both horizontal and vertical inputs. To be safe,

turn

down

the

brightness level until the signal volt-

turn

ages have been applied, then

just

far

the brightness,

the

pattern

clearly.

enough to see

up

(3) A void placing the oscilloscope where

light will fall directly on the cathode-

ray

tube. This would require

turning

the brightness up higher. The higher

the brightness the shorter the life of the tube.

If

necessary, use a shield

to block out the surrounding light.

( 4) Locate the oscilloscope away from

strong magnetic fields.

Such fields can distort the image on the cathode-ray tube and also magnetize the oscilloscope case or the deflecting plates.

If

this happens, the oscilloscope may become after

permanently inaccurate, even

the source of magnetism is re-

moved.

52.

Helpful Hints for Using Oscilloscope

The following hints will aid in the use of the

oscilloscope.

a.

It

is

good

practice to set the horizontal fre-

so

that

on

at

the

quency control on the oscilloscope least two cycles of the wave form appear screen. Figure wave.

Oscilloscopes vary in the number of verti-

28

shows two cycles of a sine

cal sweep amplifier stages they include, with

that

the result

some oscilloscopes may produce

an inverted image because normally each am-

plifier stage inverts the signal in polarity. dinarily, this

is

no

handicap when examining

Or-

sine waves or other symmetrical wave forms. However, when doing alinement work with a sweep generator the required response curve will appear inverted

on

the screen of some oscil-

loscopes.

TM4000-15

Figu1·e 28. Oscilloscope

11atte1·n

of

a sine wave.

showing two cycles

AGO 10016A

31

b.

Another characteristic which may vary

from

one

oscilloscope to another

of the slope of the trace for comparison of

is

the direction

inphase and out-of-phase signals. This can be checked easily

by

applying the same signal to both vertical and horizontal terminals with the internal sweep turned

off.

The signal source can be the 60-cycle voltage taken from the test terminals usually located

on

the front panel of the oscilloscope. The trace produced will slope either to the it

slopes represents the in-phase condition.

right

or to the left. Whichever way

If

in-phase signals produce a trace sloping to the

180

left, then input signals

° out of phase pro-

duce a trace sloping to the right.

c.

Refer to figure

b above shows a

that

is the in-phase or 0° phase-shift condition.

29.

pattern

If

the

test

similar to

described in

that

of

A,

Then when two signals of equal amplitude and

frequency are compared, B indicates a shift; 135° phase nal voltages

C indicates a

shift;

and E shows

are

of opposite polarity,

90

o phase shift; D shows a

that

there is a 180° phase shift between them.

45

o phase

the two sig-

or

that

If

however, the test described in b above shows a

that

at

E,

that

pattern similar to condition. Then D would indicate

90

still be

°; B would

be

135° and

is the in-phase

45

o ; C would

A,

180

°.

ray tube. Such fields also can be picked

up through the technician's body as he handles the test leads. This condition becomes more some when examining

low

level signal s with the gain of the oscilloscope amplifiers near mum.

To

minimize the effects of s

avoid holding the

test

leads in the hands; in-

uch

trouble-

stray

maxifields,

stead, clip them into the circuit and then ex-

amine the pattern. A void using too much gain in the oscilloscope amplifiers and keep the input

low,

otherwise overloading may occur with the

that

result Examples of overloading Shown torted; the others are flat either bottom or

Figu1

the wave forms appear distorted.

are

shown in figure

at

A is the only

pattern

that

at

both.

A

c

·e 80.

Oscillos

co1Je

patterns

and distort ed wave

show·ing fonns.

is not dis-

the top or

8

0

T

M4000·!8

und

·isto1·ted

30.

A B

0

F

igure 29. Phase angle

d.

Sometimes misleading information is pre-

sented

on

the cathode-ray tube because of inter-

pattents.

E

T

M4000

c

-62

ference from external sources. The oscilloscope

on

the s

tray

cathode-

the

should be grounded to a ground point equipment under test. Be careful el

ectric fields

do

not

come

close

that

to

the

32

e. Sometimes

divider probe such as

31. This probe is useful voltages and prevents overloading of the cope

amplifiers. The probe cuts down the signal

by a known amount

it

is necessary to use a voltage-

the

one

shown in figure

for

high input signal

oscillos-

so

that a true

evaluation of

the signal amplitude is possible. The probe also

isolates the oscilloscope from the circuit under

test

so

that

loading is held to a minimum. As in the case of multimeters, capacitive loading can be

reduced

with the hot test lead,

53. Frequency Measurement with Lissajous

a.

tions of the oscilloscope frequencies. When two sine-wave voltages :fed

to

ube,

t

by

the addition of a resistor in ser ies

if

a probe is not available.

ures

One of the most import

ant

and useful func-

is

the measurement of

the deflection system of a cathode-ray

the resultant

pattern

is known as a Lissa-

AGO 10016A

Fig-

are

PROBE HOUS lNG

~

Jlliil~&li

·-:-:-:-:-:-:-:-:~-:-:-:-:-:-:-:-:-:-:-:-:-:-:-::

~~=~-~~""!

lli

;-.:-:-:...-

:--:-:-:-:--

~

!

'j~lfiii~t~~~

:-:-:-:-:-:-:-:-:-:-

:-:-:-:-:-:-:-:-:

-:-:-::-:-:-.:--

----- - --

TO

VERT.

INPUT

CABLE

AMPL

OF

SCOPE

TM4000-18

A

B

Figu1·e

jous figure. Figure patterns for ratios commonly encountered in frequency measurements. The ratio of the two frequencies can be determined by counting the number of loops along the top (or bottom) edge of the the sults in the formula below.

A

GO

pattern

right

(or left) edge and substituting the re-

Frequency

10016A

32

shows three Lissajous

and the number of loops along

on

horizontal axis

on

vertical axis

81. Input

p1·obe

jo1·

oscillosco1J

b.

The accuracy of measurement s method is limited by the accuracy of the reference, or known frequency. These patterns sometimes change form because of slight variations in phase and frequency between the reference signal and the signal under test. This constantly changing countered in counting the loops.

e.

Number of loops

pattern

increases the difficulty en-

on

right

edge

top edge

Pattern

by

this

drift

33

and the consequent difficulty is counting the loops limit this method of frequency measurement a practical ratio of

10

to

1.

However, if extreme

care is taken in counting, and if the gain of the

it

oscilloscope is increased,

30

92.

Lissajous

loops.

figu?·es

ft·equency mtios.

Polarity

as many as

Figut·e

54.

Determining

Some patterns, such as a simple sine wave

is possible to count

TM4000-63

showing some common

of

Deflection

on

the screen may show a distorted upper or lower

be

half. The trouble cannot its source unless the input polarity of the

definitely traced to

oscil-

loscope is known. A method of determining the

polarity of deflection is described below.

a.

Turn

off

the horizontal sweep and reduce

the intensity (to protect the screen from being

burned) to the point where the spot of light

be

to

can barely position

seen. Turn the focus control

that

causes the spot to be as large

possible. Turn the vertical sweep gain control

to its maximum clockwise position.

b.

Connect short pieces of insulated wire

the

vertical amplifier terminals. Touch the

leads momentarily and

at

the same time to the terminals of a battery of from 4 to 6 volts. spot will jump either

It

slowly.

jumps because the

up

or down, then return

battery vertical sweep input blocking capacitor charge, then slowly discharge.

c.

Touch the wires together to discharge the capacitor completely. Reverse the connections to the vertical terminals and connect them the battery. The spot will now jump in the posite direction. Note the polarity of the battery

that

makes the spot jump up.

If

the side of the battery is touched to the hot terminal of the vertical amplifier of the oscilloscope and the positive side of the input polarity is negative.

battery

If

is grounded, the

the true, the oscilloscope has a positive input larity.

to

as

to

two

The

causes the

to

op-

negative

opposite is

po-

a

55. General

For practical use in the

field,

a tube tester

Section

X.

must provide a simple and quick appraisal of a tube. Tube testing equipments, however, have certain limitations. Although they compare

do

tubes to a predetermined standard, they

not reveal how a tube may operate under a specific set of conditions. The final and most accurate indication of the condition of a tube is its ability to function in a circuit designed for its though tube testers

do

not test a tube under

use.

Al-

actual circuit conditions, they are still considered important as an aid to fast troubleshooting.

56.

The

Emission-Type

a.

The emission tester measures the condition

Tester

of a cathode emitting surface. The end of the useful life of a tube usually is preceded by a reduction in cathode

b.

The emission tester has certain limitations

and disadvantages.

emissiOn.

Since the manufacturer does

TUBE

TESTERS

not state a definite 100 percent emission point which

test

could

be

used for reference, the emission

is not conclusive. High emission does not necessarily indicate a good tube, because condition might faulty grid structure, or gas content. emission has been

fails; therefore, the emission

that

a tube is in perfect condition when

be

present in a tube with a

Very high

ob

served

just

before a tube

test

could indicate

about to fail.

c.

A further disadvantage of the emission

is

that

gas is liberated within the tube when ac

test

voltages are applied unless

the

test is made quickly. In addition, because the tube is not operated ages,

it

is not tested under actual operating

conditions.

its recommended

It

is possible for a tube to show nor-

de

electrode volt-

at

mal emission and still not operate properly. One reason for this is depends control

on

the

plate current.

that

the efficiency of the tube

the

ability of the grid voltage to

thi.:;

it

test

is

34

AGO

10016A

57.

The Transconductance-Type Tester

a.

The transconductance-type tester provides a more accurate evaluation of the condition of a tube than the emission-type tester because

it measures the amplification ability of the tube under simulated circuit conditions. The transconductance is measured and compared with ratings supplied by the tube manufacturer.

b. The meter scale of this type of tester is

usually calibrated to indicate the transconductance (Gm) either directly in micromhos or in

good,

terms of considered defective when its decreases to standard tube tables. From this that

the

mined only by testing

weak, or bad. A tube usually is

tran

70

percent of the value stated in

it

true

condition of a tube can

it

in terms

of

sconductance

can be seen

be

deter-

mutual

con-

PIN STRAIGHTENERS 7 PIN 9

ductance. A tester for testing the is shown in figure

58.

Use

of Tube Tester

33.

Gm

of a tube

Follow the instructions given in the technical

manual supplied when using a tube tester.

Perform the tests in the order given. Observe the following precautions to avoid damage to the tubes or to the tester.

a.

Be sure the controls

to the proper positions

on

the tester are set

before a tube is inserted

for testing.

b.

Be especially careful with the filament voltage control. being tested, the filament will

If

it

is

set

too

high for the tube

be

burned out.

As a safety precaution, the filament control

be

should

set to the lowest voltage position when

AGO

lOOlGA

Figure 8

8.

Transconductanc e-typ e tube tester.

TM

4000·1

1

35

the tester is not in use. This will prevent the

that

is

next user from burning out a tube

in-

serted before setting the controls.

c.

Take special care when inserting miniature tubes in the tester because the pins bend easily.

d.

Be

sure to connect grid or plate cap leads

before beginning any tests.

e.

Reset the line voltage control for each type

of tube tested.

reading

on

This is necessary to give a true

the meter, and also to protect the tubes from excessive voltage. This is especially true after checking high-current tubes such as power output or rectifier tubes. These types

require that the line voltage be adavnced when they are tested. Unless the line voltage is re-

duced,

excessive voltage may be applied

to

the

next tube checked.

f.

The usual order used in checking a

is

as follows: test for shorts and filament tinuity. Then test for leakage, and finally mutual conductance or emission.

shows a

shoh,

make no other tests. They might

If

the

tube

con-

for

tube

damage the tube tester.

g.

If

a tube tests slightly below normal in

Gm

tests,

try

a new tube in the equipment.

less

there is a noticeable increase in perform-

the

Un-

ance, the original tube can be returned to the equipment.

h.

If

tap

a tube is suspected

it

lightly during all tests.

of

being intermittent,

If

the tube

gives

erratic indications when tapped, replace it.

Section

59.

General

a.

Basically, a frequency meter is a tuned

circuit with a dial

that

is calibrated directly

XI.

FREQUENCY-MEASURING

in kilocycles or megacycles.

b.

The frequency meter is an accurate instrument used for frequency measurements and calibration of signal generators and receivers. Some

types

of

frequency meters can

be

used in place of a signal generator for signal tracing or other troubleshooting procedures.

60. Heterodyne Frequency Meters

(fig. 34)

a.

An RF heterodyne frequency meter has excellent stability and accuracy. tains· an electron

..

coupled oscillator which is

known for its stability. Another

that

it

is

generates strong harmonics. A hetero-

It

usually

good

con-

feature

dyne meter of the more elaborate type includes

a crystal-controlled oscillator which is used to check

the accuracy of the divisions

on

a calibrated dial. Most heterodyne frequency meters have provisions for connecting a headset.

b.

The accuracy of a frequency meter depends

on

the individual heterodyne frequency meter

it

calibration charts supplied with

used.

tal switched from Each chart is calibrated for only

The calibration charts must not

one

frequency meter to another.

one

and cannot be used with others.

and the crys-

be

particular meter

METERS

c.

The heterodyne frequency meter can

be

used to calibrate both transmitters and receivers.

During receiver calibration, a portion

of

the frequency meter output is radiated and is picked up

by

the receiver. When the heterodyne frequency meter is used to check the frequency a transmitter,

it

acts as a receiver and picks

of

up the signal radiated by the transmitter. Many types of heterodyne frequency meters are available. Although the operating procedure for all frequency meters is similar, a frequency meter should not

used until

the

technical manual

be has been read and thoroughly understood. This is important because accurate results cannot

be obtained unless the operator knows how to read the dial and set

61. Proper

a.

To

prevent damage to the frequency meter,

it

properly.

Use

of Heterodyne Frequency Meters

never connect it directly to a transmitter output circuit. Many transmitters generate enough RF energy to burn out the

input

quency meter placed too

it

Usually,

is only necessary to place the fre-

quency meter near the transmitter, and the

nal strength can

If

tance.

the transmitted signal is weak, a piece

of wire may

for better pickup.

be

varied by changing the dis-

be

connected to the frequency meter

It

may

circuit of a fre-

close

to a transmitter.

run

close to the trans-

sig-

mitter but must not be connected directly to it.

b. Before using the frequency meter for

frequency-measuring purposes, allow

at

least

30

36

AGO

l0016A

Fig

ure

84.

Heter·odyne fr·e

quency mete

1·.

TM4000

- 64

minutes as a warmup period, because the equip-

does

ment

not reach a stable operating temperature for some time. After the warmup period, the temperature does not change and the frequency of the frequency meter

c.

If

the unit is battery-operated, remove the batteries before placing have a tendency

it

to

corrode, and if corrosiol1

does

not drift.

in storage. Batteries

occurs while the batteries are in the unit, the corrosive action will produce a sticky substance that

will leak out and ruin component parts.

62.

Using Heterodyne Frequency Meter When

Tuning Transmitter

a.

If

the oscillator in a transmitter is crystal-

no

controlled, there is frequency.

If

the oscillator is not crystal-controlled, the frequency meter may insure its operation

b.

Set the frequency meter to the frequency

at

which the oscillator is to operate. Couple the

problem in keeping

be

on

the desired frequency.

it

on

used to

frequency meter output

loosely

to the oscillator, and tune the oscillator until a beat note is heard in the headset. Continue tuning the oscillator until the beat note finally reaches zero. Zero

beat has then been reached and the oscillator is

on

frequency.

c.

The same procedure

is

used

to

tune buffer amplifiers, frequency multipliers, and final amplifiers to the desired frequency.

63.

Using

a.

Heterodyne Frequency When Cali-

brating Receiver

Under normal operating conditions, a receiver should maintain calibration for long periods.

There are times, especially after certain

ome

er

com-

when

tubes have been replaced and after s ponents ha when alinement is

b.

to the procedure us the frequency meter is us

ve

changed physically and electrically,

nece

The procedure us

ed

ssary.

ed

for receivers is opposite

for transmitters. That is,

ed

as a receiv

AGO 10016A

37

setting a as a

c.

receiver antenna terminal.

transmitter

co

·uple

the

on

frequency,

but

is used

when calibrating a receiver. frequency meter loosely to the

.Set the frequency meter to the frequency required to calibrate the receiver. With the receiver bfo turned up the frequency meter signal

it

and tune

to zero beat.

If

on

the receiver dial

on,

pick

the receiver

doe

not indicate the frequency to which the frequency

meter is set, the receiver requires adjustment.

64. Wavemeters

a.

One of the simplest and most useful types of frequency-measuring devices is the wavemeter. held in one hand while being used. There no operation is extracted from the circuit checked.

Some units may be small enough to be

are

operating voltages required; all power for

being

For

this reason, the use of the wave-

meter is restricted mainly to transmitters.

b.

The wavemeter is the forerunner of

the modern frequency meter, and even though some of them have dials cycles, they are often referred to as rather

than

frequency meter the wavemeter frequency

meter; extent in detecting the presence of mitter circuits and to give

that

are

calibrated

s.

does

not

compare

it

is therefore used to a

an

in

mega-

wavemeter.:;

The accuracy of

with

that

of the

great

RF

in trans-

approximate frequency reading. Wavemeters cannot be relied on for an accurate measurement because they tend to detune self-excited oscillators to which

are

they shunted by a variable capacitor.

coupled. Wavemeters consist of a coil

Some units have a milliammeter or a small lamp connected across either the inductor or capacitor.

65. Reaction-Type Wavemeter

(fig. 35)

The reaction-type wavemeter absorbs very

little power from

circuit to which

it

is

the

coupled, and is preferred to other types of wave-

for

meters circuits. dication of resonance is usually supplied by a milliammeter in circuit under resonance is indicated by a deflection on

measuring frequencies in low-power

It

has no resonance indicator· an in-

'

the

test

device under test.

includes a

current

If meter,

the

s

Figur

e 8

5.

Reaction-type

wav

emete

meter as the wavemeter is tuned through reson-

not

ance. The wavemeter should

be placed too close to the circuit under test, because the resonant

frequency may change.

66. Absorption-Type

a.

The absorption-type

to

the

reaction type except

or

meter

lamp

ser

in

tor

ies

has

·connected across a fixed' capacitor

with

the

a much

Wavemeter

tuning

higher

wavemeter

that

it

has

capacitor. Thi s capaci-

capacitance variable capacitor. The high capacitance permits a very

that

will

but

has cause of more accurate

lar

ge voltage to be

light

the

lamp

or

operate

practically no effect on

it

s low reactance.

than

This

the

reaction meter,

built

up across

the

wavemeter

absorbs more power from the circuit under test. Its

use is generally

vices becau se

b.

The

brightest

the highe

cator For coupled lamp less

st

reading

signifies

greatest

loo

sely to the

barely

the

loading on the circuit.

accuracy,

glows.

restricted

it

tends to load

to high-power de-

the

circuit.

indication on

on

the

meter resonance indi-

that

resonance has been reached.

the

wavemeter should be

tank

circuit

The

loo

ser

the

coupling,

If

the

so

wavemeter contains a sens itive meter, be careful to avoid close coupling test. energy

In

addition to load

may

be absorbed

to

the

circuit

ing

effects, enough

by

the

wavemeter

damage the resonance-indicating meter.

TM

4000

-25

1·.

is

similar

a small

than

the

it

the

meter,

circuit

be-

is

but

lamp

or

that

the

absorption

under

to

38

AGO 10016A

67. Combination Absorption and Reaction Wave meter

(fig. 36)

coils

can

to

be

This wavemeter uses a set of plug-in

cover the tuning range of the unit.

It

used either as a reaction-type or an indicating-

type (absorption) wavemeter. A pilot lamp is

be

included and may

screwed into a socket to make the unit an indicating type. With the lamp in use, the wavemeter is slightly less sensitive because of the power consumed by the lamp.

68. General

a.

The

field

strength of a radio wave is deter-

mined by measuring the

Figu?'e

RF

36.

Combination

Section XII.

FIELD-STRENGTH

voltage induced in

an antenna.

b.

The field-strength meter is not generally

classified as test equipment because

it

is used primarily for selecting transmitter and receiver sites. Since the main purpose of a transmitter is to produce optimum radiation

at

the correct frequency, the field-strength meter actually measures a providing a

part

of the radiated

true

indication of the amount of

field,

thereby

energy being radiated.

c.

Some types of field-strength meters measure only the relative magnitude of field intensity;

other types measure the absolute magni-

tude of the field intensity in terms of microvolts

per meter. The

latter

type field-strength meter

is more widely used.

abs01·ption-1·eaction me

METERS

69. Field-Strength Meter Operation

a.

A field-strength meter usually consists of a receiver that picks it

with a reference voltage generated by a self-

te?-.

up

the signal and compares

contained calibrated oscillator.

b.

When this equipment is used, locate

or

from persons source and the

objects near the radiating

test

equipment. This precaution

it

is necessary to prevent erratic meter readings.

c.

To

determine the best site for a trans-

mitter, move the field-strength meter from place

at

to place, and compare the readings

each location. Place the field-strength meter in the desired receiver location and move the mitter, if portable, to produce the best results. An absolute field-strength meter is shown in

figure

37.

away

trans-

AGO 10016A

39

'

FigU?·e 87.

Section

70.

General

a.

Substitution of test equipment can

at

times when the designated test equipment is

XIII.

Absolute field-stt·ength

SUBSTITUTION

be

used

unavailable or inoperative. The use of substitute test equipment can lead to a false and mis-

it

leading analysis of a circuit or

can result in

misalinement and erroneous sensitivity read-

40

TM4000·20

metet·.

OF

TEST

EQUIPMENT

ings. Substitution of

be

done haphazardly.

se

intelligent

b.

If

lection.

the technician cannot obtain the test

test

equipment should not

It

requires careful and

equipment specified in the technical manual for

the equipment being tested,.he should determine

test

whether other

equipment is suitab le as a

AGO 10016A

substitute. stitution of one

very

be

Other types of repair.s require special

ment

for

71. Multimeter Substitution

a.

If

For

certain types of repair, the sub-

test

equipment for another may

simple and may cause

which

there

is

no

substitute.

no

difficulty.

test

equip-

the multimeter specified in the technical manual is not available, another with similar voltage and resistance ranges and the same

just

sensitivity will probably work

the

sensitivity of the substituted multimeter is

far

less

than

that

of the recommended one, the

as well.

If

technician can expect some wide variations in the

voltage readings made in high-impedance circuits. higher sensitivity the higher in

the technical manual.

timeter has a lower sensitivity

If

the substitute multimeter has a

than

that

called for, some of

voltage measurements will be considerably

than

those specified in the voltage

If

the substitute mul-

than

chart

the one specified in the technical manual, lower voltage readings will be obtained in certain circuits.

b.

A substitute multimeter can taining accurate voltage readings even if has a higher designated in

or

lower sensitivity

the

equipment technical manual.

be

used for

than

ob-

it

the one

Use the substitute multimeter to measure voltages in a normally operating

being repaired. Compare results and

use the readings on the good

rect

ones.

72. Signal Generator Substitution

a.

If

the specified signal generator is not available for troubleshooting by the signal stitution method, with

similar frequency coverage will

An

fm generator can ·be used to apply an un-

modulated signal to

another

an

set

similar to

on

the two sets

set

as the cor-

type of generator

do

am receiver by

that

sub-

as well.

turning

off the sweep.

b.

If

the receiver is being alined or checked

for sensitivity, the signal generator specified for the

equipment should

be

used

if

at

and the technician should follow the

tions in the technical manual carefully.

•

all possible,

instruc-

If

the

alinement is performed with a substitute signal

any

generator,

or all of the problems listed be-

low may be present.

(1) The substitute generator may not have

to

be

allow

fre-

high

the necessary frequency range complete

aline~ent

on

all bands of the

equipment. The accuracy of the

quency calibration

may

not

enough to produce an alinement job

that

which will guarantee

the receiver will give the required dial calibration accuracy.

(2) Another important

factor to be con-

sidered is the output impedance of the

signal generator. This is when measuring the sensitivity of

important

an alined receiver. The receiver and signal generator impedances matched. the

signal generator is nearly the same

If

the output impedance of

must

be

as the input impedance.of the receiver, the

error

will not

be

very great. An example of a matching network is shown in figure 25.

(3)

It

may be .necessary

at

times to use a harmonic from a signal generator, whose fundamental frequency is not high enough. of the harmonic output is less accuracy of

(4)

If

the accuracy of the substitute signal generator is not as accurate as should be, ous frequencies near

In

this case, the accuracy

the

generator.

it

can be calibrated

the

than

~he

it

at

vari-

ones to be used. This can be done by comparing the frequency of its

output· with

that of a heterodyne frequency meter or a crystal oscillator.

A

GO

10016A

41

CHAPTER

4

GENERAL

Section

73.

Importance

shooting

a.

Logical Thinking. Troubleshooting el

tronic equipment requires logical thinking.

be

effective, troubleshooting must and swift. Therefore , combined with ingenuity and

of

logical,

Accurate

'be

it

calls for clear thinking,

common sen

TROUBLESHOOTING

I.

Trouble-

ec-

To

accurate

se

and

logical working procedures. The technician must

be

st u

l,

se

ar

the

think for himself and thereby make the of the information gathered, in

ea-ch

particul case, from the equipment technical manua operator of the equipment, and from other repairmen who may have worked nical manual furnishes the technician with tailed information

on

the equipment, but not supply basic maintenance practice technician must have this as p

on

it. The tech-

it

s.

ar

t of his back-

de-

does

The

ground.

b.

Knowledge and E xperience. Troubleshoot-

ing electronic equipment is not learned in a

short time. The man with experience is limited

if

he has had

no

training

in theory. Practical experience can be gained only by working with equipment over a period of time. To

be a go

od troubleshooter calls for an effective combination of practical experience and equipment theory.

74.

Why

Technical

a.

Sp ecific Troubleshooting

Manual

Is

Needed

Informa

tion. No

technician knows everything about the various

equipments

He

pair.

may

that

may

be

brought to him for re-

be

familiar with the block diagram of the superheterodyne receiver and the modulat

ed

tran

at

the

smitter. This knowledge-will he

start

of troubleshooting, but there are

lp

him

INTRODUCTION

certain circui ts requiring additional knowledge on

his

part

if he is to understand the ir functions. The equipment manual then becomes useful, because

it

is often the only source of information describing the unit and the exact stepby-step procedures for its operation, disassembly, and alinement.

In

addition, the repair

manual includes the tube socket voltages and

resistance charts, and the resistances of

formers and coils. Al

so,

the technical manual shows pictorially the location of each component part, and it may reveal whether a circuit is common to both

transm

itt

er

and receiver and

whether a defect in one can affect the other too.

It

al

so

includ

es

schematic diagrams, wiring dia-

grams, and interconnections.

b.

Specialized Technical Data.

(1) Commun

be be possible, and cial m For troubl

ic

ation equipment may often

intended for specialized use.

as compact and self-contained as

it

may have certain spe-

ec

hanical and electrical features.

es

hoo

ting specialized equip-

ment, the repairman must equip him-

self with correct specialized information.

(2) The repairman must know the method

used for interconnecting the components of a radio set for connecting the

and, in particular,

transmitter ceiver to a power source. A operates or volts

one

de

on

either 115 or

that

operates

can

be

230 volts ac,

on

6,

seriously damaged by

the incorrect use of a switch or plug.

trans-

It

and re-

set

12, or

will

that

24

75.

Tube

Testing

a.

Before testing the tubes

Section

Techniques

II.

CHECKING

at

TUBES

all, test the

cables and external ,connections. Isolate the

42

AND COMPONENT

PARTS

trouble to a unit or section of the equipment.

b.

Before removing any tubes,

power to see whether they

warm

turn

on the

up properl

AGO 10016A

y.

If

the envelopes are of glas will show whether any is burned out. envelopes are of metal, attempting volt

or

to feel them with your fingers. (One-

other low-current metal tubes, however,

s,

a visual inspection

tu

rn

off the power before

If

the tube

will not generate sufficient heat for this purpose.)

c.

If

a tube tester is available, first

turn

off

the power, and then remove and test the tubes

one

at

a time. Substitute new tubes only for

those

If

that

are shown to be definitely defective.

a tube is suspected of being intermittent,

should be tapped gently while being checked, to bring

out the defect

d.

If

a tube tester is not available, trouble-

if

it

exists.

shoot by the tube substitution method.

76.

Tube

Checking

a.

Replace the suspected tubes with new tubes

one

at

a time.

by

Substitution

If

the equipment begin s to operate normally, discard the last tube removed, and return

the other original tubes to their sockets. Some circuits, such as oscillator circuits in very high-frequency units, may operate with one good tube and not with another. This is because of the difference in the interel ectrode capaci-

tance between the tubes, which plays a large

part

in determining the resonant frequency.

Therefore,

lator circuit,

be

to

Caution:

if

a tube do

not discard

definitely bad.

By rocking or rotatin g a tube, you

doe

s not operate in an oscil-

it

until

it

is known

may bend the pins, you may break the weld wire

if

where the pin enters the glass, or, even

the

weld does not break, you may cause a high-re-

joint

sistance tubes, allow them sufficient time to

b.

In

some equipments,

to develop. Before handling large

cool.

it

is possible to remove

a tube from one section of the equipment with-

d.

out affecting the section being checke a case,

it

is possible to troubleshoot the defective

In such

section by using a tube from another section as

a substitute,

if

sufficient spares are not avail-

able.

Note.

If

a r eplace

tive immediately, check

circuit.

tuning. An off-resonance condition with

high

at

In

the

plate

current

c.

If

a component has more than one bad tube

the

same time, substituting tubes

ment

case

of

can easily

for

a bad tube be

the

compone

a

transmitter

ruin

nt

, che

a tube.

com

part

ck

its

for

one

es defe

s in

that

proper

extremely

at

time and reinserting the original tube before substituting for a second tube will not locate the defective tube. The original tube may have been

it

defective, but is another defective tube mal operation.

was not evident because there

that

is preventing nor-

To

correct this trouble, install new tubes, and keep putting in new tubes until normal operation is restored. The last tube replaced is defective and should be discarded. To determine whether another original tube is bad,

return a noticeable change in operation, discard the

it

an

original tube to its socket.

last original tube installed. Another method is to install all new tubes, then replace them with the original tubes,

one

at

a time. When failure or change is noticed, discard the last original tube installed.

if

the equipment operates satisfactorily with the

original tube.

Do

not leave a new tube in a socket

If

none of the above procedures restores the equipment to normal operation, further

troubleshooting is necessary.

If

ment is to be sent to a higher echelon for repairs,

return

all of the original tubes to their sockets, even if the tubes are suspected of being defective.

d.

A tube should never

be

discarded unless a tube tester or other instrument shows defective,

broken glass envelope, broken base pin. because

or

it

can be seen

Do

not discard a tube merely

it

has been in operation for a long time.

that

the tube has a

an

open filament, or a

Satisfactory operation in the equipment is the

final proof of

77.

Checking

a.

Tube filaments connected in series present

tube quality.

Series

Filaments

a problem. An open filament in a tube will cause all other filaments in the This makes

it

difficult to detect a burned-out

string

tube quickly.

b.

One

way to test the tubes for open filaments

at

is to remove them one filaments for continuity with

a time and check the

an

ohmmeter,

this procedure usually takes too much time.

c-

addition,

it

can cause burnouts in the 1-volt or

other low-current tubes. The ohmmeter should

be set

usually the

through the tubes

a

high enough to

on

a scale other than the lowest, because

current

burn

the ohmmeter can pass

on

its lowest scale is often

out the filament.

If

there is

the equip-

it

to

be

to

go

out.

but

In

AGO 10016A

43

VI

Figw·e

V2

88.

Finding open filament

with

a voltme

OPEN

Fl

LAMENT

V3

te1·.

V4

c.

If

the bottom of the tube sockets is accessible, the tube found by measuring filament terminals kets. All good tubes

zero voltage across that

is burned out will have the full voltage is applied across the filament will have 6 volts across it.

tube has

with

the

open filament can be

the

voltage across the tube

with

all tubes in their soc-

in

the

string

their

filaments,

string

1%

volts across it, all filaments

(fig.

38).

will measure

but

the

one

that

The open

If

any

one

are good, because the 6 volts will be divided equally among the four tubes.

d.

Equipments using series-parallel filament

circuits often have

some tube filaments in tain

the correct value of

tube.

In

this

type of circuit, the voltage meas-

shunting

the

ured across a burned-out filament

resistors across

series circuit to main-

current

flow in each

may

be nearly the same as the voltage across a good tube. This is because the

shunt

resistor may be intact.

That is why measurements should be made carefully and

not too rapidly.

e.

In

some field equipment, the keying

must

be

closed to apply power to the

filament -circuits.

39) can be closed manually

For

example, relay

and

across the various components in

If

the

circuits measured.

bottom of the tube

transmitter

the voltages

the

KlOl

filament

relay

(fig.

sockets is not accessible, filament circuits can be tested test

through

prod can microphone only a single series points,

the

another circuit. The ohmmeter

be

connected to the chassis

input

filament

connector (fig. 39) and, string

existed between these

circuit

could be checked

and

the

if

for

continuity. These points left-hand corner of

78. Testing Parts

When a section ment mu means testing

the

trouble has been

and

then

to a

or

simp

le

shortcut

st

be pinpointed to the defective

the

capacitors, or inductor

ing can be accomplished

procedure is usefu l

mounted in a

79.

Checking Resistors

a.

Before checking

an

ohmmeter, the

unit

circuit determine whether one

le

ad of

the

resistor.

part

that

other

can

ance indicated will be low

are

shown in

the

right-hand

narrowed

stage

by us

ing

methods,

suspected

s.

In

with

whether

or

have been removed.

the

suspected

parts-resistors,

many

cases, the test-

a multimeter. This

the

should be examined to

it

is necessary to disconnect

If

it

is s

hunted

form

a de

path,

er

than

resistance of the resistor, because

more

sistance of two or

less

than

the

resistance

the

branch. When

it

can be checked

It

is advisable to use the

will give a midscale

b.

In

figure 40,

the

grid

winding tinuity resistance

meas

urement

path

through

resistances in parallel is

of

the

lowest value in

the

for

continuity

resistor

is disconnected

and

ohmmeter

reading

resistor

of

transformer

is

to

insure

R4 is shunted across

Tl.

made

across R4,

Tl

in

parallel will be indicated on the ohmmeter roneous

reading

will result.

It

the

diagram.

down to

test

equip-

the

trouble

part.

parts

resistor

by an-

the

resist-

the

actual

the

total

resistance.

range

accuracy.

If

a con-

the

with

and

an

is

therefore

upper

This

are

with

re-

that

low-

R4 er-

44

AGO 10016A

4

l

-44

·

L?

~309

+6V

ISO

P306

IS~~I06

Cl44 200UFL1:.

VII

:-rr+

304

0

5301

4 0 X

200

VIOl

IL4

..l..CI6S I .

OIUF

7

'1109

IR!i

Cl63 .

OIUF

!I

Rl42 6,800

II

6 6

0

P308

~108

•

,

..l..

I

Cl64 1,500

N

<jl

J309

+6V

KIOit

l

7

TO

VI06

IRS

Rl23

22

v v

MICROPHONE

I 5

J

CIRCUIT

Cl35

3,000

Rl24

.A

·v

Lll6

18

70UH

~~~9:::

:>

=:

Rl25 6,800

I

~108

P308

V104

3A5

7

I

4

5

I

9301

pper

m.

vn

to

JUip)Uble

This

:tors,

testThis

are

with

re-

that

NOTE

Ll3

Lll

6

V105

7

IRS

15

NOTE

C71 1,500

Cl36

500

FC49

3

2

7

500

7

VII

2E24

$

Rl21

>121(

V7

IAE4

R34

27

-Wv

VB

IAE4

R35

A21A

7

1,4,6

1,5

2

Jl08

NOTE

*m

·vvv

NOTEfE

~=:cu

500

C42

500

2

P308J8

R56 6

10

R577

1

0)

VV6

33A4

1

I

E 5

Rl22

100

v

II

1

Cl34

3,000

6

5301

V9

384

5t

~4

6

.A

R44 120

R45

120

II

~~I

500

J

<

R47

(

NOTE

?R48

~

NOTE

>

~

6,200

R28 6,200

C43 500

I I

2

J!i

2 2

2

J!i

R25

A~OA

3

J5

P305

I

3

P305

C60

:::

500

5

:::

f:

gg~

I

5

::f:

~6~

.A

2

9301

VIO 384

~~~

1'14

6

~

3

5301

V!i

304

~I

d

5

4

5301

-

TM4000-46

V102

304

.....

Cll2 3,000

Rl48

10

Cl!i4

.OIUF

VII& 3AII

7

5301

Cl95 3,000

I

14 6 P304

14.

~4

1

4r

P308

~3~---------+-----------------+----------------------------------------------~

V114

3

~108

IA3

3

304

Cl90 3,000

V4

NOTE 4

RIO!i

6,800

~108

9

V103

ISS

7

4

e-----o-----o

P308

IT

Cl74

.OIUF

V113

IA3

Cl71 3,000

~

I

~E313

9

5301

Rl50 6,800

Rl9 7,1100

C22

500

Ll28

-~

J~

~-

Cll6

3,000

01

7

·yr·Yv

10

P308

4 4

P305

~~~

L 107

70UH

RIO

33

I ,

VIII

IU4

7

Cl79

.

OIUF

7 7

P304

..J...

Cl91

T .

OIUF

8

S!OI

V107

IU4

I,

Sf

Cl49

.OIUF

l7

Vll2

IL4

'

Jl07

P307

P304

~4

~

Cl78

.O

IUF

~

4

L5

50UH

.OIUF

V3

3A5

71 I •. ' I

4

Cl85

Cl3

51

Rl4 150

Rl59

150

50UH

L4

~

V2 IR!i

R9

180

*g6~

-

Figure

39.

Radio set schematic diagram, showing series-parallel filament circuits.

AMPLIFIER

OUTPUT

necessary to disconnect

R4

at

checking it. Plate load resistor

it

in parallel with

and can

be

checked while

Figu1·e 40.

one

R5

has

end before

no

Audio

parts

it

is

in the circuit. The resistance from the screen

R1

grid to ground is measured through

R2

and

R3

in parallel with

in series.

resistance and voltages are different from

which is

If

the

that

called for in the TM, any one of the three re-

sistors could have changed in resistance.

check one of these parts, disconnect it

from the circuit.

It

(1)

is important to use the

one

right

To

end of

scale of the ohmmeter when measuring resistance or ,continuity. is used, a low-resistance connection will show or a closed-circuit reading.

If

a high range

part

or a poor

up

as a full-scale

Use the

high range only when checking high-

If

a

low

resistance circuits.

range is used, a fairly high resistance will give the same reading as an open circuit. The resistance value will

be

known approximately, either by its markings or circuit information; therefore, the

that

range

scale indications should

other precaution is to

fingers test

prods, because the resistance of

will give approximate half-

be

used. An-

be

sure

do

not touch the ends of the

that

the

TM

R5

4000-45

am1Jlifie1·

R2

ci1·cuit.

the body will cause an inaccurate indi-

on

cation

the ohmmeter.

(2) Sometimes a resistor will have normal

it

is

cold,

but

resistance when

will

change value as its temperature rises.

it

Measure the voltage across as the power is turned

it

after

warms

up.

If

as soon

on,

and also

the voltage

changes considerably over a short peri-

od

of time, the resistor is changing in

value and should

c.

If

voltage

or

resistance tests indicate a variable or adjustable resistor may tive, a final test will have to

be

replaced.

be

that

be

defec-

made, and for this purpose two of the three leads will have to be

disconnected, thus effectively isolating the suspected resistor from the rest of the set. check the resistor then, measure the resistance

one

from each of the two ends separately. breaks

end to the other and from the top to

To

test for

that

show

up

only as the resistance is

varied, slide the movable member back and forth while testing

d.

A resistor measured with an ohmmeter

it

to each end.

will usually measure a small amount higher or lower than the marking or color code specifies.

This is because of the tolerance of the resistor.

20

For example, a 1-megohm unit with a tolerance will measure anywhere from

percent 800,000

To

AGO 1001GA

45

ohms to 1,200,000 ohms. In addition, the ohm-

meter will not

be

100 percent accurate, and its deviation from accuracy can cause a further error

in measurement. A resistor having a tol-

erance of 5 percent is marked with a gold band;

one

and

with a tolerance of

with a silver band. Resistors

10

percent

is

of

greater toler-

marked

ance are not marked.

80. Testing

a.

Coils and transformers include RF and

Coils

and Transformers

audio chokes, power transformers, relay coils, audio transformers, and any component except wire-wound resistors. The be

checked for resistance values and the readings compared with the normal values. essary,

one lead should vent errors in readings. suspicious, each winding should

IF

transformers and coils,

that

is wound with wire,

se

items should

be

disconnected to pre-

If

the readings

be

checked for

If

nec-

look

shorts or leaks to ground or a leak to another winding within the same component.

b. Refer to figure

41

for an example

of power-transformer-winding schematic diagram. The following chart shows the check points,

nor-

mal readings, and the points to check for shorts.

Pow

er-tran

sformer res istance chart

Te

st

Points

Primary

5-volt filament

High-voltage 5-6

High-voltage 6-7

6.3-volt filament

1-2

3-4

8-9.

c.

The condition of low-voltage windings is

Normal

indication

5 to 10

less

50

to 100

50

to 100

than

less

(ohms)

than

one

T

est

for short

Frame Terminals

Frame Terminals Frame Terminals 8, 9 Frame

3,

5,

(gro und)

4,

5,

6, 7,

6,

8,

7,

9

8,

to :

8,

9

often difficult to determine because usually the

is

so

low

that

resistance

the readings appear as short circuits. However, these windings need never

be

suspected of short circuits unless there is evidence of a blown fuse or severe overheating

of

the transformer. The

o~en

because the wire is

Will

blow

before the winding opens.

se

windings rarely

so

heavy

that

One

a fuse

method

3

~

5

VOLTS

4

2

115

VOLTS

AC·

~

oo(

I

Figu1·e· 41. Pow

of

testing a power transformer for shorts

e?·

·t?'ansform

connect a 115-volt lamp of series with the primary winding. voltage is u secondaries.

a

sed

. Remove all tubes to take the load

d.

Turn

220

volts, a 220-volt lamp must

on

the power; if the lamp lights

~

400

~

6 . 3

VOLTS

TM4000-~5

m·

50

VOLTS

~

6

7

8

9

windings.

to

100

If

brightly, the high-voltage secondary winding is

If

probably shorted. after the power had been turned time, the test will not show mediately.

It

will then be necessary to run this

test for several minutes to

the condition existed

on

up

the trouble

an

hour; if the short circuit takes time to develop, the lamp will glow when the defect appears. This

test

does ply to a defective primary; if the primary were shorted, the line fuse would have blown. The troubles

therefore

that

will cause the lamp to glow will

be

in the secondary windings. lamp lights only when the tubes are in their sockets, the transformer is not

e.

The windings used

in

RF

at

and

fault.

IF stages are subject to some of the defects found in power transformers. The windings can tested for open circuits with

an

ohmeter. They

rarely develop short circuits, but if they are

suspected, resistance checks can be used to tect them. When a coil has a very low normal resistance, the avai lable ohmmeter may not

able to indicate accurately the difference between a normal and shorted coil. The only way to sure is to install a new part. This information applies also

to

RF

choke coils.

is

to

watts in

the line

be

off

the

only

for

some

im-

not ap-

If

the

amplifier

be

de-

be

46

AGO 10016A

81.

Checking

a.

There are several ways in which a capacitor can fail. a leak or

Capacitors

It

may become shorted,

an

open circuit, or its capacitance may

it

may develop

change. In most cases of leaks or shorts these

be

failures can

checked with a multimeter or a vtvm. A capacitor checker or an ac meter must be

used to detect a change in capacitance.

the capacitor is suspected

of

being

open, a good method of double checking it, while circuit, is to bridge

it

with a capacitor known

it

is

If

in the

to be good.

(1)

To determine whether a capacitor is leaky or circuit and

shorted, disconnect

test

it

with an ohmmeter.

it

from the

There are occasions when a leak will

not show

up

unless the capacitor is subjected to the voltage appearing in the

set;

therefore the ohmmeter test will not indicate a defect. Disconnect coupling capacitor

in figure

42

at

Cl the low-voltage side. Connect the voltmeter between the free end of the capacitor and ground and turn the set on.

If

there is a short or leak in

part

of the

other side of

de

voltage applied to the

Cl

will

be

indicated on

Cl,

the voltmeter.

(2) A suspected bypass or filter capacitor

(C2,

fig.

42) should

the

ground side. Connect the voltmeter between the With

the set turned

low

the screen-grid voltage will

be

side of

on,

disconnected

C2

and ground.

part

or all of

be

indi-

at

cated shorted. capacitor ( power

(3) Whenever a capacitor, such as

to shunt a

(

4)

Capacitors of comparatively 1 a r g e values, such as electrolytics, can

on

C2

in figure

the meter if

Use

an

(1)

off

first.

42,

above), but

C2

is leaky or

ohmmeter to test the

turn

is suspected of being

open, the quickest way to check

good

capacitor across it.

Cl

it

the

or

is

be

tested for open circuits with an ohm-

the

meter. Be sure to connect

positive lead of the meter to the positive lead of the capacitor.

Connect the ohmmeter terminals across the capacitor terminals and watch the meter needle.

If

the capacitor is

good,

the needle will rise rapidly as the capacitor is charging, and will fall slowly as the capacitor becomes charged.

(5) Another way of testing is to connect

the capacitor across a source of power where less than, the capacitor. be

sure to connect the plus side to the

the

voltage is equal to, or

de

voltage rating of the

If

the capacitor is polarized,

de

plus side of the power source. After a

of

few seconds

contact, remove the

capacitor and bring its terminals close together. tor

is not completely

If

a spark results, the capaci-

open.

(6) Capacitors may change in value, there-

by producing abnormal results. To check the capacitor accurately,

it

is

AGO 10016A

CAPACITOR

~OER

:~~E~-+--+--..:_,

PLATE

TEST

/DISCONNECT

FiguTe 1,2. Method

IF

AMPLI

Fl

ER

HERE

[o1·

checking capacito1·s

VOLTMETER

with a voltm

ete

+e

TM4000·43

T.

47

necessary to disconnect move

it

entirely and check

one

end or re-

it

with a

capacitor checker. Electrolytic capacitors lose capacitance with age because the electrolyte dries out.

b.

A variable tuning capacitor or air trimmer

can become shorted or leaky.

one

end

of

condition,

the capacitor must

connected from the circuit to prevent a

through a

coil

or other part. The operation is the same as for other capacitors; ohmmeter one

set

test

leads must

of

plates to the other. Any accidentally

To

test for either

that

be

connected from

be

de

is,

dis-

path

the

bent plates must be straightened and foreign matter removed from between them. While observing the meter needle, move the rotor plates through their complete range. When the abnormal condition has been remedied, the

ohm-

meter will read infinity.

82.

Cable

a.

Gene?"al.

sets used in

Troubles

A large number of communication

the

military forces are composed of several basic components connected together with multiconductor cables

to

form an operating unit (fig. 37). These cables conduct signal, power and control voltages between the various units

that

form the complete installation. Some-

times cable in vehicular installations are

diffi-

cult to inspect because they are inaccessible, hav-

at

ing been built into the vehicle

manufacture.

Cable troubles in this type of

the time of

installation are. often difficult to find and spare cables of sufficient length are not always avail-

that

able. A radio set may have a transmitter

is being keyed constantly, but the key is not being operated. The trouble

could

be

caused by a shorted capacitor across the keying conductors, or the conductors could be shorted internally.

b.

Types

of

Cable Troubles. Troubles in cables

occur in the same forms as in any electrical circuit;

that

is,

cables may develop short circuits,

open circuits, or leaks. In addition to short cir-

cuits and leaks between conductors, the same

conditions could occur between a conductor and any point along the length of the outer shielding. Often, a visual inspection will reveal cable

troubles, especially in vehicular installations, where vibration can cause a cable plug to work loose

or cause one or more conductors to break.

A defect in a cable may result in a trouble

that

appears

to

be

in

one

of

the

basic components. Although the trouble appears to be in the transmitter,

it

could

be a shorted pair of conductors in the cable. The cabling in vehicular installations is especially likely to cause troubles and

should always

be

checked before removing any

of the units for further checking.

83. Testing Cables

a.

Testing

cable

at

of trouble in

troubles. An ohmmeter,

be

used to check a cable for shorts.

range is used, a high series resistance will

fo?·

Short

Cir·cuits. Disconnect the

both ends, to eliminate any possibility

the

units showing

on

its high range, can

up

as cable

If

a

low

be indicated as an open circuit. When testing the cable for intermittent

short

circuits, check

be-

tween the shielding and each conductor while

shaking the cable. Also each conductor to

b.

Testing for Open, Circuits. A simple

the

test

for shorts from

metal plugs and shields.

continuity te st will reveal an open circuit. Use the low

range

on

the ohmmeter.

If

the high range

is used, a high resistance will be indicated as a

closed circuit. The continuity one

end

of the conductor to the other. Also, as

in

a above, shake all conductors while they are

test

is made from

being checked to reveal any possible intermittent open

circuits.

c.

Testing for Leaks. As with the

short

tests, test for leakage between conductors and from conductors to the shields. Where the leak is a

the

high resistance, the voltage supplied by

be

meter may not

sufficient to force current through it, and the meter would indicate there is and a

of ohmmeter operate

no

leak;

it

will read infinite resistance

megg

er must

it;

this voltage is usually supplied by

be

used. A megger is a form

that

uses several hundred volts to

a built-in, hand-operated generator. As with short tests, test each conductor for leakage

ohm-

that

the

tq,

the metal shield. The leakage resistance between

be

at

any two conductors in a cable should 100

megohms. An example of a cabling system

in a keying circuit is shown in figure

d.

Checking Long Cables.

for open circuits, connect

To

one

44.

test a long cable

end of the ductor being tested to the shield or chassis. nect one end of the conductor and

test

prod of the ohmmeter to the other

the

other

test

least

con-

Con-

prod

to

the shield. Test each suspected conductor in the

48

AGO 10016A

AUX

RECE I VER

SET I

SET

I

POWER SUPPLY

RF

ASSEMBLY

MAST

SECTIONS

CABLE

___

_..

VEHICLE BATTERY

(1

2V

OR

24V)

LOCAL

CONTROL

FigU?·e

49.

Cabling diagmnt.

POWER

-TO

OTHER CONTROL BOXES

HEADSET-MICROPHONE

TM4000-21

AGO 10016A

49

RF

OSCILLATOR

_JG

+<

-+-<

Me(

~

F

~

E

~

I

0 (

c (

I

A (

= I

I

INTERCONNECTING

CABLE

I

I I

JUNCTION

BOX

I

~

<

(

~

(

f--!-

I

<

(

~

I

(-j-

(

KEY

~

1 = I

I

Figu1·e .H. T1·ansmitte1·

I

J

1-<

i--<

I

-I"(

-+-<

I

keying

INTERCONNECTING

CABLE

~

( (

(

c-i1·cuit.

I

JUNCTION

I

BOX

I

q-

<

f-t--

<

(

~

(

iD1

I C3

-

[~~~TE

TM

4000-39

TO

same manner. connect a conductor known end of the conductor being tested. Having an

84.

Signal

a.

Signal substitution

the stage or section

(fig.

45). The output speaker, remains connected to the same point, while the point of signal injection during the tests.

b.

Feed a signal from an audio signal genera-

tor to the output of the audio stage.

no

output from the speaker, the trouble is in

is the speaker. speaker, the speaker is in operating condition.

c.

Feed the signal into point the audio stage. speaker, the trouble is in the audio stage, or the power supply.

the speaker, the trouble is not or the power supply.

If

the cable has

Section

Substitution

is

that

is causing the trouble

device,

If

there is an output from the

If

there is

If

there is an output from

no

to

be

Ill.

SIGNAL

used to determine

in this case the

1,

the input to

no

output from the

in

the audio stage,

outer shield,

good

to

one

SUBSTITUTION

is

changed

If

there

assistant make the necessary connections

far

end will reduce the length of time for

ing long cables.

AND

SIGNAL

d.

Feed a modulated signal into the detector

at

point

2.

If

there is the detector is defective. the detector is operating normally.

e.

Feed a modulated signal into the other

stages, working toward the antenna. When a

point is reached where there is defect is in the last stage tested.

85.

Signal

a.

where the trouble

(fig. 45). The point of signal injection remains

the same, but the output indicator is moved from

point to point.

b.

Feed a modulated tector of an audio signal tracer (fig. 46) to the chassis.

Tracing

Signal tracing is al

at

point

TRACING

no

output from the speaker,

If

there is an output,

no

so

used to determine

is,

but the method is different

IF

signal into the

4.

Connect the ground terminal

at

the

check-

output, the

de-

50

AGO 10016A

\7

~

3

"

RF

t~

MI XER

I

LO

CAL

OS

CIL

LATOR

Figu1·e 45.

Connect the other terminal of the signal tracer to

the volume control

at

point 3. output indicated in the audio signal tracer, the detector is not operating.

If

there is an output,

the detector is operating.

c.

Connect the signal tracer to the input of

at poi

nt

2.

If

the audio stage the

vo

lume control is defective.

output, the

volume

control

there is

is

in operating condi-

tion.

d. Connect the signal tracer to the output

the audio stage

the au

dio

If

there is an output, the audio stage and power

supply

the

are

e.

Connect the signal tracer to the output

pa

stage. is not operating. stage is in operating

f.

Thus, the signal tracer can

at poi

nt

1.

If

there is

stage or the power supply is defective.

operating.

If

there is

If

no

output, the pa stage

there is an output, the pa

cond

ition.

stage to stage closer to, or away from, the point of signal injection, to locate the defective stage.

If

a signal tracer is available

RF

late an and

RF

in

fr

om the antenna.

g. Another method of signal tracing uses a

signal,

stages to pick

it

can

up

be

connected to the

a signal

signal generator and an oscilloscope

Feed a modulated signal from a signal generator into the input of the

vertical amplifier terminals

RF sta

of

Su7J

e1·hete1·odyne 1·eceivm

If

there is

no

output,

If

there is an

no

output,

be

moved

that

can demodu-

that

is coming

(fig.

ge.

Connect the

the oscilloscope

no

of

from

IF

45).

I

~

2

IF

DET ECTOR

rv

I

rv

AUDIO

~

t

POWER

SU

PPLY

·,

block diag1·am.

Fig

ure 46. Audio signal t1·ace1·.

to the output of the various stages, beginning

at

the

RF

stage. The wave forms

at

of signal present

the various stages. When a

point is reached where there is

the oscilloscope, or the signal is not stronger as

it

progress

between

es

toward the output, there is trouble

that

point and the antenna.

cuits are operating normally, an audio signal

that

similar to will appear

shown above the audio

on

the oscilloscope. This is true whether the output is taken from the RF, IF, or audio sections. When the oscillos nected to other

than

the audio section, a demodulator (detector) probe similar to the in figure 4

7,

must

be

connected between the

oscilloscope and the test point.

50

0

UUF

------~1\

TO RECEIVE

Figu1·e

ll

R

47.

Dete

68K

-:-

cto1·

..

~

pTob

e,

lO

schematic dia

TM4000-22

HIGH-IMPEDANCE

HEADSET

TM4000-53

show

no

indication

If

cope

one

OK

OSCIL

TM4000-22

gmm.

the type

the cir-

sec

is con-

shown

TO

LOSCOP

0

on

tion

E

1

AG"

10016A

51

Section IV. GENERAl TROUBLESHOOTING

PROCEDURES

86. Sequence

a.

The quickest and most logical method

of

Troubleshooting Techniques

troubleshooting is to follow an organized plan of attack. All troubleshooting procedures are

based

on

the observation

of

symptoms, the

com-

bination of which may tell an experienced re-

pairman exactly where the trouble lies. A vital

is

rule of troubleshooting, therefore,

to observe and analyze. The presence of certain symptoms and the absence of others enable the repairmen

to eliminate

at

once the impossible, or

at

least the improbable, causes of trouble. Thus, if a multiband receiver operates

on

all bands but

one, the repairman discards immediately the possibility of a blown fuse or the possibility of a

defective power supply rectifier, because either of these two faults would prevent the receiver's operating

on

any band

at

all. With two possi-

bilities out of the way, the repairman concen-

trates more narrowly upon the probable causes

of the trouble.

b.

The normal sequence of troubleshooting

procedures is as follows:

(1)

Obtain the history of the equipment.

(2) Preliminary examination by sight,

hearing, and smell.

( 3) Sectionalization. ( 4) Localization. (5) Isolation. ( 6) Testing

87. History

after

of

Equipment

repairs.

Before attempting repairs, the technician must

learn as much as possible about the nature of the complaint by questioning the operator as well as by reading the complaint notice attached to the equipment. The more information he

can gather, the more accurate his diagnosis

will

be

and the sooner repairs can

88. Preliminary Examination

be

made.

Preliminary examination by the use of the

senses will help in gathering evidence

that

may

lead to the defect. Arranged in proper sequence

below

are

things to look for by sight, smell, and

sound.

a.

Visual. Improperly connected cables, blown

fuses, burned-out tubes, broken cords or plugs, tripped circuit breakers, abnormal meter indi-

cations, broken burned-out resistors, arcing, and smoke. In

of

dition, switches and dials mu

see

that

they have been set to the proper posi-

transm1s

swn lines or antennas,

st

be inspected

tions.

b.

Smell. The odor of burned insulation,

charred resistors, overheated transformers, and overheated dry rectifiers.

c.

Hearin

g.

High-voltage arcing between wires and between wires and the chassis, and the

cooking of overloaded or overheated trans-

formers. The hum, or lack of hum, in vibrators.

89. Sectionalization

Sectionalization means tracing the trouble to a component of a system, or to a component of a radio set .

a.

Sectionalization

(1)

If

receiver 4 (fig. 48) is not producing

a signal

in

System.

at

its output, receiver 4 operator requests the operator ter

4 to use the order line to ask

receiver operator 3 to alert

2 operator.

Transmitter

will check his transmitter by observing

the meter indications. mitter

is

normal, receiver 4 is probably

If

defective.

At

(2)

the same time, the technician receiver 4 begins to troubleshoot the receiver, by following the procedure in

b below. He can also make some simp

tests.

For

example,

if

the receiver is completely dead, the receiver is fault. ceiver output when

If

there

is

a noise

the

gain control is

advanced to maximum, the defect is in the RF, mixer, local oscillator, or antenna system.

(3)

In

this way, by using the order line for communicating between terminal sets and the relay station, the trouble can be sectionalized

part

of

in any

b.

Sectionalization in a Radio Set. Many

the

to

system.

a component

radio sets have a common antenna and a mon power supply (fig. 49).

(1)

If

the

transmitter

and receiver

produce any output, the power supply is probably

at

fault.

ad-

at

transmit-

transmitter

2 operator

the trans-

at

the re-

com-

do

not

to

at

le

at

52

AGO 10016A

RADIO

CONTROL

STATION

TERMINAL

SET A

RELAY

STATI

ON

TERMINAL

SET

B

(2)

(3)

( 4)

(5)

I

RECEIVER

RECEIVER I

TRANSMITTER I

Figu1

If

the receiver operates, but the transmitter defective. but is defective. The fact

does not, the transmitter is

If

the transmitter operates,

the

receiver

does

not, the receiver

that

TRANSMITTER

RECEIVER

·e

48.

Radio-1·

one of the

3

2

elay

components operates, eliminates the power supply as a possible source of trouble.

If

the receiver produces

the

transmitter

the transmitter

is

meters indicate

only

noise and

that

operating normally, but the signal is not being radiated, the antenna is disconnected from transmitter

If

the

mally, but

noise, the receiver

and receiver.

transmitter

the

receiver produces only

is operating nor-

RF

section is

· both

at fault, or the antenna is not connected to the receiver.

If

the receiver operates normally, and

the

transmitter meters indicate that

is operating normally but there is

[7

\

1

TRANSM I

it

no

TTER

RECEIVER

3

TRANSMITTER

2

syst

em, block

diagmm.

TRANSMITTER

4

RECEIVER

4

TM4000-231

signal being radiated, the antenna is

not connected to the transmitter.

90.

Localization

Localization means tracing the trouble to a

stage of a section. Before removing the

chassis

frorri the cabinet, and prior to disconnecting

be

sure

that

cables,

tubes are not

connected, and

that

can

be

corrected without disassembling the

equipment.

For

pletely inoperative;

power is being supplied, the

at

fault, the antenna is

that

the defect is not something

example,

turn

if

a receiver is com-

the sensitivity and

intact

audio gain controls up to produce maximum output. tor

RF

If

only noise is heard, the local oscilla-

stage,

or

the mixer stage is defective. Check the tubes in these stages. The following tests are used in tracking down trouble:

a.

After

the cabinet, check the resistance

where the

the stages, to tell whether there is a

the chassis has been removed from

at

the point

de

from the power supply is applied to

short

cuit in the power supply.

b.

Find

out which portion of the equipment

IF,

(RF,

or audio) is faulty. Use signal sub-

stitution (par. 84).

Not

e. Localizing the trouble to a subchassis

portion of the main chassis is al so referred to as sectionalization.

c.

Check the suspected tubes with a tube

or

tester

or by substitution.

and

the

cir-

to a

~

AGO 10016A

Figu1·e

PO

WER

SUPPLY

49.

Radio set, block diag1·am.

TM4000-232

d.

Make voltage measurements in the plate,

screen-grid, and bias lines and

at

the tube soc-

kets of the suspected stages.

e. Measure resistances

at

the points where

the voltages are abnormal.

f.

If

the complaint is a weak receiver and

other tests fail to detect the troublesome stage,

53

stage-gain measurements means feeding a signal of a given

must

be

made. This

strength

into

the suspected stages and measuring the strength

at

of the signal comparing

the output of those stages and

it

with

that

of a good set.

e.

Stage-gain tests may not be necessary at

this point,

paragraph

f.

but

if

they are, the information

90

applies.

Bridging the suspected

part,

such

capacitor, with one known to be good.

as

in

a

91. Isolation

Isolation means tracing the trouble to a part, such as a capacitor, resistor, transformer, or relay. Isolation is usually done by:

a.

Using the senses as was instructed in the

paragraphs

localization (par. 90),

on sectionalization (par. 89) and

but

in this case

it

may lead the repairman directly to the defective part.

b.

Making voltage and resistance measure-

ments

at

the tube sockets. Examples of this are

covered in the next section.

c.

Signal substitution and signal tracing as

used in localization.

d.

If

a tube is defective

have been located in

paragraph tion. However, there may that

has short circuited and

it

probably would

90

be

a component at

the same time

on

localiza-

part

has damaged a tube.

Section V. ISOLATION

OF

TROUBLES

92. Repairs

Although shooting and come do

come before testing can

pairs.

refer

to chapter 10.

93. Testing After Repairs

After

the equipment may not ne faults. vhf

unit

though the new tube is in

repairs

For

information on how to make repairs,

faulty

For

example, a weak tube replaced in a

are

after

parts are

not a part

of trouble-

troubleshooting, they

be

loc

ce

made

ated

ssarily

and

after

replaced,

be

can cause operation to cease even

perfect

condition. The

re-

free of

difference between the interelectrode capaci-

and

that

of

the

old

parts

one

no

or

be

tance of the new tube can detune the circuit so

longer

at

resonance. Therefore,

that

the

after

circuit is

tubes are replaced, the equipment should

given an overall test and the results compared

with the

tests

data

specified for a good set. These

are

covered in chapter 11.

IN INDIVIDUAL

STAGES

94. General

a.

The isolation of troubles in individual

the

stages is basically

-same regardless

type of stage in question.

Once the defect is

of

the

known to be in a certain stage, voltage and resistance measurements

must

be made. The information gathered by making these measurements will, in most cases, pinpoint the trouble to a particular

b.

General information ance measurements is given in paragraphs and

96.

c.

In

the several

part.

paragraphs

on

voltage and resist-

that

follow, those

95

on voltage and resistance measurements are examples of isolating defects in various types of stages used in both receivers and transmitters.

95. Voltage Measurements

a.

The usual procedure is to make the voltage

checks .first. This will

of

the

trouble. When

l<;>cate

an

the general

area

abnormal voltage con-

dition has been found, the power off and resistance checks ma short

or an open circuit.

b.

In

most equipments, voltage measurements

are

made with the negative lead

must

be

de

to determine a

of

the voltmeter

turned

connected to chassis ground. The positive terminal is then connected to

In

ured.

this way, a

with respect to the chassis are measured.

me

voltage to be to ground, the an

electronic multimeter is being used, the com-

as ured is negative

po

sitive

mon terminal is left connected to the polarity reversing switch is

po

tive

sition for meas

the neg ative position voltages. The meter range switch to its highest range present

is not known. This prevents possible damage to the meter. served on the meter,

the

points to be meas-

ll

voltages

terminal

uring

for

that

are

positive

with

respect

is grounded.

the

chassis and

set

to the posi-

positive voltages

mea$

uring

negative

must

if

the approximate voltage

Once

an

indication is

the

range

switch

&hould

If

be set

ob-

set to obtain a midscale indic ation.

a

If

or

be

54

AGO 10016A

c.

There normally measured with respect to the and

it

and

possible damage to the meter. the ac power from the chassis. This is a safety feature

are

is

important

certain voltages

that

to avoid incorrect readings

For

input

voltage usually is isolated

are not

chassis

example,

that prevents grounding of the power line when power line polarity is not observed, and keeps personn is touched. This is especially is grounded to the

el

from getting a shock when the chassis

the

earth. To measure accurately

voltages present, the

true

test

leads must

if

the chassis

be

connected directly across the points where the voltage is present. of a power supply rectifier tube may de voltage with respect to the chassis. rectifier filament voltage is measured with spect to ground, high

voltage present. When checking the fila-

ment

voltage in this case, the meter terminals must without vent

be connected to

touching

meter

tube, thereby removing

d.

If

the measured voltage is high or low by

than

more voltage source is equipment is operated from batteries. A meter

can lower sensitivity make the measurements. This must be taken

In

another example, the filament

be

at

a high

If

the

meter will

the

chassis. One way to pre-

be

ruined by the

filament terminals

damage is to remove the rectifier

any

de voltage present.

20

percent,

it

is possible

the

cause, especially

that

if

volt-

be inaccurate also, or

than

the one originally used to

it

can have a

the

re-

the the

in-

to consideration when making measurements.

For

the most accurate results, the voltmeter

should have the same sensitivity as the one

that

was used to make the original measurements.

96.

Resistance

Before making resistance checks important filter capacitors be discharged. Resistance

Measurements

that

the power be turned off and all

it

is very

measurements can be made between certain points and the chassis, or between any two points

that

are connected by wiring or parts.

97.

Isolating

a.

The circuit in figure

ponent

parts troubles encountered cuits where

Trouble

in

Audio Circuit

50

contains the com-

referred to in this paragraph. Most

are

due to failure in cir-

de

is present. This means

that

the trouble can be located by voltage and resistance measurements.

b.

Assume

There must be

that

the stage becomes inoperative.

certain de potentials present for the stage to operate. The first step is to measure the voltage from the plate to ground. The meter range switch must

be

set

to the maximum

volt-

voltage position because the voltage present is unknown. This precaution is necessary to vent possible damage to the meter. no

voltage present, move the hot voltmeter prod

B+.

If

to the point marked

a voltage is indi-

If

cated, there is an open circuit between

pre-

there is

that

point and the plate of the tube. The screen-grid

AGO 10016A

C3

INPUT

~1--~t----+t-

Rl

R2

Figure

AMPLIFIER

X X

50.

C2

Audio amplifier ci1·cuit.

R4

\r--+B+

TM4000·42

55

voltage will

across screen dropping resistor

than

the normal drop. Figure the plate voltage is applied through resistors and R5. other side of

probably open, or check will verify this. normal, move the A voltage reading on the other means

be

below normal because the drop

R3

50

Move

the meter hot test prod to the

R5.

If

the reading is zero,

C4

is shorted. A resistance

If

the reading is about

test

prod to the plate again.

on

one side of R4 and none

that

R4

is probably open. A

is

greater

shows

R5

that

R4

is

resistance check completes the test. Refer to paragraph

c.

plate voltage from the tube even though

79

for

A shorted

checking resistors.

C4

could cause the removal of

~he

B+ voltage is about normal. The screen-gnd voltage also will the same source as the plate voltage. A ity check of short or a partial short to ground. In R5

will be overheated and may even char and

smoke, because

be

affected because

C4

will then show either a

if

C4

is shorted,

it

is taken from

continu-

add1t10n,

R5

will be

.d.ead

placed ,directly across the power supply.

d.

Another cause for complete failure of the

stage is the lack of screen-grid voltage when the

plate voltage is nearly normal. A shorted

screen-grid bypass capacitor voltage to zero. This can be checked by

C2

will reduce the

measuring the voltage from the screen grid to ground, and double checked by making a continuity check between the same two points. Refer to

graph is left these conditions,

81

for checking capacitors. If the power

on

for a considerable

R3

and

len

gth of time under

R5

will overheat and

may even smoke, because they are connected

para-

di-

rectly across the 'power supply. This is a definite

indication

of a short grid has more control the plate has, the plate current will therefore, the plate voltage will

circuit. Because the screen

on

the plate current than

go

down;

be

higher than

normal.

e.

Another test urement of the cathode voltage across R2. the voltage is higher

that

can

than

be

made is the meas-

normal,

R2

is prob-

If

ably open. There is neither plate nor screen-grid current

grid voltages will high-voltage reading is present across R2

flowing; therefore, the pia te and screen -

be

higher than normal. The

because the high resistance of the meter completes the

drcuit

across R2, creating a large voltage

drop across the meter. The value of the voltage will depend on the type of voltmeter used.

R2

is shunted by

an

electrolytic capacitor, the

If

circuit from cathode to ground will be completed by

the leakage resi

cathode voltage would

sta

nce of the capacitor. The

be

only slightly higher

than normal, regardless of the type of meter

to

used

measure the voltage.

f.

If

the output of the amplifier is distorted, it is probably because the bias has decreased from the normal value. grid voltages are found to bias

·capacitor

Cl

cause the tube to operate on the nonlinear

If

the plate

be

lower than normal,

and

scree

n-

is probably shorted. This will

portion of its Eg-Ip curve and will produce distortion. Measure the bias between cathode. The voltmeter must

If

Cl

a negative voltage. will

be

zero.

If

the

is shorted, the voltage

meter needle goes

the

grid and

be

set to indicate

below

zero, the grid is positive.

g. There is only one way the

grid

can become

positive, in addition to gassy tube troubles, and

that

is for the coupling capacitor to develop a

B+

on

If

an

that

C3

it

its

is

ex-

is

short circuit or leak, allowing the other side to leak through to checked with an ohmmeter,

the

grid.

it

may show tremely high resistance, which means not leaky. However, when power is applied to the circuit, including B+

appears between

C3.

side of

As a rule,

the

preceding stage, the

the

ground and

any

capacitor having

the

input

equal, or nearly equal, de voltage from either s

id

e to ground should be suspected as being

shorted.

h.

If

all voltage and resi

are normal but the stage does

ling capacitor

C3

may

sta

nce measurements

not

operate, coup-

be open. The simplest,

quickest, and easiest method to check it, is to

it

bridge

with a good

capacitance and voltage rating.

it

bridging

the stage

i.

If

will produce a signal in the output of

if

a signal is applied to the input.

the output is

(the procedure used to determine

capadtor

not

as

great

of

the

If

C3

is open,

as

it

should

it

is described

same

be

in chapter 11) and the voltage and resistance

are

measurements

pass capacitor

normal,

Cl

is open. decrease in output would be caused by tion. Signals

around the resi

that

would normally be bypassed

stor

by a good bypass capacitor

it

is possible

If

it

that

by-

is open, the

degenera-

56

AGO

J0016A

will cause a signal voltage drop across the resis-

tor. This voltage drop bucks the applied signal and effectively decreases the amplitude of the signal applied to the stage. As a result, the output

also decreases. 3-volt ac signal is applied between grid and ground. During the positive portion of the

signal, the grid is 3 volts positive with respect to gro und. This causes an increase in plate current

through R2 and the developed voltage

across

to ground, say 2 volts. Therefore, the gr

1 volt positive volt input signal is seen as a 1-volt signa

98. Isolating Trouble

amplifier is similar to

amplifier. There will be shorted or open capacitors, open resistors, and open inductors; they can be detected by making voltage and resistance meas figure 51.

ment and fixed higher frequency. The inductors mu tuned to the mers

the output of the stage will

some equipments in which the windings of and T2 with a powdered-iron slug adjustment.

99. Isolating Trouble

fier in practically all respects, except tuned

it

makes

a.

The method of isolating trouble in an

ur

ements. An

b.

There is one exception to the above state-

that

proper

C1, C2, C3,

are

tuned by varying the inductance

a.

An

RF

amplifier is simil

drcuits

can

the

with

l.

is

that

For

example, assume

grid negative with respect

respect

the

frequency. Thus,

and

C4

in

be

adjusted to resonate over

to

by

the input

in

IF

Amplifier

that used

IF

stage

IF

stage operates

are

not set accurately,

be

RF

Amplifier

ar

cathode. The

of

for an audio

is

shown in

weak. There are

to

an

IF

that

id

is only

the tube

IF

at

st

be

if

trim-

T1

ampli-

that

·its

a wide band of frequencies instead of one fixed frequency. Therefore, the troubleshooting meth-

.

od

will also

a

ac

b.

RFC is the load. Other circuits may have a transformer winding Regard.less of the type of will cause similar symptoms.

c.

and

C1 incomin g signal. Capacitor which is set during alinement procedures.

3-

setting is disturbed, the sensitivity of the stage

will

be

weak.

100. Isolating Trouble

a.

and the local oscillator signal, to produce the difference frequency.

b.

Troubleshooting a mixer (fig. 53) is simi-

lar

in most respects to troubleshooting an stage. coupling capacitor would

a

gr

id, and the intermediate frequency would not

be

produced. As in the

sary

to keep the adjustments T2 set to the proper frequencies, otherwise, the output will T1

is tuned to the signal frequency and T2 is tuned to the difference between the incoming signal and the local oscillator frequency. 53

shows a triode mixer; a pentode mixer will include a screen grid and suppressor otherwise, the operation will triode mixer.

be

similar.

In the

The combination of the secondary of T1

The mixer combines the incoming signal

RF

amplifier stage (fig.

or

a resistor as a load.

lo

ad, defects in them

and

C2

must

be

tuned accurately to the

C2

is a

decreased and the signal output will

in

Mixer Stage

In

be

addition,

no

oscillator signal fed to the

be

weak or not present.

it

is important to check

C2

.

If

it

should open, there

RF

amplifier,

that

tune

Transformer

be

the same as the

52),

trimmer

mi

it

is neces-

T1

Figure

choke

If

its

be

RF

~~~

and

grid;

AGO 10016A

IN

'

p~

Tl

..-----+--t-~~

R4

TO

AVC

Figu1·e

r------.----,

51.

IF

a?nplifie?'

T2

rr-I.F

LLTPUT

B+

TM4000-224

stage.

57

Figut·e 52.

,---------1---0UTPUT

RF

ant1Jlifie

t·

stage.

R2

"J

C4

RFC

R3

Bt

TM4000-225

Ttt

C5

I.

OUTPUT

F.

Cl

SIGNAL

LOCAL

101. Isolating Trouble

a.

A converter stage combines the elements

in

Converter

FROM

OSCILLATOR

Figu?·e 58. TTi

Stage

of the oscillator and mixer tubes in one envelope. When making voltage and resistance

measurements, the socket measurements are

at

one

made

b.

In a pentagrid converter stage (fig. 54),

socket instead of two.

the oscillator anode is also the mixer screen grid. Therefore, absent, there is

if

the screen-grid voltage is

no

voltage

at

the oscillator

anode. The signals are mixed in the electron

stream of the tube.

c.

Anything

that

causes the oscillator to cease operating will prevent the intermediate frequency from being produced and fed to the primary

of T3. The first test to make

lator is to measure the bias. The bias must

measured across

at

one of is used,

lea

it

may load the circuit to the extent the oscillator will seem to is operating normally.

Rl.

If

a voltmeter other than

st

20,000 ohms-per-volt sensitivity

be

If

there is very little or

on

the oscil-

that

defective when

be

no bias present, the oscillator is not operating.

d.

If

trimmer

dial calibration

rate

and a signal of the wrong frequency will be

C5

has been tampered with, the

on

the receiver will be inaccu-

R2

C3

B+

TM4000·226

ocle mixet·.

fed to T3.

If

trimmer

C2

is not set so as to pro-

duce maximum output, the output being fed to

T3

will be weak.

102. Isolating Trouble

a.

The detector demodulates or removes the

intelligence from the

in

Detector

carrier

wave. Thus, when a signal contains voice modulation, the output from the detector will be speech.

b.

Figure

55

shows

an

example of a diode tector. Because this type of detector is used almost exclusively in am circuits, other types will not be mentioned.

c.

Because there is

no

high voltage present in this circuit, the chance of component breakdown is not very great. The

IF

windings of

transformer T must be tuned

primary

and secondary

the proper frequency to produce maximum out-

If

Rl

put.

or R2 should open

output. An open circuit in

it

the ouput,

but a short

circuit in one of them

C3

there

or

C5

would result in grounding the signal and put.

d.

The ave voltage is developed across load

resistor R2.

If

ave filter resistor R3 opens,

there will be no ave voltage fed to the grids of

Stage

de-

to

will

be

no

will reduce

no

out-

58

AGO 10016A

Rl

Figtt?'e 54. Pentag1·id conve

the controlled stages, and the output will probably increase on capacitor be

reduced to zero, and the output may increase,

C4

strong

signals.

If

ave filter

short circuits, the ave voltage will

depending on the value of the grid-return resistor in the controlled stages.

103.

Isolating Trouble

a.

A power supply delivers operating voltages to mitters. The ma

the

various stages in receivers and trans-

jority

in

Power Supply

of troubles in a power

supply occur in the high-voltage rectifier and

filter sections.

b.

The most

full-wave type (fig. 56). The rectifiers

widely used power supply is the

are

often

contained in the envelope of one tube. Trouble-

shooting is done almost entirely by voltage and resistance measurements.

c. An open circuit in the primary or secondary

winding of output. Also, open in choke voltage

transformer

circuit.

if

the windings

Ll

or

L2

T will result in

are

no

intact, an

will open the high-

de

:.-:nT

IN~

I.

F.

OUTPUT

1·te1·

d.

If

filter capacitor

rent

through tubes

ci1·cuit.

cs

Vl

C9

Bt

TM4000-227

Cl

short circuits, the cur-

and

V2

will be very high

and will cause the high-voltage winding to

If

smoke, or burn out the tubes. tor

C2

short circuits there will be

but

because of the resistance of the two chokes, the amount of if

Cl short

e.

The filter capacitors keep hum out of the

current

circuits.

flowing will be less

operating circuits. Therefore,

output capaci-

no

de

output,

than

if

either one should open, the output voltage will contain a ripple, and hum will result. There will be a

large drop in the output voltage

f.

Bleeder R discharges the filter capacitors

when the power is turned

If

measure. point will

the bleeder opens, the de output

be

dangerous to touch because the

if

Cl

off.

This is a safety

opens.

capacitors will be fully charged. The bleeder also keeps a

lo

ad on the output when the regular

load is removed.

1 04.

Sample

a.

The circuit in figure used oscillator. shooting oscillators. The

the

text

b.

In general, the method for troubleshooting

an oscillator is

Oscillator Circuit

It

are

marked on

the same as

57

is

that

of a widely

is used to assist in trouble-

parts

the

referred to in

diagram.

that

used for

an amplifier, because an oscillator is basically an amplifier

that

has provisions

for

positive feed-

back.

AGO 10016A

JC5

Figu1·e 55. Diode det e

cto1-.

TM4000

-228

105.

Isolating Trouble

a.

Connect the

test

in

Receiver Oscillator

prods of

an

electronic

59

r

TO

~LAMENTS

ACSUP~

'

VOLTA~

GL~~ENTS

Fig~we

C3

Cl

L

C2

-,

Figu1·e

R

57.

OS.cillato?' ci?·cuit.

multimeter across grid-leak resi

grid

The spect to the cathode

If

it voJtage meter;

will be several volts negative with re-

if

is not oscillating, there will be little or no

_present.

the

circuit will be loaded down and the

Do

not u

stage will cease oscillating ating

condition.

b.

If

the above

test

shows is not operating, voltage checks mu This is a shunt-fed circuit and there is nothing in

the

B+

line to

If

the

plate voltage is normal, make resistance

check~

on the other components.

c.

Capacitor

to the lower end

burn

C4

connects

of

out except the

indicator L. oscillation would probably cease becau feed-back circuit will be broken.

56.

Full-wave ?·ectijie

RFC

C4

stor R (fi

TM4000-

g. 57).

233

the stage is oscillating.

se

a low-resistance

if

it

had been in oper-

that

the oscillator

st

be made.

RF

choke.

the

plate of the tube

If

C4

opens,

se, the

L I

?'1JOtue?·

supply,

more power. There

be made on

b.

Bl

A rapid method for determining whether

transmitter

a a simple indicator is used

L2

DC

R

.

additional

oscillators.

tests

C2

schematic

transmitter

......._.,__--...-OUTPUT

-=

TM4000-229

diagram

are

oscillator is operating is to couple

RF

indicator (fig. 58) to it.

with

high-powered equipment,

the lamp should be mounted on a long

prevent

to

c.

(fig. 57) ; to resonance, the bulb will light. the

loop

possible shock or burns.

Hold the indicator

if

the circuit is operating and is tuned

too close to L, because

loop

near

Do

the burn out. Thi s method cannot be used with receiver oscillators because there usually will not be

enough energy to light the lamp.

cl.

Another quick check is to detune the oscil-

li

lator s

ghtly by

small amount in either direction.

rotating

Cl

or

C2

If

is oscillating, detuning will cause a change in the meter indications in

If

fier circuits. ther

e will

the oscillator is not operating,

be

no

change in the

buffer or

fin

meter

the

as the circuit is detuned.

that

can

If

the

dry

stick

inductor L

not

hold

lamp may

(fig. 57) a

the stage

al ampli-

indications

106.

Isolating Trouble

tor

a.

Transmitter

ceiver oscillators,

60

in

a Transmitter Oscilla-

oscillators

but

they

are

similar to re-

usually

generate

Figu1·e

58.

Simple

RF

indicato1·.

2"

- G"DIA.

OF

ABOUT NO.

LOOP

18

WIRE

TM4000-223

AGO 10016A

107. Isolating Trouble

a.

The troubles encountered in an

are

amplifier

similar in some respects to

found in receiver

diagram of

figure

59.

an

RF

It

is not necessary to make voltage measurements because milliammeters manent

part

of

in

RF

Power

RF

amplifiers. A schematic

Amplifier

RF

power

tho~e

power amplifier is shown in

are

a per-

the

circuit. However, if an

abnormal condition is indicated by the meter

be

readings, resistance measurements must

made to determine whether there is an open or a short circuit.

b.

If

there is no plate current flowing, be due to by

the

power supply, an open choke RFC2, a

no

operating voltages being supplieu

it

could

burned-out tube, or a defective plate milliam-

If

the

meter.

stage is of the low-power type, an open circuit in the upper half of the primary winding of T2 would open the plate circuit.

In many high-powered amplifiers, the inductors are made of copper tubing; therefore, they cannot burn

out.

c.

If

the plate current is abnormally high, the antenna loading is too heavy, the circuit is not tuned to resonance, neutralizing capacitor

is not adjusted properly, or the excitation to

C3

the grid is insufficient. Low excitation produces a low

bias, causing the plate current to be high.

d.

If

the grid-current milliammeter indicates

no

zero current, there is the secondary of circuit in

RFCl,

Tl

the grid milliammeter, or

excitation to

is open, or there is an open

the

grid,

R,

the grid-bias resistor.

e.

In

some high-powered

plate current is such

cherry · red during normal operation.

is a

that

RF

amplifiers, the

the plate of

the

If

tube

the plate gets excessively red, the cause could be a gassy tube, the tank circuit is not tuned to res-

no

onance, there is loading is too great. meter in the circuit,

grid drive signal, or the

If

there is a plate milliam-

it

will indicate an abnor-

mally high current.

c-

FiguTe

59.

RF

11owe1·

B+

am11lifie1.,

r._ro

~TENNA

TM4000-230

AGO 10016A

61

CHAPTER

5

TROUBLESHOOTING

108. Vehicular Installations

a.

Radio sets installed in tank

s,

truck

VEHICULAR

s,

and jeeps present special problems. A typical insta lation is shown in figure

60.

Frequently trouble

appears in the installation only while the vehicle

is

in motion. This is usually the result of a poor

ion

connect

which shows tion of the radio equipment. To of the trouble, check a

up

because of the vibr a-

lo

cate the source

ll

cabling for

loo

seness and improperly tightened plugs and connectors. While the should be wiggled and the basic components

the radio set should

equ

ipme

nt

is in operatio

be

rocked

so

n,

the cabling

that

any abnor-

of

mal result may be noted.

b.

Before troubleshooting

set, the technician must first

an

installed

be

familiar with

radio

the location of the basic components of the equip-

INSTAllATIONS

ment and the battery switches, radio fuses, and circuit breakers. The radio ment usually requires

l-

electr

ic

al system of the vehicle, normally

24

volts.

109. Operating Vehicular Equipment

a. Before the radio equipment can be operated,

it

may

be

necessary to

m

aster

switche s in the vehicl some tanks require switch and the radio on before power can In

such vehicles, remember to

off

equipment vehicles, battery

master switch.

when

it

may not be necessary to turn

the

same voltage as the

turn

that

both the battery master

ma

ster sw itch

be

applied to the radio sets.

it

is

not

sw

itche

s,

equip-

12

or

on

one

or

more

e.

For

exa

mpl

e,

be

turned

turn

the radio

in use. In other

on

the

62

Figure 60

. Radio set installed

in 1·ea1·

of

vehicle.

AGO 10016A

b.

When operating radio equipment in a

ve-

hicle, observe the following cautions:

(1)

To

prevent poss radio equipment, turn engine is racing. Turn

ible damage to the

it

off

while the

off

all radio equipment to prevent damage from abnormally high voltage.

(2)

Do

not

run

the vehicle battery

down

by unnecessary or lengthy use of the radio equipment when the engine is not running.

If

long periods of testing are

required, keep the vehicle engine run-

at

ning

a speed battery charge. gine and generator in vehic

that

maintains the

Use the auxiliary en-

les that

have them.

(3)

Be

sure to not in use. equipment by tery

turn

110. Electrical

a.

One of the most difficult troubles to sectionalize in a vehicular installation especially the noise generated

turn

off

all equipment when

Do

not turn

just

turning

off

the radio

off

the bat-

master or radio master switch;

off

all

individual switches.

Noise

is

noise,

by

the vehicle ignition system. Ordinarily, the vehicle electrical system is adequately shielded and bonded, to

ll

effectively eliminate a vehicle, and al

so

to permit interference to other

noise generated in the

radio equipment in the immediate vicinity.

loose

Sometimes, the bonding breaks noise. Before a radio

set

is

and causes

removed from a vehicle for repairs because of noise, first eliminate the vehicle itself as a possible source of noise

turning

by

b.

faults as loose

the engine

off.

A visual inspection can reveal such obvious

or

broken bonding or shielding, loose plugs, couplings, ground clamps, and loose or disconnected noise bypass capacitors

generator portion of the shielding might

or

voltage regulator. In general, any

on

the installation

permit exposed wiring to radiate noi

from ignition or the generator systems should

on

the

that

se

be examined carefully. After a technician has had experience with a certain vehicle he may be able to recognize its own perculiar noise characteristics. In s

cases, the noise source can be

lo-

ome cated by performing certain simple tests which are

given in the paragraphs

that

follow.

111.

Sectionalizing

a. Preliminary Instr·uctions. When noise in-

Electrical Noise Troubles

terference enters the radio set, the vehicle

be

should high-tension power lines, radio or

moved to an open space away from

radar

installations, other vehicles, and electrical equipment that

could cause noise disturbances.

b.

Int

erference Pr·esent

With

Vehicle Engine

Off.

(1)

If

noise is present in the radio receiver output when the engine is turned and all other electrical accessories are turned sets), disconnect the antenna and ground the antenna terminal of the set being checked. tinues, the trouble is in Reconnect the antenna;

the antenna terminal stops the noise,

the noise was coming in on the antenna. a similar frequency range is installed in the vehicle, repeat this test to verify that

the noise is caused externally. the noise is external, to a new location, free from electrical noise, before continuing with further tests.

If

(2)

no with the antenna connected, each of the accessories (including any

other radio equipment) and note whether any noise appears. Turn

off

on

the tery is not drained excessively. appears, check any input noise filter

capacitors the accessory producing the noise. These filter components usually located in the junction box

in the

tion of the accessory. With the acces-

.sory turned

the filter capacitor temporarily; noise level rises, the capacitor in question is

c.

Int

erference

Vehicle Stopped.

noise is heard

that

off

(including any other radio

If

the noise con-

the

radio set.

if

grounding

If

other radio equipment using

move

the vehicle

noise is present in the receiver

one

at

each accessory before turning

next one

so

that

the vehicle bat-

or

chokes associated with

that

primary

voltage for the opera-

on,

disconnect one side of

good.

With

If

Engine Running and

a rhythmic, periodic popping

changes in frequency as the

off

turn

a time,

If

noise

are

brings

if

the

If

on

AGO 10016A

63

speed of is probably caused

trouble is

ll

a

the

wiring

engine is changed,

by

the ignition system.

traced

for

to

the

proper

ignition system, check

shielding and bonding.

the

disturbance

Tighten 'an cable clamps, conduit coupling connectors, and see

in

curely

place. caused by defective bonding, loose, improperly gapped tor

points, or burned magneto

(1) Induction -coil system.

is equipped ignition system, fast ignition switch. ceiver is reduced for the interval the ignition

that

all ground wires

In

general, ignition noise is

spark

plugs, burned distribu-

breaker

If

with

run

an

the

idle and momentarily

If

the

noise in

is,

off,

the

trouble is in

are

se-

burned

or

points.

the

vehicle

induction-coil

engine

turn

off

the

at

the

re-

that

the

ignition system. A typical induction-

ROTOR WITH

RESISTOR SUPPRESSOR

IGNITOR

SHIELDED

~-

--------

DISTRIBUTOR

If

a

{BATTERY

coil ignition system is shown in figure

61.

Magneto system.

(2)

equipped the switch for receiver. reduces coming ignition is tem

d.

Generator Noise.

sound is

heard is running, tor.

Run the engine

it

slow down.

TIMER)

--,

I

..---a:::::5:.._--+

with

engine

off

any

change

the

from

system

operating.

is shown

in

the

cause

If

the

FEED-THROUGH

TO

THROUGH POINTS

If

the

a m

at a fast

the

magneto

in

If

turning

noise,

the

in

agneto

the

components of the

which

system, run

idle speed and

while listening

noise level in the

off

the

interference

the

A typical magneto sys-

in

figure 62.

If

a whining, squealing

receiver while

may

be a defective genera-

at a fast

interf

IGNITION

idle speed, then let

ere

nce decreases and

CAPACITOR

SWITCH

BREAKER

vehicle is

magneto

the

engine

is

64

SUPPRESSED

SHIELDED

PLUGS

SPARK

AND

X

GENERA

X

ENGINE

TOR

FigU?'e

X

BYPASS

FEED

CAPACITOR

61.

X

CAPACITOR

-THROUGH

I unition-coil ignition system.

AMMETER

+

BATTERY

RADIO

JUNCTION

BYPASS CAPACITOR

TM4000-4B

BOX

AGO

!0016A

goes to a lower pitch as the engine and generator

slow down, the generator

the noise. See

that

the generator Look for excessive sparking that

the bypass capacitor is properly grounded.

See

that

all leads connected to the generator are

is

probably causing

is

not

at

the brushes. See

loose.

tight.

e.

Regulator Noise.

If

an intermittent clicking sound is heard in the receiver, the interference may be coming from a faulty voltage regulator. The spring when

the

solenoid is not energized may have lost some of its tension, and ture contact to of regulators, this spring can

that

holds the armature open

be

intermittent.

it

will cause the arma-

On

certain types

be

bent back to its normal position. All wiring to and from the regulator should be checked for positive contact

and the bypass capacitors should

be

bridged with

good ones to eliminate them as a possible cause

to

of noise. All shielding and bonding tor

should

f.

be

checked for positive contact.

Int

erference Caused by Static Noises. Cer-

the regula-

tain types of interference can occur when the engine is turned motion, as when going down grade. irregular, cracking noi only when the vehicle could

be

caused

electricity generated

off

and the vehicle is still in

If

excessive,

ses

occur in the receiver

is

in motion, the trouble

by a loose

connection or static

by

friction. See

that

all ground straps and lockwashers are secure and making good contact. Any two metal surfaces that are not bonded together and ing or rubbing can cause scratch noises. this

by

connecting all suspected points with

that

are

Correct

scrap-

heavy braid shielding.

g.

Interference From Auxiliary Equipment.

In vehicles equipped with auxiliary enginegenerators ignition noise can be caused only

li

ary

when the auxi tion.

To

find

off

the vehicle engine and follow the procedure

d above to locate interference caused

in

engine-generator is in opera-

the exact source of the noise,

turn

by

a

running engine.

SUPPRESSED AND

SHIELDED

SPARK

PLUGS

BRAIDED DISTRIBUTOR

SH

-J_l__i_i

JUMP-SPARK

IE

LDING r - - - - - - -

..,__,

I

'

,)I

I

,.~

I

·-

':

,-.

I

FEED-

THROUGH

CAPACITOR I

!BREAKER

1

POINTS~r----Q:zs:---1_-,_

-

-....1.,..

CAPAC~TOR

I

L

Figut·e

____

62.

_

Magneto ignition system.

CAM

+

MAGNETO

SHIELD

..,.._---=:==:__....!_I

- - - -

---,

I

__

PRIMARY

I COIL

I

--

I STOP

-

~

-L

=-

SWITCH

" .·

_L

-J-

TM4000

·47

AGO 10016A

65

CHAPTER

6

TROUBLESHOOTING

Section

112. General

There

is

no

single procedure for troubleshoot-

ing all of the different kinds of receiver failure.

It

is possible to set each of several categories of receiver trouble. When the nature of the trouble has been discovered, the technician should select the ap-

propriate procedure. The procedures are explained in the paragraphs

successively the block diagrams of a simple

superheterodyne receiver and of a typical Army

am receiver.

113. Sectionalization

a.

Certain

test shooting any superheterodyne receiver. Figure 63

shows a block diagram of a simple superheterodyne receiver and the following points of signal injection: power amplifier grid audio amplifier grid (2), volume control detector (4), oscillator (7),

IF

RF circuit (9). The test equipment required is an audio oscillator and an

The points of signal injection and order of tests

on

are shown

b.

Apply an audio signal across the volume

control terminals

put from

the block diagram.

at

the speaker, the audio secti volume control to the speaker is defective. The power supply could also the

B+

output to find out. Feed a modulated

signal

of

the

IF

frequency into point

is

no

an

9.

RF

If

output, the

signal of the proper frequency into point

there is

IF

no

output the defect is in the

section, which includes the input circuit, mixer,

hf

and

oscillator.

I.

GENERAL

up

a general procedure for

that

RECEIVER

follow by using

points are used in trouble-

(1), (3),

grid (5), mixer grid

(6),

grid (8), and antenna input

IF-RF

point

signal generator.

3.

If

there is

on

from the

be

defective. Measure

5.

If

no

there

out-

section is defective. Feed

RF

h£

RECEIVERS

TROUBLESHOOTING TECHNIQUES

114. Localization

a.

Localization can be accomplished

test

the same diagram and

equipment used

paragraph 113. The method used in localization

that

is similar to

used in sectionalization but the

trouble can be traced to a stage.

b.

For

example, feed an audio signa l into point 1 ; fier and power supply signal into point 2;

if

there is an output, the power ampli-

are

operating. Feed a

if

there is

no

defect is in the audio amplifier stage.

c.

Feed a modulated

the mixer input.

If

stages to the right of the mixer

IF

signal into point

there is an output, a

are

condition.

d.

Feed a modulated

If

there is no output, the defect is in the

RF

signal into point

oscillator.

e.

In

general, when there is

no defect is between the point of signal injection and the speaker, or other output indicator. When a point is reached where a defect is indicated,

measure the

B+

voltage in

that

stage to isolate

the defect.

115. Types

a. Dead

of

Troubles Covered

Receiver. A dead receiver, which is

probably the easiest to trouble shoot, is one produces and

does

no

sound

not resp

at

either or all of its outputs

ond

to adjustment of its

controls.

b.

Weak Receiver. A weak receiver is one

with low output volume

that

can be heard only with difficulty when its gain and volume trols are turned

c.

Distorted Receiver . A distorted receiver

on

full.

has garbled output, and cannot be easily understood.

by

output, the

ll

in operating

output, the

using

of

the

that

con-

in

6,

6.

hf

66

AGO 10016A

>

Cl

0

....

0

e

"'

>

'

'--

INPUT

CIRCUIT

ri.

STAG

RF

ES

~

MIXER

VOLUME

LL

,__.....

CONTROL

i-4

4

I

F"

STAGES

4

DETECTOR

B+

TO A

STAGES

AUD

AMPL

10

i-4

p A

TO LOUDSPEAKER HEADSET, HANDSET

OR

i

1

,2,3

4,5

8,9.

POWER

S

UPPL

.

INJE

.

INJECT

6.

INJECT

7 .

MEASURE

INJECT

Y

ORDER

CT

AUDIO MODULATED

MODULATED

GRID

OF

SIGNAL

BIAS

TESTS:

IF SIGNAL RF

RF

SIGNAL

TM4000-49

7

HF"

OSCILLATOR

POWf:R INPUT

0. ""-!

Figure

63. S

imp

le supe1·hetero

dyn

e, block

diagram

.

d.

Int ermittent Receiver. An inter

ceiver mally in

lo

ses volume intermittent

so

me

other respect for short periods of

mi

ttent

ly

or acts abnor-

re-

time.

e. Hum. A humming receiver has either a

slight hum mixed with the signal, or a hum great that

f.

can

be

it

overcomes the signal completely.

Preliminary Rapid Checks. Certain checks

made

on

a receiver b

efo

re removing the

so

chassis from the cabinet. Chassis have often been removed from cabinets, then after the defect was located, could have been left in t checks are

li

(1)

Determine first whether the

sted be

it

was found

he

low.

that

the chassis

cabinet. Some of the

fault

is

due to the operator, especially if he is

ar

not famili th

at the power switch is in the

with the receiver. See

ON

po-

sition, the frequency-range switch is

in the correct position, the headset is plugged in, and the antenna changeover switch is in the proper position.

These and other items

mu

st

be checked before the equipment is considered defective.

(2)

If

there is the tubes, supplied to the receiver.

no

be

fil

ame

sure

nt glow

that

power is being

in any of

Also

check

the line fuse.

If

(3)

the receiver is dead, wiggle the

it

power output tube in

no

there are

clicks coming from the

s socket; if

speaker, the speaker or power s

may

be

defective. Detecting a

power supply when the receiver chassis

is in the cabinet can save t work, especially a separate unit.

if

the power s

It

can

be

worked

with out disturbing the rece iver.

( 4)

If

the receiver has an S meter or other

it

indicator, and

does not s variations as the signal is being received, the antenna. Sin

the trouble is between it and

ce

the indicator

usually in the age circuit, the trouble

between the age tube and the antenna. If

the indicator does show a variation as the signals are received, the defect is between the age t

ub

e and the

speaker.

(5) Turn

gain;

speaker, but

RF

up

the sensitivity a

if

noise is heard from the

no

signal is present, the

or mixer tube may

be

This same symptom may al

that

the local oscillator tube is inopera-

tive, or the antenna has become dis-

connected. Another cause for this symptom is misalinement to the extent

that

the proper

IF

frequency is

not produced.

If

(6)

the receiver is noisy or intermittent, shake or move the chassis. There may be a loose shield can or a

on

tion

top of the chassis.

loose

upply

bad

ime

and

upply

is

on

how

any

is is

nd

audio

defective.

so

mean

connec-

116.

a.

Section

General

T~1e

receiver shown in figures

II.

TROUBLESHOOTING

64

and

65

is more elaborate and complicated than the one

It

use

in the previous figure. on

some bands and triple convers

It

has special feature

crystal-contro

ll

ed

s, such

hf osc

s double conversion

ion

on

others.

as six tuning bands,

illators, variable band-

width, and a crystal calibrator circuit.

b.

This receiver has two audio channels. one or both of these channels is inoperati receiver is considered

de

ad. Figure

64

is a block

ve,

If

the

diagram of the am receiver.

68

DEAD

117.

AM

RECEIVER

Preliminary

Inspection

The best way to begin a troubleshooting assignment is to inspect the receiver thoroug See

that

the

power plug is in place, examine the

turn

on

fuses, and

ev

idence of burning. See

for in the

right

and maintenance personnel to

the receiver. Look and smell

that all

sockets. Check with the operator

find

out whether

hl

y.

tubes are

the set has been burning or smokin g. Burning

be

can

through the w

the result of

ir

tor, or a shorted

e insulation, an overloaded resis-

arc

ing from

transformer

windin

the

chassis

g.

A resis-

AGO 10016A

!MEGACYCLE

AND

CIRCUITS I I • 8

To

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CIRCuiTS

Z20I

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THRU

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DC

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SELECTING ANTENNA

TRANSFORMERS

RF

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V201

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NOTE:

REFERENCE 101-200: MAIN 201-400:RF 401-500:CRYSTAL OSCILLATOR

~I-600'1F

701-BOO: 801-BOO:POWER

SYMBOLS

FRAME

SUBCHASSIS

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SUBCHASSIS

SUPPLY

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SUBCHASSIS

0

e

l

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V603 8AKII

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I

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Figure

64.

Am

receiver, block diagram.

AGC

TIME

CONSTANT

TUBE

V506A

11121

5814A

T8103

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[ill)

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P\11-6 Plll-8

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MIOI

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tor

becomes greatly qverloaded and will smoke,

usually because of a shorted filter or bypass

be

capacitor, but the smoking can also by a winding shorting

to

the chassis or to

caused

another winding.

118. Checking Supply Voltage

If

the voltage of the power source is not definitely known, check Be sure to follow the instructions to make connections in the set operate on the available voltage. matic diagram (fig. 65), the connections for or

230

volts are shown

tions are made for 115 volts and the plug

it

with an ac voltmeter.

that

tell how

so

that

On

the sche-

on

TB801.

If

the connec-

it

is

can

115

inserted into a 230-volt outlet, the set will burn or blow a fuse.

volts and the plug let, the set will operate very weakly or not

If

the connections are made for

is

inserted into a 115-volt out-

230

at

all.

119.

Tubes

Do

Not

Warm

Up

When the power is applied, set all controls for

maximum output.

their

in indicating

proper positions. All tubes should glow,

that

tubes lights, the trouble

Be

sure that all

they are lighted.

is

probably in the power

switches are

If

none of the

supply. This condition may be caused by any

one of the following :

open

power input cable, de-

fective component in FL101, open power switch

(part

of FUNCTION switch), open fuse F101,

open primary or filament winding in power

If

only

one

transformer T801. quiring

6.3

volts fails to warm

of the tubes re-

up,

either the tube is burned out or there is a bad connection at

the socket.

the tubes requiring

If

the pilot lamp fails to glow and

6.3

volts warmup, the pilot lamp is burned out or there is a bad connection at

its socket.

120. All

The warmup of all tubes is an indication

the

graph does not necessarily mean

tive. The tube may have

elements, an intermittent condition, or

Tubes

Warm

Up

that

components mentioned in the previous para-

are

all good. The fact

that

a tube lights

that

it

is

not defec-

low

emission, shorted

it

could be gassy. When trouble has been isolated to a stage, however, the tubes should

be

tested either

in a tube tester or by substitution before other troubleshooting is undertaken. information from personnel

121. Sectionalizing Trouble

a.

that

Before the defective stage can be locat the section found. Turn

there is an indication

meter, the stages from through the fourth

the

operator or maintenance

the tubes are good.

that

contains the trouble must be

on

the calibration oscillator;

on

the CARRIER LEVEL

RF

amplifier

IF

stage V504 and the age circuits are operating. The defect between the antenna and the

RF

stage or in the

not rely

is

therefore

on

ed,

if

V201

Do

audio section.

b.

Set a modulated signal generator to any . frequency within the range of the receiver. nect the signal generator to J104

125

Con-

OHM BALANCED antenna terminals and set the receiver to the frequency of the signal generator.

If

a signal

input circuit between the antenna and V201 power supp

using the above tests, the defect can

alized before the receiver cabinet

122. Localizing Trouble

The following paragraphs localize and

is

heard in the receiver output, the

is operating. This also means

ly

and audio sections are

is

opened.

RF

that

good.

be

stage

the

By

section-

isol

ate trouble in the entire receiver, but in actual troubleshooting, the repairman should

121

information in paragraph which section (audio, RF, at

fault, and then

go

directly to

IF)

that

of the receiver

that

use

the

tells him

is

section and

apply isolation and localization techniques.

123. Signal Output Indication Meter but

Terminals

no

Output

on

From

LINE

LEVEL

AUDIO

LINE LEVEL meter MlOl and LINE METER

switch

8105 are connected across the LINE AUDIO output terminals. This can help determine the 8105 is rotated indicates

loc

ation of the trouble. When switch

to

position 9 or

that

a signal is present, but there is

10

and the meter

no output from the LINE AUDIO terminals, there is either an open resistor

R115 or broken leads

on

R111,

TB103.

R112, R114, or

AGO 10016A

69

124. Signal Output minals but not minals, and LEVEL

When there is

terminals and

Meter

no

output

no

From

LOCAL

From

no

Indication

at

indication

AUDIO

LINE

AUDIO

on

LINE

the LINE AUDIO

on

the LINE LEVEL meter, the trouble is probably in some circuit common to both of these circuits. The

circuits from

P120 (where the meter and audio circuits are divided) to the outputs are probably satisfactory. However, these circuits may

be

checked conclusively by disconnecting P120 from J620 and applying an audio signal across pins 8 and

tinuity across pins 9 and at

the

on

the tinuity between pins 9 and beyond ing order.

12

of P120 and checking for

10.

If

LINE

AUDIO terminals, and indicated LEVEL

meter and there is con-

10,

P120 may be assumed to

If

any of these results are not

con-

audio is obtained

all the circuits

be

in

good

work-

obtained, one or more defects exist in these cir-

cuits. resistance checks show

If

there are

no

bad contacts at P120, and

that

the secondaries of output transformer T602 are neither shorted nor open, proceed with the following signal

substiution test.

a.

Connect the output of an audio signal gen-

erator across the

primary the top of the chassis by connecting pin 5 and the other to pin 6 of .05-p.f

capacitor in

ser

ies

of T602.

Do

this from

one lead to

V604.

wit

h the hot lead of the

Connect a

generator to protect the generator in case of ac-

cidental contact with high-voltage circuits. Turn the generator output

on

full;

if

even a weak audio signal is heard, the primary winding is intact. mary probably is open. grid in

be turned

If

if

If

the signal is not heard

If

it

at

is open,

all, the pri-

the

V604 will be red hot, and the power must

off

at

once.

b.

Measure

the

voltage

at

the plate (pin the voltage is normal, the winding is not, and the voltage

at

the screen grid (pin

screen

5).

good

6) is about normal, the winding is open.

c.

Move V604. is in operating condition; signal, R623 or R625 may

generator signal output, coupling capacitor

the generator hot lead to pin 1 of

If

a signal output is present, the stage

if

there

is

no

be

le

ad to pin 6 of V602B;

faulty.

Move

if

there is a

C608

is good.

output

the

TerTer-

there is

can be verified by bridging

no

signal output,

If

capacitor.

d.

Move

the generator lead to pin 7 of an output indicates ing condition. this stage. Measure the voltage

6) ;

no

voltage may be caused by open resistor R622, because the other side of R622 has present, which was measured V604. Check R622 for meter.

e.

If

measuring the voltage across cathode resistor R621 shows a reading equal to probably is open. The higher-than-normal voltage is present because the voltage divider is

(R626, R621, and R623) and there is no plate current flowing; therefore, pears across R621 or R623, and the full voltage

other side of through, le R105;

is

between pin 6 and ground.

f.

Move

the generator lead from pin 7

C607;

C607

is open. Connect the generator

ad to terminal 1 of

no

signal output means R104 is open,

P120-3 and J620-3 tact. This to

both audio channels and therefore will affect

is

the point from which audio is fed

both channels.

125. Signal Output but not from LOCAL AUDIO Terminals

The procedure in this case used in paragraph 102, because the circuits are identical, with the exception of those portions

beginning with the secondaries of the output

transformers (T601 and T602). Therefore, in addition to troubleshooting the local audio chan-

nel in the same manner as was done for the

line audio channel,

nections to T601

;

R102, and the connections to TB102.

126.

No

Output

AUDIO Output Terminals

A defective power supply could cause both

audio channels to fail. Even though tubes

V602, V603, and

be

may not

providing plate voltage. Check for

plate voltage on these tubes.

C608

it

that

this stage is in operat-

No

output indicates trouble

at

pins 2 and 5

an

open with an

no

voltage drop

if

a signal

LINE

are

From

be

are

From

V604

GAIN potentiometer

not making good

LINE

AUDIO Terminals

is

similar to

sure P119 and P120

tight, and check RlOl,

Either LOCAL or

heat

up, the power supply

If

it

is open; this

with a good

V602B;

in

the

plate (pin

B+

of

ohm-

B+,

R621

open

ap-

B+

to

the

does

not get

or

con-

that

con-

LINE

is present,

70

AGO 10016A

apply an audio signal to pin

output is not obtained,

that

stages

(V601A, first

are common to both audio channels

AF

the· trouble· is in the audio

amplifier, and

14

of J620.

If

V601B

an

af

cathode follower) .

127. No Output When Signal

is

Fed

to Gain

Controls

a.

In

paragraph 126 the two audio channels

the

were the subject of

troubleshooting. The search now narrows down to a single audio input that

is applied to both channels.

If

there

is

no

output when a signal is fed to terminal 1 of LOCAL GAIN control control R104 and other tests indicate circuit should operate, break-in relay

at

fault.

be

b.

If

contacts 2 and 6 of relay

gether,

no

signal will enter the audio channels.

A continuity check from any point

line to ground will show a dead short if the lay is defective.

If

easily, open and close the contacts

test

it.

c.

If

there is an output when the signal is fed

R105

or LINE GAIN

that

the

K601

may

K601

short to-

on

the AF

re-

the relay can be reached

by

hand to

into the gain controls, move the signal generator

AF

lead to pin 8 of the

signal is heard, R627 is intact.

d.

Apply the audio signal to pin 7 of V601B.

If

the output is about the same as

the

audio was applied to pin

cathode follower V601B.

8,

V601B

it

was when

is

If

operating. The output is no stronger than when the signal was passed through

V601B, because a cathode follower does not amplify the signal, but reduces

it

by a small amount.

If

there is

no output, voltage and resistance measurements are necessary. Possible causes are a shorted

C603A

or C603B, or open R606, R607 or R608.

b.

If

there is an output when an audio signal is applied to V601A, no

FL601 is in

audio output, FL601 is probably defective, or coupling capacitor FL601 can

be

checked by setting S104 to the

good

C602

at

the audio terminals

pin 1 of

condition.

may

If

be

open. Filter

there is

WIDE position; this shunts FL601 and connects the output of V601B. FL601 or switch

129.

No

to

If

there is applied an

to

open

V601A directly to the input of

If

there

is

now a signal in the output,

S104 is defective.

Output When Audio Signal

2 of

V601

A

no

audio output when the signal is

pin 2 of V601A,

it

may

be

Is

Applied

caused

by

cathode-biasing resistor R604 or plate

load R605. Resistor R606 is not open because

would

cause

V601B

C603B

is not suspected either, because

ground out the

AF

to

be

inoperative. A shorted

B+

line and would have been

it

woulJ

isolated before.

130.

No

Output When Audio

Output of

Limiter

V507

Is

Applied to

Applying the audio signal to this point will give results similar to those obtained in graph the output, the line from pin 2 of 2 of be also may P120-14 and J620-14 or between

129.

If

there

V507

is intact.

caused by an open

be

caused

is

an audio signal present in

V601A to pin

If

there

is

no

output,

R601

or

C549.

by

poor contact between

Pl12-7

No

para-

it

may

output

and

J512-7.

131.

No

Output When Audio

(Limiter

7

Input) of V507

Is

Fed

to

6 or

it

128. No Output When Audio Signal to

1 of V601A When

SPONSE Switch

is

Set to

is

Applied

AUDIO

SHARP

RE-

Posi-

tion

a.

Both the block diagram and the schematic

that

show

the output (pin 1) of first AF amplifier V601A is coupled to the input (pin 7) of af

cathode follower V601B through band-pass

filter FL601 when AUDIO

RESPONSE switch

S104 is in the SHARP position.

AGO

10016A

If

the audio output has about the same ampli-

tude as

stage is operating. The output is than the input because there is

in a diode circuit. LIMITER switch is in the voltage from the switched

it

had in the preceding step, the limiter

no

greater

no

amplification

If

there

is

no

output and

OFF

position,

RF

-IF

B+

line may

the

de

not be reaching the diode plates. This may be

caused by an open R532, R533, R534, or R535

~r

by

a shorted

are poor contact

C531

or

C532.

Other possibilities

at

J512 or a defective S108.

71

132. No Output When a Modulated Is

a.

V506B OFF

This

Applied to

and

is

the stage where the audio is sep-

BFO

6 or 7

Switch

of

S101

RF

Signal Detector Is

Set

to

arated from the carrier; therefore a signal applied anywhere ahead of the detector will

the

RF

range. A signal applied to the detector

be

in

will be demodulated and the audio will appear in the output of the receiver. put during this test, it may secondary winding in open

RF

filter choke L502,

b.

Other possible sources of trouble are a

shorted

C56.2,

across terminals

voltage is applied

C563,

14

to

or a

and

this stage; therefore, few

If

there is no out-

be

caused

IF

transformer T503,

R527,

loss

15

on

by

or R526.

of the jumper

TB103.

an open

No

de

breakdowns will occur.

133. No Output When Unmodulated Signal Applied to and

a.

When the bfo (V505) is operating

BFO

6 or 7

Switch S

of

Detector V506B

101

Is

Set

to ON

at

Is

its correct frequency, the output will beat with the unmodulated signal and produce an audible beat

If

note.

there is no output, the bfo is not operat-

ing, or the signal is not reaching pin 7 of

V506B. be applied to

If

C535 is open, the bfo signal will not

V506B.

anode of the oscillator, back to

Z502

to complete the oscillatory circuit.

e.

If

the plate and screen-grid voltages are

normal, check the resistor R528; is

present. Other possibilities

Z502

or open

f.

Another possible trouble is

C526.

de

voltage across grid-leal\

if

the circuit is oscillating, bia

quency of the bfo is not close enough to the quency of the

IF

signal to produce an beat note. The off-frequency operation may caused by shorted turns in L508 leak or change in capacitance of C554, C556.

(Z502), and must

134. No Output When Modulated 455-kc Sig.

These components

be

nal

Is

Fed

Amplifier V504

a.

This stage amplifies

are

replaced as a unit.

to Input

the demodulated. The audio output should siderably stronger

step .

If

there is no output, the trouble must

than

it

was in

termina

are

that

or

L509, or a

in a sealed

(Pin

1)

of

signal before

the

l 3

o:f

defecth~e

the

fre~

fre.

audib

be

C555,

o:r

can

Fourth

it

be

con-

previou

IF

i

l>e between the point of signal injection (pin 1 o V504) T503.

and the secondary of

Therefore

the

possible defect could be an

IF

transforme

open primary winding in T503, and open R52u, R524, or R525, or a leaky C525, C561,

C562,

()l'

C530.

e

~

b.

If

the voltages screen grid (pin 6) of is in the

RF

-IF

B+

at

the plate (pin 5) and

V505

are

low,

the trouble

line or

R529,

R530 or R531 may have changed in value. This path is through Pll2-11

and J512-11, BFO switch S101, and FUNCTION switch S102, to the power supply at

choke L602.

c.

If

there is normal voltage

L602 and

C606 would cut the

are not defective. An open choke

B+

supply to the bfo. Other pos-

at

L602, then

sible causes of an inoperative bfo are a defective FUNCTION switch S102 or BFO switch

S101, a shorted or leaky

open R529,

d.

An open screen-grid bypass

R530, or R531.

C533,

or

C534,

C533

or an

will

cause oscillation to cease, because normally

connects the screen grid (pin

72

6),

which is the

b.

If

there is no indication components is defective, Check the

primary

of L512,

try

that

retuning

part

of T503, fox proper resistance, because shorted change the resonant frequency signal cannot

135. No Output When Modulated 455-kc Sig-

get

nal

Is

IF

Amplifier

through.

Applied to Input

This stage is similar to

so

much

(Pin

the

stage considereJ in paragraph 134; therefore, the troubleshootin procedure will be similar. The components

suspect

A leaky

it

primary winding L510 could also cause

are

an open R518, R519, R520, or

C523

or

C522 or

an

open

symptoms.

one

turns

that

1)

of

or

the

AGO

of

th.e

T50~.

can

th.e

Third

t

R55

shorte

sam

10016

.

136. No Output at Jl16

When Modulated 455-kc Signal Applied to Input Follower

a.

The

IF

OUTPUT

nected to

is output

on

a vtvm when its

the jack.

IF

cathode follower V509B.

at

the jack, a reading will

If

there is

IF

V509B

RF

no

OUTPUT

(Pin

50

OHM

probe

is

reading,

50 OHM Jack

7)

of

IF

Cathode

jack

is

con-

If

there

be

obtained

connected across

C539

may

be

open.

b.

Other possibilities are an

or L504. Any opens in these parts would

the

B+

circuit to the plate. Also, if become shorted, B+,

causing the stage to become inoperative.

137. No Output When Modulated 455-kc Signal Amplifier V502

The second

the third and fourth

it

would ground the plate and

Is

Fed

to Input

IF

amplifier stage is similar

IF

the method of troubleshooting the second

open

R538,

R539,

open

C541

should

(Pin

1)

of

Second

IF

to

amplifier stages, and

IF

amplifier is the same as described in paragraphs

be

no

134 and 135. There will

if

grid voltage

either R515 or L506 is open, the plate voltage will still present,

but

it

will

be

low

plate or screen-

R521

is open.

because

R511

is

If

be

in

parallel with L506.

c.

The same procedure can

other filters, which are connected into the circuit

Is

at

other settings of the switch. The tions of filters

139. No Output When a Modulated 455-kc

8502 and 8503 are used to short out the

that

are not in use.

Signal First

If

the stage is operating, the output will

Is

Applied to Input

IF

Amplifier

V501

be

used for the

considerably louder than the output heard in the previous step because of the amplification of the stage. opened by a defective

or and 2 R103, would open the other stages because leaky screen-grid bypass screen-grid voltage to the extent be an increase in plate bypass

If

there is

no

output, the

B+

R504, R506, R508,

L505.

no

An open jumper between terminals 1

on

TB102,

or a defective

de

line and

it

is

common to them. A

C506

RF

GAIN control

it

would affect

would decrease the

that

there would

output; the same effect could be caused

R506

or a short in

C511

would decrease the plate volt-

C511.

age to zero or nearly zero.

140. No Output When a Modulated 455-kc

Signal

BANDWIDTH Switch

KC

Is

Applied to J513 or J518 and

S501

or 1

KC

Position

Is

rear

(Pin

could

A leaky

Set

sec-

1)

of

be

L501,

by

to

.1

138. No Output When Modulated 455-kc Signal

Is

Applied to Plate

IF

Amplifier

a.

The position of the BANDWIDTH switch

(Pin

5)

of

First

determines which mechanical filter is in the circuit. Therefore, if there is one of the switch settings, the trouble can

no

output

on

only

be

isolated to the mechanical filter that is connected

at

that

into the circuit

setting. A continuity

check of the circuit through the switch will re-

veal the defective part.

b.

For

example, with the switch set

1 as shown in the diagram,

8502 front connects

on

position

the 2-kc mechanical filter to the plate circuit of V501 and 8503 front connects the output of the filter to the input of the grid of

V502 to the coupling capacitor

( C553) connected to the plate of

is

·Continuity

maybe

AGO

lOOlGA

thtough

open.

V502. There is a path from

V501.

this path, check

If

there

C553.

It

a.

When BANDWIDTH switch 8501 is set to

the

.1

KC

or the 1

in the circuit and tuning is very critical.

KC

position, crystal

Y501

If

there

is

is an output signal of about the same amplitude as during the previous step, the input circuit to

the first

IF

amplifier is operating.

If

there is no

output, the possible causes of the trouble are a defective BANDWIDTH switch Y501, open Z501

cannot

be

replaced.

b.

When

tions, the crystal

C503,

R501,

be

replaced separately;

8501

is in any of the other four posi-

is

or

still in the circuit, but is in parallel with it. Thus, the crystal can eliminated as a source of trouble

switch to the 2 tion. The tuning will

KC, 4 KC, 8 KC

be

broader as the switch is

set to the higher numbers. Therefore,

8501 or crystal

L503.

The parts in

Z501

by

setting the

or

16

KC

must

C501

posi-

if

be

the

tuning becomes much sharper as the switch is

set to the

.1

KC

or 1

KC

position the crystal

circuit is operating.

73

141. No

a.

Output Signal Third Mixer V204

When

Is

Applied

Test point E211,

a 455-kc

to

on

top of the chassis is

Modulated

Input (Pin 6)

connected to pin 6 through R230. The output will

be

slightly above the level

it

was in the previous step if the stage is operating properly. A no-output condition may or shorted secondary L235 of T208.

and resistance measurements may show primary A shorted

(L234) of T208, R218, or R219 is open.

C308

will remove the plate and screen-

be

caused

by De

an

open

voltage

that

the

grid voltages, but would affect other stages also.

b.

The signal

is

fed into the cathode circuit from the variable frequency oscillator (vfo) V701

through the secondary of T701; therefore,

T701

an open secondary in circuit to

V204 and keep the stage from operating. The same conditions could leaky or shorted plate bypass cuit would not be

shorted out.

142. No

Mixer

a.

In

be

opened; the plate voltage would

Output

nal

When

Is

Fed Into Input (Pin 6)

V204

the previous step, a modulated 455-kc

could open the cathode

be

caused by a

C307,

but the cir-

Modulated

2.5-mc Sigor

Third

signal fed into this stage produced the proper

that

output, proving

the stage was operated properly. Apply a modulated 2.5-mc signal to pin 6 of third mixer

the vfo stage until an output is produced.

V204,

and vary the frequency of

If

an

output is not produced, the vfo is not operating.

If

the vfo is operating, the output from beat with the input signal to the mixer and duce a beat frequency of

455

kc

in the output of

it

would

pro-

the mixer.

b.

Measure the bias voltage across R701.

If

there is a voltage present, the stage is oscillating;

if

there is no voltage, the stage is not operating.

c.

Another way of checking the oscillator is to

note any change in the plate voltage when the

no

bias resistor is shorted out; that

the stage is not oscillating. Possible causes

of an inoperative vfo winding (T701) of shorted bypass capacitor

decoupling capacitors

s,

but

possibilitie

they would have been isolated

previously because the

are:

Z702,

C707

RF-IF

R702, or R703 or a

C705

change means

an open

or

C708.

and

C714

B+

line or the

primary

Shorted

also are

regulated 150-volt line would have been shorted

out.

of

d.

If

C705

opens, the connection for RF

tween the screen grid, which

is

the anode oscillator, and tank coil L702 would be broken, causing the circuit to cease oscillating. turns in either

L701

or L702 wou

ld frequency of oscillation, thereby producing output than

that

gives a resultant beat frequency other

455

kc in the third mixer.

If

tests s the defect is in one of the parts in sealed unit must be replaced. A large change

the resistance value of grid leak R701 will stop

oscillations.

143. No

a.

Output nal ond

When

Is

Applied to Input (Pin 6)

Mixer V203

Tune vfo stage

Modulated

V701

until an output is pro-

2.5-mc Sig-

duced. Normally, the only trouble would open plate circuit. This would be caused by open L233-1 in Z216-1 or an open plate dropping resistor should deve voltage would to zero. The plate current would if either

R216.

If

plate bypass capacitor

lop

a leak or a short circuit, the plate

go

to a low value, and possibly

be

R215

or the secondary (L404) of

opens, because they are both in the circuit

tween the cathode (pin 7) of V203 and ground.

b.

The other two tuned circuits (Z216-2 and

Z216

-3)

are in parallel with Z216-1 but current passes through them; therefore, a defect is rare.

would

If

go

to zero.

C289

or

C290

should open, the signal

If

trouble should occur in of them, the signal output would not to zero, but the pass band of the circuit will

affected.

144. No

a.

signal will

ond mixer

8-

to

Output

.Signal

Second

When

Is

Mixer V203

Modulated

Applied

8- to 32-mc

to

Input (Pin 6) of

In the signal-tracing process, a modulated

be

applied to the control grid of sec-

V203.

The sign al should be in the

32-mc range because this range will check the operation of the second mixer and sec crystal oscillator V

follow.

To

get the signa l through, either tune

the signal generator through the

range or set the signal generator

401,

as well as the stages

8-

at quency within this range and tune the receiver throughout the same range.

If

there is an out-

be-

of

the

Shorted

change

the

an

how

that

L702,

the

in

of

Sec-

be

an an

C288

interrupted

T401

be-

no

direct

one

be

reduced

be

ond

that

to 32-mc

some fre-

74

AGO 10016A

put,

it

should be about as strong or slightly

than

it

stronger

b.

If

there

from

second crystal oscillator V

was during the previous step.

is no output, no signal is coming

401.

Measure the

bias across grid-leak resistor R404 to determine

If

whether the circuit is oscillating.

there is no

bias, the circuit is inoperative.

c.

If

there

the crystal oscillator

may be

an

open

primary

does

not operate,

winding (1403) of T401, and open R405, R406, R407, or 1401. Other causes of trouble would C411,

C412,

or

switch

C413.

S401 or capacitor selector switch

A defect in crystal selector

be

a shorted

C410,

S402

could also prevent the stage from operating.

d.

Unless the defect is in the center arm of the switches, the switches can be set to other positions to connect other crystals and capacitors

the

into the circuit; if positions except one, one of

tal, or one of the capacitors

oscillator operates

the

switches, a crys-

that

S402

into the circuit is defective. An open

at

connects

sec

ondary

all

winding (L404) of T401 or cathode resistor

wo

R215

uld keep the oscillator signal from reach-

ing the second mixer, but because they are in the

it

cathode circuit of the second mixer, been detected in the troubleshooting of the ond mi

145.

xer

in

paragraph

No

Output When a Modulated 17.5- to

25-mc Signal

of

First

Mixer V202

a.

This stage is similar to second mixer

143.

Is

Applied

would have

to

Input

sec-

(Pin

V203

6)

therefore, the troubleshooting is similar. The plate

current

follow

s, out, the stage will trouble

in

the output of first crystal oscillator V207.

is

b.

A s

grounding

smoke. The block and that

the mixer through a switch S208 becomes defective,

from

reaching V203, which means there will be

flows

through the load, L232-1;

therefore,

might

hort

the

that

if 1232-1 shou

go

dead. Another cause of

be

an

open R209 or L231, which

ld

burn

circuit in C280, in addition to

plate voltage may cause R212 to

sc

hematic diagrams show

output of this stage is fed to the second

(S208); therefore, if

it

can keep the signal

it

no output.

146. No Output When Modulated .5- to

a.

In

Signal

First

the

Is

Applied to Input

Mixer V202

previous step, a signal of the mixer

(Pin

8-mc

6)

of

output frequency range produced in the loudspeaker. Now signal produces no output, output of the first crystal oscillator

that

it

an

audio signal

the

.5- to 8-mc

means

V207 is beating with the input signal to produce a different frequency in the mixer output.

b.

This oscillator circuit is similar to the one

used in the second crystal oscillator

there is

one

no

wide frequency selection because only

crystal is used. Measure the bias across grid-

V

leak resistor R207 first to determine whether the

If

stage is oscillating.

check the crystal by substituting characteristics.

If

there is no oscillation,

one

either

C324

or

C325

of identical

open, the capacitive voltage divider between the

sc

grid and

reen grid (the anode) would become

inoperative.

c.

Because this is an electron-coupled circuit, the screen grid serves as the anode. The anode is connected to the anode side of the voltage divider through

. Thus, if

C326

should open,

C326

the oscillations would cease. There is also a

possibility of R207 changing value; this could

also stop the circuit from operating . The first mixer and the first crystal oscillator

are

ative in the frequency range between 8 and

me;

that is,

V201

to the second mixer. The troubleshooting

therefore can

erates normally in the inoperative in the

;

could be in the first mixer or the first crys tal

the signal is fed from

be

simplified.

.5-

to 8-mc range, the trouble

If

the receiver op-

8-

to 32-mc range, but is

RF

oscillator.

147.

RF

Amplifier

a.

Apply a modulated test signal having a frequency between

V20

1

.5

and 1 me to test point E208; set the MEGACYCLE CHANGE switch to the proper position. Tune in the signal by the KILOCYCLE CHANGE dial.

If

is produced, repeat the procedure using signals

me,

32

2 to 4

me.

between 1 and 2

16

me,

and

16

to

any of the above frequencies, the

me,

4 to 8 me, 8 to

If

there is no output

RF

is dead.

b.

In all positions of the MEGACYCLE

CHANGE switch, plate voltage to

V201 is applied through parasitic suppressor E212 decoupling resistor R205. burns out, the stage will bilities are a defective switch

If

either of these

go

dead. Other possi-

S206, open screen-

that

the

not

401,

but

should

inoper-

amplifier

turning

an

output

amplifier

and

32

at

AGO 10016A

75

grid dropping resistor R204, or a shorted bypass capacitor

open because of a defective R103. circuit of V207

c.

S206 is set to position the circuit, and,

C229.

Poor contact

The cathode circuit could

RF

GAIN control

at

switch S206 in the plate

V207 or S207 in the input circuit of

could also cause the circuit to open.

When the MEGACYCLE CHANGE switch

11

(fig. 65), Z201-1 is in

if

defective, could cause the

trouble.

d.

When the MEGACYCLE CHANGE switch

12,

1,

6, 7,

or

8,

is set to position to suspect are Z206-1,

Z202-1, Z203-1, Z204-1, Z205-1,

respectively.

The

the circuits

MEGACYCLE

CHANGE switch may also be defective.

148.

No

Output When a Modulated Signal Applied to J104 125 OHM BALANCED ANTENNA Jack with CHANGE Switch

a.

Relay K101A, when energized during peri-

in

MEGACYCLE

Any Position

ods of transmission, break-in, or calibration, disconnects the antenna and grounds it.

close,

no

contacts on the relays fed to hand to

RF

amplifier V201. Actuate the relay by

be

sure

it

is operating properly before

signal will

If

the

be

proceeding. A bad contact

be

also cause this trouble.

b.

With the switch in position

in

switch

pected components are S201, S202, T201, C255,

and

C226.

five

other ranges, the same troubles in a

are

possible, except

applied signal will

149. No Output When Modulated Signal Applied to J 103 WHIP UNBALANCED

ANTENNA Jack

a.

When antenna

When the switch is set

that

the frequency

be

different

jack

for

each setting.

J103 is being switches S201 and S202 of the MEGACYCLE CHANGE

switch

are

not in the circuit; there-

fore, they are eliminated as possible sources

Is

the trouble. The antenna jack is connected into the circuit through relay K101B, which be defective. Resistor Rl21,

be

cannot it

is damaged;

considered as a cause of trouble unless

it

drains

electricity accumulated

b.

Lamp L103 grounds

charges built-up across

antenna is connected to the

es

S204

front

switch

and S205. These may cause

at

the antenna jack,

off charges of static

during

the

mobile operation.

stat

ic

or lightning

antenna

RF

. The whip

amplifier through

trouble and must be checked.

11,

S204

the

could

sus-

R233,

to

above

of

the

used,

could

the

Is

of

Section Ill. TROUBLESHOOTING WEAK

150. General

a.

The procedure for troubleshooting a weak receiver is basically the same as dead receiver;

that

is,

it

is a

that

matter

used ·for a

of localizing the trouble by stages. When a weak set is countered,

a dead set.

refer

But

to

the section

on

troubleshooting

note this difference: When troubleshooting a dead set, you are not too concerned about from a put;

but

will most often find

precise

nal is injected into FUNCTION of

the

set

In

sets

with

out,

the

assumed

the

particular

amount

test, so long as there is an out-

of output

that

results

when troubleshooting a weak set, you

it

amount

of

switch to

will

vary

age

age circuit

that

the

important

output

produced when a sig-

the

stage.

MGC,

when the age circuit is used.

but

no switch to

must

be grounded out.

tubes

in

the

to know the

First,

set

because the gain

cut

it

in and

It

suspected stages

the

en-

is

AM

RECEIVER

have been tested before

stage

gain

tests are

made.

b.

Before troubleshooting procedures gun, there are some preliminary steps be

taken to insure

ating fault. Listed below

that

the defect is not an oper-

are

some checks

that

are

should

that

be-

the operator can make before he calls the repairman.

(1)

See

that

the receiver is tuned properly.

(2) Be sure

that

all switches are properly

set.

(3)

Check the line voltage to see not

dropped.

(4) Determine whether signals

on

only one or more

(5) Check the

antenna

stat

to see

ion

s.

that

are

it

has

weak

is still

up.

( 6) Observe the indications on the

carrier-level meter, that

could localize the trouble.

or

other indicators

S meter,

76

AGO 10016A

151. Troubleshooting Without Stage Gain Data

It

is possible to locate the trouble in a weak

receiver without stage gain information. In such

cases, a signal is injected into the various stages

in

as

noted

instead of de For

of from applied to the input of the same s produce a stronger output nal generator output must the output output, the trouble is between the input and put

152. Troubleshooting by

the dead set procedure, the point being

at

which the signal input must

c?·easecl

to produce the same output.

be

increased

example, an audio signal applied to the plate

an

audio amplifier produces a certain output the

receiver. When the same signal is

tage,

it

should

than

before; the sig-

be

reduced to keep

output

at

the

same level.

must

be incre ased to produce the same

If

the generator

out-

of this stage.

Using

Stage Gain

Data

Sometimes

whether a stage

cause

it difference from normal output cannot Then make detailed stage-gain measurements determine which

it

is almost impossible

ha

s the proper gain, either

to

determine

be-

is normally low, or because the slight

be

noticed.

to

part

of the receiver is not

amplifying properly.

a.

The technical manual for a receiver will specify the minimum and maximum signals quired put. Tables I and

at

certain points to produce a given out-

II

are

for each type of stage-

re-

gain test and contain such information for this

receiver.

switches

b.

It

is important to s

as

the technical manual recommends.

To

test the stage gain in the

et

all controls and

RF

and

IF sections (fig. 65), connect a vacuum-tube voltmeter between the designated output point and

ground. Connect a lead of the signal generator to protect the

erator from possible damage

in contact with a high

.05-p

.f capacitor in the hot

if

the hot lead comes

de

voltage point.

gen-

Con

nect a vtvm between DIODE LOAD terminal 14 and ground. Apply the unmodulated nal

to

test point E211,

at

the top of the chassis.

455-kc

sig-

The signal generator output should be between

20

nd

40

microvolts to produce

-7

volts on the

v,tvm.

c.

As

the tests progress toward the antenna section, there is less signal input required to produce the same output; this is because the signal is being amplified by passing through more stages. The output of the signal generator should be

compared with the information in the

at

all test points shown in table

that

are outside of these limits by a small amount

do

not necessarily indicate

that

I.

the receiver is

chart

Readings

not operating properly.

cl.

The gain of the individual stages of a receiver over a period of time will vary. difference is

great

and the receiver's overall

output is weak, the stage probably is

at

If

the

fault.

When a stage lacks gain by a considerable

amount, voltage and resistance checks may be

necessary to locate the faults.

-

Sign a l gener

o

utput connecti

Fr

equency (me)

Signal generator

volts

).

e.

To

test

tion,

the

procedure is identical with

the

RF

in

the

same terminal s,

ator

stage-gain tests. Connect the vtvm to

at

a frequency of

ato

r

on

output

(micro-

the stage gain of the

but

leave the signal gener-

455

kc

the points shown in table II.

AGO 10016A

BA

L-

AN

CED

AN-

TENN A

connector

.5-

32

less than

4

fixed

Table I.

that

Test E208, g

of

.5-32 4 to

IF

point

V201

16

used

RF

rid

sec-

for injection into Compare the out-

Stage-Gain Tests

Test

point

E209, g

.5-8 15

to

of

60

rid

V202

17.5-25 15

to 16

-3 13

T

to

est

g

rid

40

poin t E210,

of V203

8-32 2-3

20

to

65

50

put in microvolts of the signal generator needed to produce the required on

the vtvm with the information in table II.

great

Any the table means checks must

deviation from the requirements of

that

resistance and voltage

be

made.

-7

Test poin t E 211,

g

rid

of V204

.455

to 125

20

to

that

volts output

40

is

77

Tabl e

II.

IF

Stage-Gain Tests

Signal

generator

connection

1st

if

Amp!

(V501)

2d

if

amp! (V502) grid, pin 1

3d

if

amp! (V503)

4th

if

amp! (V504)

f.

To

test the gain in the audio section, con-

output

grid,

pin 1

Signal generator

(microvolts)

100

to

200 250 to 500 10,000

to 20,000 300,000

to 400,000

output

nect the output of a calibrated output audio sig-

nal generator through a

.05-p,f

DIODE LOAD terminal of TB103 and

capacitor to

turn LOCAL GAIN control fully clockwise. Turn RF

GAIN control fully counterclockwise. Ad-

just

the frequency of the generator to

400

cycles.

Set the AUDIO RESPONSE switch to the WIDE

position. a 600-ohm noninducti AUDIO terminals 6 and of the generator until rms across

Connect

an

ve

600

ohms) is indic ate d

ac

vtvm in parallel with

resistor across LOCAL

7.

Increases the output

500

milliwatts (17.3 volts

on the

vtvm.

The output of the generator should be approxi-

mately 1 volt.

g. Transfer the vtvm and the 600-ohm r

to

the

LINE

AUDIO terminal s

LINE

GAIN control fully clockwise. Adjust

10

and

the signal generator to produce an output of

milliwatts (2.45 volts across 600 ohms)

m.

vtv le

The output of the generator should

ss than 1 volt.

h.

In the case of a weak receiver, the tub

13

and

on

es

istor

turn

10

the

be

es

should be checked before any other action is

ur

taken. Voltage and resistance meas

ements will

show such faults as leaky bypass capacitors, and

that

resistors

ditions can reduce the plate and scr

have changed in value. These con-

ee

n-grid

vo

ltages to the extent

crease and cause the signals to become weak.

i. Capacitor C603A can cause

bl

es

in the plate circuit of first

V601A and

j.

An open bypass capacitor is

se

becau dition in the But

there are other symptoms th

AF

cathode follower V601B.

there is no telltale indication of its con-

de

voltage and resistance readings.

tected.

the the the

(1) Capaci

V601A, the first should open, there would be a signal vo

tor

ltage drop across cathode-biasing sistor R604, the volt degeneration, and th is in turn will duce the output noticeably.

(2) Capaci

in

tor

V501, the first should open, the signal wou siderably weaker

in

been cause

the

any signal present in the tion of a no considerably weaker

AF

section, and any l

the

IF

section is therefore the more

noticeable.

lc.

An open coupling capacitor between stages

ll

y will cause the output to drop to zero,

usua

the

but in signal may

case of a very

get by the open capac

duce a weak output. However, si the very strong ones will not very much strength; sidered wea

k.

that

the

output will de-

the

above trou-

AF

amplifier

hard

to locate,

at

can

be

C609

is the

cathode

af

amplifier.

bypass

If

age drop will cause

C505

is the cathode bypass

IF

amplifier.

than

it wou

ld

If

be

ld

cable of C609. This is be-

IF

rmally operating set is

than

it

is in

oss

occurring

stro

ng signa

it

gna

ls other

ge

t throu gh

l,

or and pro-

than

with

then the set can be con-

de-

in

C609

re-

C5

05

con-

have

sec-

the

in

the

Section IV. TROUBLESHOOTING

153. General

Only a few defects can cause distort

ion, and

they can usually be identified by the sound of

sent

the receiver output. Distortion is pre

ra

the output signal is muffled or

it

sound as

should. The experienced technician

can often tell from the sound

di

stortion is present and

what

spy, or does

ju

st what

causes it.

when

not

type of

In

most

cases the distortion will be in the audio sect ion.

mo

st

Distortion, in

cases, is caused by an upset

in bias, or by overloading of a stage.

78

DISTORTED

154. Types

a.

Frequency distortion occurs when a quenci For

es

examp l cies origina in the output of the receiver the

are

notes

AM

RECEIVER

of

Distortion

ll

fre-

are not amplified to the same extent.

e,

if the high and l

ll

y were of the same

ow

audio frequen-

strengt

flow

frequency

h,

but

reproduced louder than those of the

high frequencies, frequency distortion is present.

(In

this receiver this type of distortion will not

be

present, because

it

is designed for a limit

ed

frequency response.)

AGO 10016A

b.

Amplitude distortion is present when there

the

is a change in after

it

passes

harmonic content of the signal

through

one or more stages. This type of distortion is the more bothersome because the signal sounds unpleasant, whereas frequency distortion is only a matter of some frequencies being

It

others. but

will be noticed during voice reception

probably

stronger

not

when cw signals

or weaker than

are

being

received.

155. Common Causes of Distortion

a.

Leaky Coupling Capacito'

r.

One of the most common causes of distortion is a leaky audio coupling capacitor such as couples the signal V602A to If

C605 becomes leaky,

in series

the

with

from

local

grid

resistor R613 and plate load

C605 (fig. 65), .which

the

local

AF

output amplifier V603.

it

will act as a resistor

amplifier

R611. This series circuit is connected across the B+

line,

tive

than

making

it

was,

the

grid

end of R613 less nega-

or

even positive. The tube now operates on the upper portion of the Eg-Ip curve, producing distortion.

b.

Gassy Tube.

or

any

other tortion will result. grid

may

draw

c.

Other Causes. Other causes of distortion

If

the

local output tube

V603

tube becomes gassy, amplitude dis-

The

current

bias will

and produce distortion.

be

reduced, the

are misalinement, poor power supply filtering, warped speaker diaphragms, oscillation, excessive

strength

of

input

signals, and interference

from cross talk.

156.

localizing

Distortion

of the output

it

from in good condition. Move

transformer

and all components

to terminal board TB102

the

hot prod from pin 5

are

probably

to pin 1 of V603.

c.

If

the output is not clear, the plate circuit of V602A is not operating properly, and the trouble could

d.

A more positive method of detecting distor-

tion is by using

be

in the

an

oscilloscope and

input

circuit of V602A.

an

audio sig-

nal generator. The equipment setup is shown in

figure

Connect the signal generator to

the

66.

vertical amplifier terminals of the oscilloscope by

S1

setting position 2 to connect the oscilloscope across

to position 1. This switch is set to

the

plate load resistor of the stage being checked.

e.

The testing procedure follows:

400

signal generator to and

adjust

the oscilloscope frequency controls

cps. Set S1 to position 1

to produce on the screen two sine waves look like those

at

A figure

67.

Adjust

the oscil-

the

that

loscope controls to show clean sine waves. We are assuming present. Compare the results shown in figure

f.

Assume

to the local audio channel.

of the signal generator across

control R105 and set

at

this point

that

with

trouble is

the

67.

that

the trouble has been isolated

Connect the output

LOCAL GAIN

S1

to position

2.

not

patterns

Connect

the test probe to the plate (pin 5) of V603 and

observe the wave form. If, for example, the wave

form is unlike

pattern

A when the probe is con-

TEST

PROBE

a.

Localizing distortion is more difficult

than

troubleshooting a dead or weak receiver. Signal

but

it

substitution can be used, to use a form of signal tracing. tenna

to

the

receiver

input

constructed audio signal

is more convenient

Connect an an-

and connect a locally

tracer

(fig. 46) to vari-

ous points to determine where the trouble lies.

b.

Assume

AUDIO terminals is normal

that

the

output

but

at

the

the output

LINE

at

the

LOCAL AUDIO terminals is distorted. This

the

pinpoints

trouble to which includes V602A and gain control R105. Connect the prods of Rignal the

AGO 10016A

tracer

to

signal is present

the

local audio channel,

V603, and LOCAL

5 of V603 and ground.

at

the output, the secondary

the

If

AUDIO

SIGNAL

GENERATOR

Fi

2

gu?-e 66.

OSCILLOSCOPE

Setup

/o?-

checking di

st01-tion_

TM4000-

51

79

TOO HIGH BIAS

TOO

LOW

BIAS

UNDISTORTED SINE

OR

TOO

LOW

PLATE

OR

TOO

HIGH

PLATE

WAVE

OR

OUTPUT

OR

SCREEN

VOLTAGE

nect

ed

to pin 5 of V603 and is like pattern A

when the probe is connected to pin 2 of the distortion is between those two points. patterns in

that

will appear on

B,

C,

and D show the wave

the

oscilloscope screen

there is distortion; they also indicate

some

V602{\,

T~e

foriQs

wh~n

of

tile

causes of distortion.

g.

Diagram D sine wave as such a condition arises,

in the age system, because the age

prevents overloading by keeping the output

all signals

at fault, switching to mgc will tortion. The operating point about which age works is determined by the RF

GAIN which is

in

figure

it

appears on

an

the

a constant level.

the

sole control when

67

shows a

oscilloscope.

trouble is

If

the age is

not

eliminate

sett

distor~d

Wh~n

probably

normally

frotn

di

the

ing

of

the

age is turned off.

h.

An example of a trouble would

R516, the age decoupling resistor, or the

be

an

opeh

age decoupling capacitor C519, both of which are the grid circuit of the 3d

One of these troubl tion from controlling could be the resu lt.

es

would prevent the

the

Any

IF

amp

lifi

er

V503.

age

ac.

stage and overloading

trouble in the age sys. tern would prevent the age voltage from affecting the controlled stages, which include the 1st, and 3rd Other controlled stages

mixers, V202,

IF

amplifiers V501, V502, and

are

the

1st, 2nd, a

V203 and V204, and

RF

2nd

V503.

nd

3rd

amplifiet

V201.

at

.

ih

,

OVERLOADED

Figm·e

157.

General

A

re~eiver

time to time

times goes dead or develops any oth trouble. to which a receiv and disappear

tervals. Such troubles

do

they

mally, and because the

STAGE,

67.

CAUSED

Osci

llo

scope patte1-ns showing clisto1·ted

Section

sin

BY

e wave.

V.

TOO

MUCH

TROUBLESHOOTING

is operating intermittently if from

it

operates normally

Intermitt

not exi

ent troubles include all

er

is subject, but they appear

at

irregular, or even regular, in-

are

hard to trace, because

st

when the set is operating nor-

set

may resume normal

SIGNAL

TM4000-52

but

between

er

type of

operation before the technician can finish

ing it.

80

'NPUT

types

test-

INTERMITTENT

158.

Causes

a.

Capacito1·s. A frequent cause of intermit-

tent

operation is

AM

RECEIVER

of Intermittent Operation

the

haphazard opening and closing of a connection within a fixed capa citor. For

example, the circuit would open

ll

ed

loo

lead pu

jarring made

again;

se from the foil, and

of the set

or

might

that

same effect might be caused

cause

by a sudden switching of the voltage off and

If

capacitor C607 (fig. 65) which couples the audio from of line

AF

LINE

GAIN control R104 to

amplifier V602B should become in-

then

the

contact to

if

a pigtail

a slight

the

AGO 1001

on.

grid

be

6A

termittent, the signal level would vary

If

down.

the opening and closing condition is a rapid rate, the effect may appear as noise. the screen-grid bypass plifier

V504 should open and vals, oscillations will occur. iri

rapid succession, noise will

C529

in the 4th

clo

se

If

it opens and closes

be

produced. Vari-

at

slow

up

IF

inter-

and

at

If

am-

able tuning ·capacitors can short intermittently because of dust, dirt, or other foreign particles becoming lodged between the plates. The plating on

the plates sometimes peels

are

often long enough to cause intermittent short

off

in slivers which

circuits. The rotor wiping contacts may have improper spring tension, or corrosion that could cause intermittent high resistance contact to the capacitor frame.

In

the very small types of variable capacitors the plates have extremely close spacing; these plates can become bent and may short

if

the frame should warp because of heat

or

twisting of a subchassis. Small

air

capacitors, used as trimmers, and commpression-type trimmers also collect dirt. The troubles will

be

of

the same types as those in tuning capacitors.

b.

Loose Connections._ A

loose

connection in

any portion of the set can cause intermittent

operation. Vehicular vibration may shake loose

a connection, or the cause of looseness may be a factory fault, such as an unsoldered or poorly soldered joint.

c.

Resistors. Wire-wound resistors sometimes

at

develop intermittent open circuits

of the resistance wire and the terminals.

the junction

Carbon

resistors may develop opens, but they usually

occur

after

the resistor becomes hot during a

long period of operation. Some carbon resistors are

insulated and have the resistance element in

the form of a carbon rod in the center. The

carbon rod can crack and cause intermittent op-

eration.

d.

Tubes. Normally, in trouble shooting, the

If

tubes are suspected first.

it

may

be

normal when tested, and the test will

a tube is intermittent

be of no value. An intermittent tube can sometimes Elements in a tube may expand because of and filament may expand and break, then as the ends may flows,

be

found

short

to other electrodes momentarily. The

by

tapping the suspected one.

come

together again, and current

it

cools,

producing normal results. Depending on

heat

conditions, this may occur several times in a

minute for a particular tube.

e. Inducto1·

are especially susceptible

The form

s.

on

which the

RF

and ac coils

coil

that

carry

to

intermittent opens.

is wound may expand

de

from the heat and snap the winding. Moisture on

the surface of the wire produces a chemical

action which causes corrosion that will eat away

close

the conductor. An arc may form and

the circuit momentarily; then when the carbonized area breaks down, the circuit opens again.

f.

Potentiornete1·

controls often have resistance strips become pitted because of wear. tion of the moving arm may

s.

Carbon volume or gain

that

Only a small por-

be

in contact with

may

the strip, and a slight jarring of the set may break the contact momentarily. This condition

be

will

current

present especially in controls where flows,

and arcing could occur.

g.

Solder Joints. Original solder joints often

de

appear good to the eye, but under the surface there may

be

a looseness which,

if

subjected to vibration, could cause intermittent operation. Technicians often introduce intermittents by poor soldering. Therefore, carefully examine all solder joints, particularly those

that

have been

made during repairs.

159. Isolating Intermittent Troubles

a.

Audio Signal Substitution. In this receiver

the

CARRIER LEVEL meter can

be

of

great

assistance in troubleshooting. The meter is in

IF

the output circuit of the fourth Therefore,

if

during

intermitte~t

amplifier.

operation the

meter needle remains steady, the trouble is be-

IF

tween the fourth

amplifier and the audio output. An arcing in the power supply would have an effect

to the audio section, the signal tracer

on

the output.

If

the trouble is traced

that

was used to trace distortion can be used by following the instructions given in paragraph 156. Rather than

use

the incoming signal from a transmitter

it

is

in this procedure, however,

better to apply

an audio signal from a signal generator between

15

on

TB

103

terminal

and ground. This

because the signal from the generator will nor-

be

mally

at a fixed

signal may vary over a wide range.

amplitude, while the incoming

If

the CARRIER LEVEL meter needle varies in step with the output signal,

somewhere between the fourth

the· intermittent condition is

IF

amplifier

and

the antenna.

is

AGO 10016A

81

b.

RF

Signal Substitution. The same proce-

dure is used as in troubleshooting a dead set. A signal of the proper frequency is fed into

J104 125 the modulated signal can

ulated, the signal can

If

the signal is unmodulated, a vtvm connected across the detector load, (between and

set variations erator grid.

intermittent antenna the the output

OHM

BALANCED antenna jack with

antenna disconnected. A modulated or un-

be

used.

be

heard in the speaker.

If

15

it

is mod-

on

TB103

ground) will indicate an output. When the

is intermittent, the trouble will show up as

on

the meter.

lead to pin 1 of V201, the

If

the meter needle is now steady, the

Move

the signal gen-

RF

amplifier

is between pin 1 of V201 and the

jack.

If

the output is still intermittent

trouble is between the signal generator and

detector load. Move the signal generator

to the grid (pin 6) of

the

first mixer

V202. Test point E209 (on top of the chassis)'

do

this.

If

is a convenient place to

needle does not fluctuate in step with the mittent point and the

signals, the trouble is between this

RF

amplifier grid.

needle follows the variations, the trouble is

the meter

If

the meter

inter-

between the first mixer grid and the detector load. This procedure is used to localize the

de-

fective stage, working toward the meter connection.

c.

Fo1

·cing Tr-oubles to Reappea1·. There are

times when the intermittent condition does not reappear be made to

over the receiver to concentrate trick by shorts or opens

the ring, the

intermittent

for hours

reappear

or

even days. Often

it

by placing a cardboard box

the

heat. This

works best when the condition is caused

that

are due to heat under

chassis.

rap

If

the receiver is sensitive to

the chassis

at

several points to make

reappear.

can

jar-

If

(1)

one end of

sensitive to rapping, the trouble

the

chassis seems

more

is probably, though not necessarily, at that

end. Moving resistors and

pacitors around with

an prod will often reveal poor contact th

e components.

If

there is a sharp

disappearance of the signal or a

ca-

insulated

in

sudden change in noise, move the wiring around with the prod.

the wires too

far

out of place, as this

Do

not

move

may ·cause other troubles. Poor solder connections can often be found wiggling the

wiring

at

the sockets

by

and

other terminals.

'

ay

arring

ssary

seem to

at

to keep prodding

(2) The chassis m

sensitive to j is then nece around, stage by stage, until the point is found.

(3) Certain components will open

mittently

during

line voltage surges and will later be restored to normal. Remove and replace tubes one

If

time.

the receiver becomes insensitive to tapping when a is removed, the trouble is in or a stage closer to

the input voltage to power transformer T801 can

a variable

be

increased by connecting

transformer increas ing the voltage to 125 volts or more. This increase in often will show up

intermittents ordinarily would not appear for hours at

the normal

input

voltage.

be

equally

all points.

inter-

at

particular

that

tube

stage

antenna. The

in

the

line and

the

voltage

that

It

bad

a

Section VI. TROUBLESHOOTING

160. General

a.

Before the receiver can be freed of hum,

it

is first necessary to know

what

hum is and how to recognize it. The experienced technician recognizes Hum

and

is usually

82

it

as a steady low-pitched sound.

is produced

60

by

power line ac variations,

or

120 cycles.

It

is a tone

that

RECEIVER

has a constant amplitude quency·.

This distinguishes

consists of an unpleasing sound of many

FOR

HUM

and

is of one fre-

it

from noise which

;andom frequencies and is constantly changing in amplitude. Hum can also be regarded as a

frequency audio voltage. most

other ac receivers,

60

frequency of

or 120 cycles.

In

this receiver, as in

the

hum will have a

low-

AGO I00!6A

b.

Hum

frequency

(1)

may

develop directly in the audio

section of a receiver because

Inadequate

filtering in the power sup-

of-

ply.

(2)

Stray

coupling

from

the

ac power

leads.

(3)

Short

~athode

( 4)

The circuits being modulated by

161.

Causes of Hum

a.

Filter Capacitors.

of

hum

in capacitor. cuit

for

removing

the

output

and

C606B (fig. 65) pactors. hum

level would rise considerably. The schematic are

diagram

mounted

develop between would be similar to connecting a choke L602. This would reduce

of

the

choke

of

the

filter,

be forced

C606A becomes leaky. the

choke causes core

the

inductance,

tive

as

a filter. These

that

the

hum

but

the

actual

b.

Filter Chokes.

chokes becomes will hum.

test

required

defective should be

circuit

between

of a vacuum tube.

signal

any

The

of

the

If

either

in

and

producing

through

present

ac

receiver is

capacitors

the

The

hum

in

most likely cause

are

that

rectifiers. Capacitors C606A

are

examples of filter ca-

~apacitor

shows

the

that

same can.

the

two capacitors,

should open, the

C606A

If

cause a ripple in

hum. Excessive

choke L601

The

if

excessive de

saturation,

and

makes

is

apparently

the

last

two examples show

caused by

trouble is a leaky capacitor.

If

any

one of the frlter

shorted

Internal

to determine

internally,

shorting

carried

whether

out

the

heater

the

RF

an

open filter

in

the filter ciris

present

and

the

and

hum.

C606B

leakage should

the

result

resistor

the

across

effectiveness

the

output de

can

filter capacitor

through

which lowers

choke less effec-

the

choke,

the

receiver

seldom occurs;

the

coil is

only

after

other

IF

in

componnts have been checked. Iron-core chokes usually core wedge copper, drop place, the saturation

ing.

have

an

air

gap

in

the

saturation.

of

non-magnetic

or

brass.

out

or

if

the

gap

The

gap

material

If

the

gap

it

were

not

could close up

core to

is

kept

such as paper,

material

there

from

prevent

open

should

in

the

vibration

core laminations. This would allow core

which

c.

Power Transformer High-Voltage Wind-

This

power

would produce hum.

supply

uses a full-wave rec-

by

first

tifier, which means rectifier cycles

If

output

if

the power line frequency is

the

winding numbered

between points

that

the frequency of

(input

5 and 6 or between 6

of

the filter) will be 120

5,

6,

7 should open

abnormal hum would result. Testing

and

placing all chokes correct

the

trouble, because the opening of one

filter capacitors will not

leg of the high-voltage winding changes circuit to put

and

that

of a half-wave rectifier. The out-

frequency is now

capacitors

are

60

cycles and

not

large enough to filter

such a low-frequency hum. This condition could

by

also be caused

a cathode-to-plate

full-wave rectifier tube.

d.

Tube Cathode-to-Heater Leakage. A 60-

cycle hum can be caused by cathode-to-heater leakage in a tube, especially in the audio stages, which can readily pass low frequencies. As example, the first audio frequency amplifier V601A would produce a 60-cycle hum

cathode were to

is shown in figure 68. should touch, resistor

the

R604 would be in parallel as shown by

connection

the

at

short

to the heater. This tube

If

the cathode and

heater

point

and the cathode-bias

A.

This would

volts ac across R604. The 60-cycle voltage

the

would modulate

electron hum. This could happen in an in

which case the 60-cycle signal would mod-

the

RF

ulate

demodulated by

cathode-bias

when a signal is tuned in and

the

detector.

resistor and the cathode is

stream

RF

If

a stage has

grounded, the same defect would not be noticed

V601A

a

C609

R604

6 . 3 V

of

TM4000-54

Figu1·e 68. H eate1·-to-cathode shoTt.

the

60

cycles.

and

7,

and

re-

the

chokes

short

in a

if

the

heater

put

and cause tube also,

AC

an

6.3

be no

AGO 10016A

83

because there is

cycle voltage can

162. Determining Frequency

a.

When a hum is heard in the

receiver, the first step is

quency. In a set using a half-wave there can

the line voltage. In this receiver

no

be

be only o.

place across which the

developed.

of

Hum

~utp~t

to

determme Its .fre-

rectifier,

ne

hum frequency,

(fi

g.

that

65)

60-

of a

of

'.the

rectifier is a full-wave type, and the rectifier

output quency is

experience the audio signal tracer shown be

source. In this set to the chassis and the other grounded such as heard in the headset. sounds like the hum

frequency; if the frequency

is

120

cycles when the input voltage fre-

60

cycles.

b.

If

the technician

to

be

able to recognize the frequency,

does

not have sufficient

in

figure

46

may

used. Connect the tracer across a 6.3-volt ac

one lead

fi

lament terminal of a 6.3-volt tube

V603

. A loud 60-cycle hum will

just

can

be

connected

one

to the un-

If

the hum in question

heard it is of the same

is

higher,

it

is a

be

120-cycle hum.

c.

If

a 120-cycle hum is present,

it

means

that

the defect is probably in the power supply filter

that

and

the filtering is inadequate.

If

a 60-

cycle hum is present, the defect is not in the

It

power supply. heater short in a tube,

tube

gr

id. The

covered in paragraph

163. Tracing Power Supply

a.

Sectionalization.

is probably a cathode-to-

or

stray ac pickup by a

one

exception

to

this rule is

161c.

Hum

or Audio

In this receiver there are

Hum

two audio channels; therefore they must covered separate

trol R104 and the

ll

the way counterclockwisa.

a

heard,

still

ly.

Turn the LINE GAIN con-

LOCAL GAIN control R105

If

the hum is

it

is coming from lthe power supply or one of the audio channels. Disconnect the speaker from the LINE AUDIO terminals and

12

on

TB

103

.

If

the hum stops, it is in line audio channel. audio channels and due to madequate filtering in the power supply.

If

. '

the hum is present in both audio channels with both gain controls turned 60-cycle hum,

it

If

the hum

it

is a 120-cycle hum

could

is

be

from a cathode-to-

present in both

on,

and

it

the

it

is a

heater short in a tube, or stray coupling to a

grid, but

it

is more probably

in

the audio section. The common audio sect composed of V507, fir

st

the

detector, V506B,

AF

amplifier, V601A,

and

cathode follower, V601B.

b.

Isolation.

If

the

hum

has

been traced to the audio section and is a 60-cycle hum, it will be nece

inates. A quick and easy method tion is one

ssary

that

to find

uses

the

stage

an

audio signal

where

of

as the one shown in figure 46. Connect probe of the

chass

is.

AF

output tube V603.

it

is originating between

speaker.

somewhere between this

tracer

to a convenient point

Touch the other probe

If

the

hum

this

If

the hum is heard,

point

to

is

point

it

is coming froyn

and Touch the probe to pin 2 of V602A. is not heard, the

this point and the previous hum is heard,

tween this point probe successively to stages toward heard. Thi s system eliminates tween the

test

hum

it

is

originating

and

the

detector until

point

and

is

originating

test

detector.

input

of

the

speaker

point.

somewhere be,

the

the hum is heard.

164. Tracing

Modulation hum modulates,

carrier

is RF,

at

it tions; the detector will signal and the

a.

Sectionalization.

controls to

be

tion.

If

turned on

are

off, trols and the antenna. from the detector to

paragraph

in

11

plifier tube, V504. This will stop getting through. Any RF

is

section can

the

RF

60

cycles.

below will have a frequency of

b.

Stmy

import

ant

Modulation

the

hum

can pass

frequency. Since the carrier

through

Hum

then

hum

will be

heard

Turn

the

maximum clockwise (on) po,i-

the hum is

but

the

hum

present

is

not

is

originating

present

The

the

141. Pull

out

hum

get

through

carrier,

and

Therefore

will

the

have

Coupling Hum. One of

causes of

hum

or

varies, the

the

RF

demodulate

.

both audio ga'n

with

either

when

between the

entire

output

the

fourth

the

originating

only by modulatil

a frequency

hum

discussed

60

cycles.

is induction into

common

ion

is

the

limitel',

the AF

it

orig-

hum

isol

a-

tracer

such

one

on

the

1 of

Joc

1

3

not

heal'd

and

th~

the

detector,

If

the

hum.

between

If

th~

Move

remainin~

th~

hum is 11o

stages

be...

whenevet

:Rl?

and

IF

se~

th.e

contr I

the

conb'ol·

con-

audio secti n

was

co\·er

d

IF

atn-

hum

fr

n1

in

the

~

of

in.

the

n1

st

h

84

RF

or

IF

section. There cuits or capacitors prevent this.

If

a capacitor in this circuit

are

at

the ac power

usually filter cir-

input

to

should open, hum would result. Capacitors

C104, C105,

Cl06, and C107 are examples. The simplest method of determining which of them is

at

fault is to bridge them, one

at

a time

with

a good capacitor.

Section VII. TROUBLESHOOTING

165. General

A receiver is noisy when the output, in addition to the desired signals, contains crackling, sputtering, or two general

frying sound

s.

Noises fall into

categories-external

and internal. External noise is from a source outside the receiver. the

quencies it

Internal

receiver. Noise is made up of many fre-

ranging

can pass

does not modulate

166.

Causes

External

atmospheric and

a.

Atmospheric noise is caused by lightning

and

other electrical disturbances

duced by nature. Little can

from

noise

noise is from a source inside

from

audio to

through

of External Noise

the

any

stage even though

carrier.

noise is divided into two

man

made.

rf;

therefore,

classes-

that

are pro-

be

done to reduce

atmospheric conditions; usually

it

can be avoided by moving the receiver to an-

other location, or by changing the operating fre-

that

quency to one

is relatively free from inter-

ference.

b.

Man made noise can be produced from

loo

many sources, such as lines, gasoline-engine ignition system

se or arcing power

s,

electric

motors and generators, other radio sets, dia-

s,

thermy machine

noise can

c.

tenna

be

suppressed.

Loose or corroded connections in the an-

and ground systems

etc. Frequently, this type of

are

a frequent cause

of external noise.

167.

Causes

a.

Trans/O?-mers. Noise in transformers is

of

Internal Noise

frequently caused by corrosion-coated breaks

in

the

and

windings.

radio

transformers

Primary

are

windings in

IF,

RF,

the worst offenders. The corrosion causes the winding to open, leaving a sma be conducted by

ll

gap across which the current may

the

corrosion itself

or

an

arc

NOISY

AM

RECEIVER

may jump intermittently, producing sharp, rapid changes in current, and therefore noise. Transformer windings can work loose from

their

terminals, producing noise resulting from

intermittent connections.

b.

Wi1·e-Wound Resistors. Wire-wound re-

are

sistors that

occur in transformers. When a resistor

winding works

is a rapid

subject to

loose

rate

of

the

same noise troubles

from a terminal,

intermittent

the

result

operation which

causes noise.

c.

Potentiometer

s.

Potentiometers

are

among

the main sources of noise. The constant friction

arm

and

between the sliding

the resistance ele-

ment causes wear and noise. As the resistance

element becomes badly worn, the

-contact becomes very poor and, ultimately, intermittent, and noise results even when

the

control is not

being adjusted.

d.

Band Switches. When the contacts on a

band switch or a similar switching device become corroded and worn, a noise will be generated

other. When the switch is dirty,

~hen

it

is

set

from one position to an-

it

may be noisy. The contacts may become bent, causing an intermittent condition

e. Tuning Capacitors. Though

that

produces noise.

it

seldom happens, tuning capacitors can become noisy, especially when the rotor plates

are

turned. Warping of the plates, shifting of the rotor shaft, and particles of metal

sl

ivers peeling from

the plates are common sources of noise. Dust and dirt often carry fine metal particles, and

if

they become lodged between plates, noise results as the rotor is turned.

f.

Tubes. Electron tubes generate noise

that has several possible causes.

Shot effect is produced because elec-

(1)

tron current consists of separated ticles

that

leave the cathode in a ran-

dom

fashion, producing fluctuating

par-

currents uniformly distributed over all

frequencies.

AGO 10016A

85

(2) Flicker .

caused by

effec-t

is

.a

low-frequency .noise

.small ·emitting .areas ·of cathode .constantly ·changing the:ir emission ·Characteristics.

(3) In tubes having more than ·one ·

lector ·element, .

plate

·of a pentode,

s-uc

h as -the

the

scr-een

random ·division of ·current produces uniform CtN'Tents

spectrum of

Mic

( 4)

produced by-motion ·of a tube. These are heard -when is .su'bj ected

( 5)

Other sources ·

pos:itive-~on~ernission

tiv·e-ion

'l'YJPe

·over -the .a

-tube out.Put.

-whole

"fr.equency

.rophonics .are low-freql:lency

the

elements ·

the

-to

vibration.

of

-n

·oise

in

tubes .are

·

CI!lrrents,

currents -produced

·

<>f

noise

.as

no.ises

the r.e-

'Wh

the

col-

.and

noi.se

Of

tube

posi-

en

nobiced

.sult ·

Of

gas .ionizati•

·eiectron emission.

g .

.Poor~y

joints may

.Such

no.ise

j·oint

'ben<eath

on

th.e .smace

.

h.

M·echan.iwUy

that

.are not secureiy

.and ·

Calilse

.Soldered ./·Oi1'vts. Pooriy soldered

be a very seri.ous cause

wou·

ld

resalt

.a

soldering j·

but

scraping noises when

j.arr·ed. Lo0se screws

wi11

-prod-uce

thing

to

by

the

rapid m.aking .and

somewhere.

tbeir

-probah1e

the

remember

.Some

·causes, .ar·e listed

same scraping noises.

is

below.

oR,

.and seconda

from

mov-ements of the

ob

which iooks

which

CGLus

w.a

s n·ot weB done.

ed Noise

lock'Bd

.and

s.ubchassis cover plates

that

noise

s.

in

place can

is

always caused

breaking

eommon

types

Pr

obabl e ca u

ses

of

Tube s

the

set

of

.a

circuit

of noise,

in

the

ry-

noise.

good

hields move

is

The

.and

chart

Scratching ________________ Scratching Scraping

Intermittent

______

______________ When

____________________ ___

•cr

.wckling, scraping

__ __ W.hen si,gnal

adjust

When

chan.ging

___

'When chassisisjafl!'ed.

js

ing .

l68. lsolaH·n·g External Noise

a.

Refer

control in the headset. Disconnect the J104 125 OHM :BALANCED

terminals. .Short

jumper. siderably, ceiver. Remove the antenna. Shake the -transmission line;

to

Rl-05

figure ·

to

65.

a point where

out

If

the

noise stops

it

is originating outside

jumper

'Turn LOCAL ·GArN

the

noise is heard

the

.antenna

ANTENNA

the

terminals or

is

with

reduced ·

of

the

:and reconnect

if

from

noise gets worse, the transmission 1ine has

break or tenna tion between

in

it

-or

it

is rubbin-g .a-gainst

other

opject.

It

is

al

so

possible

is rubbing against something or

the

.antenna and

..a

tree,

that

.a

the

transmission

the connec-

line is loose.

b.

If

th-e

.Preceding tests indicate

sourc-e

noise is probably tenna. The trouble vicinity and may power lin

is

outside

radiated

e,

veh'icle ignition system, motor, gen-

of

the

noise picked up by

now

is

be

coming

erator, or hospital equipment.

mobile receiver is availab

le,

it

.receiver,

in

the

from

If

·can be

that

the

noise

the

immediate

.a

nearby

..a

-portable or· taken

being ·tuned jn,

gain

control.

bands

.

various .areas When incr-eases

is

.a

decreases,

COn-

re-

the

poor so coming

the

.a

169.

pole,

an-

.PO.int

.antenna from .antenna terminals. with

the

-originatin-g

.an-

-extreme counterclockwise position.

·continues, control and

-continue,

to

and

.a

near

by.

.c.

Disconnect

or

urc

e.

in

localizing

.a.

Turn

where

.a

jumper.

b.

Turn

the

gain

Dirty

tulling

eapacitors.

W-oTn

g.ain control.

Worn

w.ave-band s

Loose tube (!!ements, s

that

may

point

is

reached where

in

the

test

receiver,

the

ground

the

ground connection is probably

the

ground lead is too close to a noise

If

the

noise incr eases,

over

the

power line.

Internal Noise

LOCAL GAIN control

the

noise is heard. Disconnect the

the

J104 125 OHM BALANCED

Short

If

the

in

the

receiver.

witch

.

cTew

oT

shield ean.

be

radiating

the

noise level

the

noise source

wire;

if

the

it

is probably

Rl05

out

the

termina

noise continues,

LOCAL GAIN control R105 to the

If

the

it

is originating between

the

speaker.

it

is

originating between

control. Use

If

the

method described

the

noise does not

the

antenna

noise.

noise

to

ls

it

is

noise

gain

.a

86

AGO

10016A

in

paragraph

170.0

noise.

170.

lso~ating

Noi·se

a. Signal Tracing.

audio used to

~round

venient AF in the

signal

to

trace

r shown in. figure

localize noise

localize distortion

terminal point

outp.ut tubes.

pin 5 or 6 of

powe1

· supply

on

of

the

Insert

the

power-supply tubes

connections,

all plugs

and

the

AF

output

probe

If

the

to

noise originates the

speakers.

the

in gain same from

primary

control. Move

tube.

the

jacks..

pin 5

is

in

If

V60·3 stage.. Keep touching the probe·

particularly

If

tube

of

not

heard

the

secondary of

If

the

of

T60•1

the

nois·e

for

locating:

In

by

the

and

the

signal

chassis ..

the

socket.

may

be

by

subs.titution,

the

this

audio stages,

same method used

hum.. Connect

tracer

Remo;ve

sign.aJ

at

trn.cer probe

If

noise is. heard,.

fault

.. Check

the· connecting·

noise· is.

not

heard, replace·

. Touch the· signal

local

AF

output. tube· V603.

in. the· head

set

'F601

noise· is. heard,.

or

between

the

probe to- pin 1 of the

is

not heard,

source

46.

can

to. a. con-·

one· of

and

inspect

tracer

, the noise

or

between

it

originates

5 and

it

is coming

o.f

the

be

the

the·

the

to

the

input

or

successive:ly

output

ious. stages,. working toward the

b.

Stage Blocking. The· signal tracing method

just

described

in

the· audio· circuits.

can

be used

only..

wi:th

If

of the var-

gain control.

the

signal

a signal

tracer

with tuned circuits. and a. demodulator is avail-

it

able A simpler which is. similar to hum

can be used in the RF. and

and

quicker meth.

that

used in troubleshooting

defects·,.

It

can be·

used

section .. Connect a. clip-to· a.

od

test

IF

sections.

is stage blocking,

also-

in

the audio

lead and fasten

the· clip- to- a. convenient point of the chassis.

'FI:lrn

the· LOCAL GAIN. control

noise·

can be· heard·

the lead

to-the· grid. noise· continues,. noise· stops,. Touch. grids. antenna_

it

is. between.

the

probe· successively to

of

the· various stages) working· toward

If

the· noise· continues when a point

it

with

good volume. Touch

(pin

1) of V603.

is

in

the· V603. stage.

pin 1 and

R.105

so

If

the

the the

the antenna.

the

control

the

is. shorted out, the nDise is originating between that

point

and

the· l

ast

point

that

was shorted

out

If

the·

nDise

· stops when a point is shorted

out,

the

· noise· is. originating between

that

point

and the antenna ..

Section

Yilt.

TROUBLESHOOTING

AM

RECEIVER

171. General·

Squealing times noises in the quency sounds because

used

they

and

motorboating

to

describe unwanted sounds or

output

of receivers. Very low-fre-

are

classified as motorboating

are

terms some-

sound like· the "put-put" of a motorboat. Motorboating is usually the result of a component the

receiver

back

in

the produced feedback ceiver;

by in

however, interfering tenna. to external bles.

and

determine

interfering

If

the

disconnected.

172. Squealing Caused by

The

squealing sounds described

failure· in

that

produces regenerative feed-

the

audio section of

audio amplifiers . Squealing may

anything any

radio

short

whether

that

causes regenerative·

of

the

amplifiers of the re-

it

is. sometimes. produced by

. signals. Disconnect the· an-

circuit the antenna. terminals.

the

squeal is. caused

signals or internal trou-

squealing stops

the

trouble is usually external.

with

the

antenna.

Interne~

Conditions.

in

this. sec-

be

by

THAJ

SQUEALS

OR MOTORBOATS

tion. are· high-pitcliled. audio sounds. They are

usually· causedor· more stages. of the receiver.. Squealing:

by· unwanted. oscillations in one

RF

or

IF

section of the

may-originate· also in

the

audio· section .. In. either case, the squealing is

the· result.

ence· is. cuits, would occur squealing: sounds. While the· osci-ll'ations. would occur ql:lency

of

unwanted oscillations. The differ-

that

the oscillations,

at

the· same frequency as the

·

to

-which the circuits are tuned. The

if

in the audio cir-

in

the

RF

at

or near the fre-

or

IF

circuit

audib le· squealing sound is then produced by the heterodyning· unwanted oscillations. wa:ntecl

1-73

· oscillati-ons· with· a. received signal.

:.

Causes. of-

The· causes.

but

tliley

of

Unwanted Oscillations

of

unwanted oscillations are many

have· one·

two-

thing

or

more frequencies of

with

each other or the un-

in common. They each

:produce regeneration. Component failure in de-

coupling: ftlters. sometimes. causes regeneration.

If

the· capacitor· becomes open

or

reduced in

AGO 10016A

87

value or the resistor changes to a lower value, the filtering action is reduced and signal varia-

that

tions will occur in the voltage viously decoupled pling filters are used in

by the filter. These decou-

de

voltage sources, such as ave, age, plate, screen, and bias supplies are

common to two or more stages of amplifica-

tion.

Poor shielding, a tube shield left

after

tube

replacing the tube or lead dress not

being restored to its orignal condition

was pre-

that

off

its

after

replacing a component, are all causes of undesirable coupling

that

may

be

regenerative and

cause unwanted oscillations.

17

4.

Sectionalizing Source

a.

Audio Signal Tracing. Refer to figure

If

the oscillating condition is present whether

a signal is tuned in or not, and

when the set is tuned,

it

probably

of

Oscillations

does

is

65.

not vary

originating

in the audio section. The audio section includes the detector put

V603, or the line audio output, V604. Turn

the

LOCAL GAIN control R105 to the extreme

counterclockwise (off) position.

stops, and the detector.

V506B through the local audio out-

If

the squeal

it

is originating between the gain control

If

the squeal continues,

it

is

originating between the gain control and the

on

local audio output terminals

terminal board

TB103.

b.

RF

Signal Tracing.

present only

after

a signal

If

the squealing is

is

tuned in, and

it

is

heard with all signals, the oscillation

likely in the

would

be

and the detector, V506B.

RF

section. In this receiver it

between the first

If

IF

amplifier

the

squeal is heard

is

most

V501,

only when the set is receiving a signal and it occurs RF

175. localizing Squeal or Motorboating

mo

stly

at

one end of a tuning stage, the

amplifier, V201 is probably

at

fault.

The stage blocking method described in paragraph thought to 6 of the detector,

170b can be used.

be

in the audio section, s

V506B.

the trouble is between the

it

if

continues,

it

is between the detector and

the audio output. The shorting probe can

If

the

antenna

the

trouble

hort

out pin

squeal stops,

and

detector;

is

be moved to any of the audio tube grid terminals. Whenever a point is found where the squeal stops, the squeal is originating ahead of that

It

point.

is possible

by troubles in two stages. Therefore,

for

the squeal to be caused

if

the trouble is corrected in one faulty stage, and it still squeals, another stage nearby may also causing the defect. Stage blocking can also accomplished by removing tubes one

AF

Remove the local

amplifier

at

"'Y602A;

be be

a time.

if

the

squeal stops, the trouble is originating between this stage and the detector. not stop,

it

is originating between this stage

If

the squeal

does

and the output.

Section IX. TROUBLESHOOTING CALIBRATION-OSCILLATOR SECTION

176. General

The receiver being considered (fig. 65) provides for accurate indication of the frequency to which erate calibrating signals

over the receiver tuning can check and

177. Receiver

If

FUNCTION switch is in the

tion, the normal receiving circuits

it

is tuned. A system is used to gen-

at

100-kc intervals

range;

adjust

Picks

Calibration Signals Are No'l Present

the frequency indicator.

Up External Signals, but

the operator

external signals are received when the

MGC

or

AGC

posi-

are

in operating condition; therefore, calibration signals are not being applied to

88

RF

amplifier V201.

Assuming

that

FUNCTION switch S102 is in position 5 (the CALIBRATE position), the 100-kc multivibrator V206, and 100-kc cathode

follower first. Te

178. Trouble

V205B, stages should be suspected

st

the tubes.

in

V206

Connect an ac vtvm between pin 6 of the 100kc multivibrator and ground. age is not present, check the

If

a sig nal volt-

de

voltages

at

the tube socket. Possible causes for the absence of de

voltages

be

open resistor R224 or R226.

open irr L211 or any one of the

at

the plates (pins 1 and 6) could

In

addition, an

grid

resistors

could cause the stage to be inoperative. An

C314

open coupling capacitor

or C315 would

AGO l0016A

open

the

feed-back circuit an d oscillations

would cease.

179. Trouble

a.

Connect

100-kc cathode follower

in

V205B Stage

an

ac

vtvm between pin 8 of the

and

ground.

If

the required signal voltage is present, the stage is operating;

stage

the at

the present, open L211. circuit, for example the plate

if

there

is no signal voltage present,

is defective. Measure the de voltage

plate,

6.

it

could be due to

An

open component in the cathode

current

from

If

there

L210

flowing

is

no

de

an

open R228 or an

or

R229, would keep

and

stop opera-

voltage

tion.

b.

If

the

stage

is found to be operating, check

the

connections between pin 8 of V205 and pin

1

of

V201. This

part

of

the

circuit

includes coupling capacitor C228, which could be open or have a broken lead.

180. Calibration Signals Present in Output, but Not

a.

heard

Are

The

means

fact

that

Stable

the

calibration signals

the

100-kc cathode follower

are

stage is operating. The multivibrator, however,

is not a stab le circuit

le

a stab

device. The calibration oscillator V205A is a stable crystal oscillator the multivibrator on frequency by the multivibrator fore,

if

the calibration oscillator becomes defec-

tive,

there

will be

tor

and

it

would become unstable.

b.

Measure the R220 with a vtvm. sent, the stage is operating. not present, operat of

V205A (pin

or

ing. Measure

completely, the possible

an

be

open R221 or a shorted C312. bias resistor R220, crysta

l would also cause the circuit to be in-

and

must

be controlled by

at

precise intervals. There-

no

control of the multivibra-

de

voltage across bias

If

the proper voltage is

If

the voltage is

is very low,

the

1).

If

an

the

oscillator is

de voltage

the voltage is lacking

so

urce of trouble can

open C311,

that

triggering

resistor

at

the

An

or

a defective

keeps

pre-

not

plate

open

operative.

c.

There is also a possibility leaky; this would s stop oscillations. but

the calibration signals

hort

If

the oscillator is operating,

the grid to ground

are

that

still

C310 is

not

being

and

controlled, an open coupling capacitor C313

would

prevent

the si

gna

l from reaching

the

multivibrator. ·

Section

181. General

a.

A frequency-modulation receiver is bas-

the

ically like

The audio are

identical

\

1

~

standard

stages

and

RF

in

the

f---

am

the

two types of receivers

only differences between

MIXER

OSCIL-

LA

X.

superheterodyne.

I--

TOR

Fi

TROUBLESHOOTING

th

the stages and procedures described in previous sections covering and

IF

f---

LIMITER

.

TO

ALL

POWER SUPPLY

gu?'e 69.

F1n 1·eceive1·,

block

FM

RF

and

p~ss

band characteristics. Because these

am

receivers

are

not repeated.

1--

STAGES

1

diag1·am.

RECEIVERS

IF

stages

parts

are

applicable to fm receivers

DISCRIM -

INATOR

are

the frequencies

and

similar, troubleshooting

f---t

AUDIO

K

TM4000-58

AGO 10016A

89

b.

The fm receiver has two stages not found

in the am receiver: the limiter and

discriminator stages (fig. 69). Troubles and troubleshooting procedures peculiar to these stages

are

covered in this section.

182.

Signal

a.

The signal substitution method can

Substitution

used in troubleshooting a weak or dead fm receiver.

For

troubleshooting by signal substitution, a sweep signal generator is preferable, because its signal can of the receiver.

If

not available, the same type of in strument

be

heard in the output

such a signal generator is

that

is used for am receivers may be employed, although that erator must produce frequencies

it

is not as convenient as the type

produces a sweep signal. The signal gen-

that

are equal to the tuning range of the receiver and to its intermediate frequency (or frequencies).

b.

When testing a weak receiver, checking

stage gain by listening to the output is not a

dependab

le

procedure. An increase in signal

input level may not result in a corresponding increase in output level because action of the limiter stage. Also, identical put

levels for a signal injected in the plate cir-

cuit and for a weaker signal

not indicate

that

a stage is amplifying. The

of

the limiting

at

the grid

out-

do

limiter may have reduced the signals to the same level. The best way to check the gain of the

RF

and

IF

stages is to measure the

of the signal

that

is applied to the control grid

of the limiter. This can

(l)

Use

a.

vtvm with an

measure the

(2)

If

the limiter stage is

develops

grid

be

done in two ways:

RF

voltage directly.

leak bias, use a vtvm to

probe and

the

type

strength

that

measure

the

de

voltage between trol grid and ground. The voltage increase with strength.

an

increase in signal

An

alternative method this type of limiter is to open the grid circuit and in

sert

a

0-200

meter. The meter will indicate grid

be

current;

an increase in signal strength

will cause an increase in grid current.

c.

Begin signal substitution by feeding unmodulated signal from a signal generator the grid of the limiter stage . Set a vtvm indicate

de

volts,

with

the needle

at

Connect the vtvm across the discriminator load resistors; these the connection points are put

frequency of the signal generator

are

Rl

and R2 in figure

A and

C.

resting frequency of the receiver (the

mediate

frequency);

the vtvm should read

Alternately tune the signal generator above and

If

below the resting frequency.

to one side and then

the

the vtvm swings

other of zero as the frequency is varied, the limiter and the inator are operating.

no

there is

d.

output from

If

the vtvm needle

(At

the

resting

the

discriminator.)

doe

s not follow fre-

quency variations, the trouble is in either the

er

limit

multimeter reversed each

of the discriminator and described in c above.

or the discriminator.

Note.

If

a

vtvm

is

not

available, high-resistance

can be

substituted,

time

the

e.

Connect the signal generator to the input

polarity

but

the test

of

the

voltage changes.

vary its frequency

In

figure

70

the signal applied between point D and ground . voltage indication

on

the

vtvm does not follow

the frequency variations, the discriminator or

coil

Ll

is defective.

con-

will

with

microam-

an

zero center.

70

and

Set the out-

to

the

inter-

zero.

discrim-

frequency

leads

must

be

as

is

If

the

to to

IF

OUTPUT

FROM

LIMITER

90

Figure

r------.--------.----.

70.

Discriminator, showing test points.

Rl

8

R2

A

c

TM4000

AGO 10016A

OUTPUT AUDIO AMPLIFIER

- 59

TO

183.

Receiver Troubleshooting Charts

a.

The

receiver troubleshooting follow will be of qu ickly. The app

lied to

an

idea as to

the

of

any receiver. The technician can

equipment.

assistance

inform

what

ation is gene

the

Ty

pical Capacito1·, Resisto1

charts that

in locating troubles

ral

and can be

trouble is,

regard

when the general been decided upon, use the specific

in the equipment technical manual to pinpoint

get

the defective circuit

less

·,

and Inductor Fai

b. Consult the

lu1·

es

in

a R

chart

loc

eceive?

when troubles arise, and

ation of

the

troubles has

inform

and

component

· Stage

part.

ation

Symptom

Fe

edback or

1.

output

No

output

2. Decreased

motor-boating charge

No

output

output. Hot

resistor.

Weak

Di s

torted

No

output

put

Distort

Severe h

No

output

No

output

No

output

No

output

3.

4.

5.

6.

7. No

8.

• Tes t to be made w1th one end of p

ed

um

s

hum

or

very

output

or

very weak

output

or weak

output

or blocki

or

very

or

very

or

very

or

very weak

or

ve

De vol

tage

meas ur

ements

a. Normal a.

weak

b. Zero or

and a. Normal a.

b.

Zero

screen

ve

ry low

plat

e to ground from the

ve

ry

or

screen to ground

low from

a. Normal

No reading across capac-

b.

itor

a. Normal

out

-

High reading from grid

b.

ound.

at

part

Positive

grid

cir

cuit .

ng

weak

weak a.

weak

ry weak

gr

to pola

rity

No reading across

a.

ponent

b.

reading across

No

pone

nt

reading from plate

No

to ground

b.

High reading from

to ground

High reading across resistor

No reading from screen

to

ground

art deLa

ched !rom

from

corn-

plate

grid

Resi

stance

measurements. Defec ti ve

Capacitor

charge *

Partial or

b.

ground

Capacitor

Partia

b.

a.

b.

a.

b.

l or

from screen to ground

Capacitor

ch

arge

Partia

l or

from cathode to ground

Capacitor

charge *

Low read i

pling capacitor

a. Open

from or to bias line

Partial

b.

from

to bias line

No reading from

a.

B+

Partial

b.

from pl

Op

en circuit reading across

terminals of resistor •

circuit

Open

scr

een grid

direct

B+

'-'

direct

*

direct

ng

circuit

grid

direct

or

gri

d to ground

line

or direct

ate

reading

to

do

es

does

across cou-

reading

to

B+

not

short

line to

not

sh'ort

not

short

not

ground

short

plate

short

line

B+

from

line

pa

r~

Pl

ate

bypass capacitor

a. Open

Short

b.

Scr

a.

b. Shorted

Cathode capacitor

ed

ee

n bypa ss capacitor

Op

en

a. Open

Shorted

b.

Coupling ca

a.

·Open

Shorted

b.

Series grid resi stor,

secondary winding.

Op

a.

Shorted

b.

or

Series plate resistor, coil, or

prima

a.

to

b.

Cathode resis Op

en

Screen-grid

tor

Op

en

pacitor

en

ry

winding

Open

Shorted coil

tor

dropping

co

I

or

il

resis-

ACO 10016A

91

CHAPTER

7

TROUBLESHOOTING

Section

184. General

All the general principles of troubleshooting

( ch. 4) apply to the troubleshooting of am trans-

mitters.

In

the case of a large transmitter, the many built-in meters are of particular aid to the troubleshooter. Defects can often alized and localized without the use of additional test

equipment.

185.

Key

Test

Points

a.

All transmitters have key

are

places where, for testing or troubleshooting

purposes, currents and voltages are measured

or

signals are injected. When the defective

I.

GENERAL

test

points. These

TRANSMITTER

be

section-

AM

TRANSMITTERS

TROUBLESHOOTING

stage

is

found,

B+

voltages

are

measured to aid

in isolating the trouble. In the simple trans-

71,

mitter represented in figure points

are:

the key test

(1) Oscillator grid circuit.

(2) Oscillator plate milliammeter. (3) Buffer plate milliammeter. (4) Power amplifier plate milliammeter. (5) Antenna ammeter.

(6) Modulator plate milliammeter.

(7) Driver output. (8) Driver input. (9) Speech amplifier input.

b.

Test

No.

7 in figure

71

requires the injec-

tion of an audio signal. Usually, this is practi-

9

OSC

ILLATOR

SPEECH

AMPL IFIER

2

1----1--+t

8

B+

Figt,re

3

BUFFER-

DOUBLER

DRIVER

TO

AL:L STAGES

l

POWER

SUPPLY

71.

Simple transmitt er, block diagram.

POWER

AMPLIFIER

7

MODULATOR

I.

MEASURE OSCILLATOR BIAS.

2. MEASURE OSCILLATOR PLATE CURRENT.

3. MEASURE BUFFER PLATE CURRENT.

4. MEAS!JRE

5.

MEASURE ANTENNA CURRENT.

6.

M~ASURE

7. INJECT

8. INJECT AUDIO SIGNAL

9.

INJECT

6

POWER

MODULATOR

AUDIO

AUDIO SIGNAL.

4

TRANSMITTER

AMPLIFIER PLATE

SIGNAL.

TESTS:

PLATE

5

CURRENT

TM4000-60

CURRENT.

.

92

AGO 10016A

Military surplus Troubleshooting_and_repair_of_radio_equipment_TM11-4000_1958.pdf troubleshooting guide

Specifications and Main Features

Frequently Asked Questions

User Manual