Tandy CM-4 Service Manual

TANDY

8

Service

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COLOR

MONITOR

CM-4

Catalog

Number

:

25-1021

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CONTENTS

Page

SPECIFICATIONS

3

IMPORTANT

SERVICE SAFETY

PRECAUTIONS 4

THEORY

OF

OPERATION 6

DISASSEMBLY INSTRUCTIONS

12

BLOCK

DIAGRAM 13

ALIGNMENT

INSTRUCTIONS

14

TROUBLESHOOTING

GUIDE

18

P.C. BOARD (Top and Bottom Views)

CRT Socket PCB

22

LED PCB

23

Main

PCB

23

WIRING

DIAGRAM

AND

PARTS

LOCATION

27

CABINET EXPLODED

VIEW/PARTS

LIST

1.

Cabinet

Exploded View

28

2.

Electrical

Parts List

29

3. Cabinet

Parts

List

39

SCHEMATIC

DIAGRAM

41

WAVEFORMS

44

SEMICONDUCTOR

LEAD

IDENTIFICATION

44

SPECIFICATIONS

Description

Nominal

Limit

1. Power input

AC 120V, 60Hz

2. AC input current 0.75A

+

10%

-30%

3. Input signal

a)

RGB Video RGB

separate

T.T.L.

Level, positive white

2.4

-

5.0Vp-p

~^"~

(default)

b)

Synchronous

"

"

T.T.L.

level,

positive going

(default)

c)

Intensity

T.T.L.

level, positive

going

(default)

4.

Resolution

a)

Horizontal

640 dots

b) Vertical

(non-interlaced)

225 lines

6.

Brightness

30

fl.

min.

(at

intensity

white)

8.

Display color

1

5

colors

9. High

voltage

23 kV/O^A

27.5 kV

maxJOfJK

10. Picture

linearity

a) Horizontal

10%

max.

b) Vertical

10%

max.

11.

Synchronous (Pull in

range)

a)

Horizontal

15.701 kHz

:i>

b)

Vertical

60 Hz

>

12.

Dot

Pitch

0.64 mm

NOTE:

Nominal

specs represent the

design

specs;

all units

should be

able

to

approximate these

—

some

will exceed

and

some

may drop

slightly

below these

specs. Limit

specs

represent the

absolute

worst condition

which still

might

be considered

acceptable; in

no case

should

a

unit

perform

to

less than within any

limit spec.

IMPORTANT

SERVICE

SAFETY PRECAUTIONS

Service

work should

be

performed

only

by

qualified

service technicians

who

are

thoroughly

familiar

with

all

of

the

following safety checks and

servicing

guidelines:

WARNING

1. For

continued

safety,

do

not

attempt

to

modify

the

circuit.

2.

Disconnect the

AC

power

before servicing.

3. Semiconductor

heat

sinks are

potential

shock haz-

ards when

the

chassis

is operating.

SERVICING

THE

HIGH

VOLTAGE

SYSTEM

AND PICTURE TUBE

When

servicing the high

voltage

system,

remove

the

static

charge by connecting a

10k

ohm

resistor

in

series

with

an insulated

wire

(such

as

a test probe)

between the chassis

and the

anode

lead.

(The

AC

line

cord should

be disconnected from

the

AC

outlet.)

1. The picture

tube

in this

display

monitor

employs

in-

tegral implosion

protection.

2. Replace with a

tube

of

the same

type

number

for

continued

safety.

3.

Do

not

lift

the

picture

tube

by the

neck.

4. Handle

the

picture

tube

only

when

wearing

shatter-

proof goggles

and after

discharging

the

high vol-

tage

anode completely.

X-RADIATION

AND

HIGH

VOLTAGE

LIMITS

1.

Be

sure

all

service

personnel are

aware

of the

procedures

and

instructions

covering

X-radiation.

The only

potential

source

of X-ray

in

a

current

solid-

state

display

monitor

is the picture

tube.

However,

the picture tube

does

not

emit

measurable

X-ray

radiation

if the high

voltage

is as

specified

in the

"high-voltage

check"

instructions.

It

is only when high

voltage

is

excessive

that

X-

radiation

is capable of

penetrating

the

shell

of

the

picture

tube, including

the

lead in

glass

material.

The

important

precaution

is

to keep

the

high vol-

tage below

the

maximum

level

specified.

2. It

is essential

that servicemen

have

available

at all

times

an

accurate

high

voltage

meter. The

calibra-

tion

of

this meter

should

be

checked

periodically.

3.

High

voltage

should

always

be kept

at the

rated

value

—

no higher.

Operation

at

higher

voltages

may cause

a failure of

the picture

tube or

high vol-

tage

circuitry

and, also,

under

certain conditions,

may

produce radiation

in excess of

desirable

levels.

4. When the high

voltage

regulator

is

operating proper-

ly

there

is

no

possibility

of

an X-radiation

problem.

Every

time

a

color

chassis

is serviced, the

bright-

ness

should

be

tested

while

monitoring the

high vol-

tage

with

a

meter

to be

certain

that

the high

voltage

does

not

exceed

the specified

value and

that it is

regulating

correctly.

5.

Do

not

use

a

picture

tube other

than that

specified

or make unrecommended

circuit

modifications to

the high

voltage

circuitry.

6.

When

troubleshooting

and taking test measure-

ments on

a

display monitor

with

excessive high vol-

tage, avoid

being

unnecessarily

close

to the

display

monitor. Do

not

operate

the display

monitor longer

than

is necessary

to locate

the

cause

of

excessive

voltage.

BEFORE RETURNING

THE DISPLAY MONITOR

Fire and

Shock

Hazard

Before

returning

the display

monitor

to the

user,

per-

form the

following

safety checks:

1.

Inspect

all

lead dress

to make

certain that

the

leads

are

not

pinched

or that

hardware

is

not

lodged be-

tween the

chassis and

other metal

parts

in the dis-

play

monitor.

2. Inspect

all

protective

devices

such as nonmetallic

control

knobs,

insulating

materials,

cabinet backs,

adjustment

and

compartment

covers

or

shield,

iso-

lation

resistor-capacitor

networks,

mechanical in-

sulators,

etc.

3. To be sure

that

no

shock hazard

exists,

check for

leakage

current in

the following

manner:

•

Plug

the

AC

line

cord

directly

into an 120 volt

AC

outlet. (Do

not

use

an isolation

transformer

for

this

test.)

•

Using two

clip

leads, connect

a 1.5k ohm,

10 watt

resistor paralleled

by a 0.15/iF

capacitor

in

series

with all exposed

metal

cabinet

parts

and a

known

earth

ground,

such

as

electrical

conduit

or electri-

cal ground

connected

to

earth

ground.

•

Use

a

SSVM or

VOM

with

1000 ohms

per-volt

or

higher

sensitivity

to

measure the

AC

voltage drop

across

the resistor.

(See

Figure

1.)

•

Connect

the

resistor connection

to

all

exposed

metal

parts

having

a return path to

the

chassis

(metal

cabinet,

screw

heads,

knobs

and

control

shafts,

escutcheon, etc.) and

measure

the

AC

vol-

tage

drop across

the resistor.

All

checks must

be repeated

with

the

AC line cord

plug

connection reversed.

(If

necessary, a non-

polarized adapter

plug

must

be

used

only for

the pur-

pose

of

completing

these

checks.)

Any

reading

of 0.3 volt

RMS

(this

corresponds

to

0.2

milliamp.

AC.) or

more

is

excessive

and indi-

cates a

potential shock

hazard which must

be cor-

rected

before

returning

the display

monitor to

the

user.

SAFETY

NOTICE

Many

electrical and

mechanical

parts in display

moni-

tors

have special safety-related characteristics.

These

characteristics often

pass

unnoticed and

the protec-

tion

afforded by them cannot necessarily

be

obtained

by

using

replacement

components rated for higher vol-

tage,

wattage,

etc.

Replacement

parts

that

have these

special

safety

characteristics are identified in

this manual; electrical

components

having

such

features

are

identified by a

A

and shaded

in

the Replacement Parts Lists and

Schematic

Diagram. For

continued protection, replace-

ment

parts must

be identicaltothose

used

in the

origi-

nal

circuit. The use

of

a

substitute replacement part

that

does not

have

the same safety characteristics

as

specified

in this service

manual, may create shock,

fire,

X-radiation or other hazards.

TO

EXPOSED

METAL PARTS

CONNECT

TO

KNOWN

EARTH

GROUND

Figure

1. Leakage Current

Test

Circuit

THEORY

OF

OPERATION

1.

RGB

Drive

Circuit

The

RGBI

signals

are applied to

HEX inverter

IC491

used

as

a

buffer). The

outputs of

IC491 are then

ap-

alied

to

IC492

which

is

an

open

collector HEX invert-

er.

The

resistors

tied

to

the

outputs of IC492 are

used

:o

set

the

appropriate

voltage

level

for the color

sig-

nals.

The

signals

are

then applied to

the base

of RGB-

Amp

transistors

Q451,

Q461 and Q471.

The

brightness

and

sub

brightness controls

(R41

7

and

341

6)

are used

to

adjust the

bias of

the

RGB-Amp

tran-

sistors.

These

controls are

connected

to the

base of

Q401

and

Q402.

When the intensity

signal

is

LOW,

transistor

Q421

is

turned

ON.

The

contrast

control

(R422) is used

to

adjust

the con-

trast

between

high

and low intensity color

signals.

2. Video

(RGB) Output {Fig.

2)

An RGB

drive system

is

utilized

in

the

video output

cir-

cuit of

this

unit.

The

function

of

this

circuit

is to

com-

oine

the

color signals and the brightness

signal,

and

amplify them

sufficiently

to

drive

the

cathodes. 1

45V

DC

must

be applied

to

the collectors

of the

output tran-

sistors

(Q851, Q861,

Q871).

When

the horizontal

output circuit is

operating, pulses are

developed

and

fedtothe

1

16V supply

where they are applied

to

a

winding

of the

horizontal output transformer (T602).

This pulsed

DC

voltage

is

then

taken

from terminal (s)

of

T602 and applied

through

D71

7,

and

R865

or

R866

or

R867 to

the

collectors of

Q851,

Q861

andQ871,

respectively.

The brightness

signal

from the

Blanking

(Q402)

is

ap-

plied

to

the emitters of

Q851,

Q861 and

Q871.

C853

and C855

are

peaking capacitors.

Color signals from

the

outputs

of

Q451,

Q461 and

Q471

are

applied

to

the bases of

Q851,

Q861 and

Q871 . The picture

tube

used

in

this

unit

is

a

precision,

inline

gun-type.

The control grid

(G1)

and the

screen

grid

(G2)

are common

with

respect to the red, green

and blue cathodes. Consequently, the

emitter circuits

of Q85

1

, Q86 1

and

Q87

1

are

provided

with bias con-

trols

(R862, R863 and

R864,

respectively)

for picture

tube

cut-off

adjustment. Drive

controls

(R856

and

R858) are provided

in

the emitter circuits

of Q851 and

Q871 for white balance

adjustment.

FROM

EMITTER

>

OF Q451

Q851

RED

OUTPUT

FROM

EMITTER

>

OFQ461

FROM

EMITTER

>

OFQ471

FROM

VIDEO

DRIVE

>

CIRCUIT

<

+145

Figure

2.

Video

Output

Circuit

3,

Vertical

Deflection

Circuit

The

vertical

sync,

signal

with

positive polarity

is ap-

plied

to

pin(7)of

the vertical

and

horizontal

IC

(IC601).

Pin

(8)of

IC601

is connected

to

the vertical oscillator

circuit.

The

frequency of the oscillator can

be

con-

trolled

by the

voltage

of pin

(8)

which

can be varied

by

V.HOLD

VR

(R514).

The

sawtooth signal is

ob-

tained by

the integrating

circuit which

is connected be-

tween

pin

(5)

and pin

Qj)

.

The

oscillator output

is

fed

to the vertical

drive circuit

through a

buffer

circuit.

Its

output, derived

from pin

(2)

,

is

applied to

the

vertical output.

The

sawtooth

wave

is

applied

to

pin

(3)

of

IC601 as

an AC

feedback

signal.

The

emitter circuit of Q501 is controlled by

V-SIZE VR

(R507)

to vary the vertical size of the raster.

The

vertical

linearity

control

(R526) is part of

an

in-

tegrating circuit

which controls the sawtooth

waveform.

4.

Horizontal

Oscillator, AFC

and

Drive Circuit

The

horizontal

sync,

signal

with

positive

polarity is ap-

plied to

pin

(15)

of IC601.

The output

from the fly-back

transformer (T602)

is

in-

tegrated

and

connected

to pin

(^3)

of

IC601 as part

of

the

automatic frequency control circuit.

H.

CENT

control

(R623)

determines

the relative posi-

tion

of

the raster and

picture.

The

horizontal

oscillation

frequency can

be controlled

by H.

HOLD VR

R607

connected

to

pin

(j~2).

The

horizontal frequency is

obtained from pin

\[Q)

of

IC601,

and is

fed

to

the

next

horizontal

drive circuit.

The

pulse-switching

mode of the

driver and

output

stage is a

reverse

polarity

type;

that

is,

when

the

driver

transistor

Q601 is

ON,

the

output

transistor

Q602

is

OFF.

5.

Horizontal Output

and

HV

Rectifier

(Figs.

3-5)

Horizontal drive voltage,

developed at

pin

^6)

of

the

deflection

processor integrated

circuit

(IC601

),

is

am-

plified

through

the horizontal drive

stage

(Q601)

and

coupled to the base

of

the horizontal output

circuit

via

the horizontal drive transformer

(T601 ).

Refer

to

Fig. 3.

The horizontal

output circuit generates the

horizontal

scan and

high voltage

to

be applied to

the picture tube.

The function

of the

horizontal output

stage

(Q602)

is

to

serve

as a

switch for the horizontal

output

circuit.

Refer

to

Fig. 4.

During

the horizontal

scanning

period, Q

operates

(S

1

is

closed,

S2 is open) and the current

is

applied

in

one

direction through the

horizontal coils

of the

deflection

yoke

(LY)

and the

capacitor

(C).

During retrace

time,

Q is inoperative (S 1

is

open, S2 is

closed)

and

the

cur-

rent is applied in the opposite

direction

through

the

damper

diode

(D),

the horizontal

coils of

the

deflection

yoke (LY)

and

the capacitor

(C).

The

high

voltage

required

to

be

applied

to

the

anode

of the picture tube is generated by

boosting

the

pulse

from the collector of Q602

through

T602

during

the

flyback

(retrace) period and applying

this

boosted

pulse

to

a

series of

silicon

rectifiers. Refer

to

Figure 5.

High voltage regulation is accomplished

internally

in

T602.

T601

HORIZ. DRIVE

From

Pin (TS)

^

of

IC601

^

C609

T

I

To

BASE

of

Q602

Q601

HORIZ.

DRIVE

^+12V

Figure

3.

Horizontal

Drive Circuit

Q(S1)

~©

1

DAMPER

DIODE

(S2)T

C

r'COIL)

LY

(DEFLECTION

VCC

(A) (B)

Figure

4. Equivalent

Circuit

of Horizontal

Output

Circuit

I

—

'

'

—

IR71

18V

«

•

*

H

*

>W

C72izt

°714

R717

^T

TO

CRT

SECOND

ANODE

**

TO

CRT

FOCUS

*-

TO

CRT

SCREEN

**

TO ABL

CIRCUIT

145

V

To

Blanking

and

orizontal

AFC

DET.

Circuit

*•

TO CRT Heater

(THIS

AREA

IS

COLD

GROUND)

D713

AAV-

p\

—

f

18V

Figure

5.

Horizontal

Output and

HV

Rectifier

Circuit

6.

High

Voltage Shut-down System

The

shut-down

circuit

prevents

the

high

voltage from

rising above a

preset

level.

Under normal

operating

conditions,

this circuit is

in-

active.

Operation

of

the

protector

circuit depends

upon

a heat-

er

pulse

which

appears at

pin

{3)

of

the horizontal out-

put

transformer

(T602).

It monitors

a

heater pulse

subjected

to

rectification by D603.

Being in

proportion

to

the

voltage of

that heater pulse,

if

the

incoming

high voltage

increases and exceeds its

limit,

the heat-

er pulse

voltage

also

increases. As

a

result,

there is

a

larger voltage

produced to

charge

C617

so

that

its

potential

will eventually

be higher

than

the

voltage

(

+ 22V) of

Zener

diode

(D605)

turning

it

ON.

With

D605

turned

ON,

the

X-ray

protector

(of

IC601

)

oper-

ates

to

stop

the

horizontal

oscillator circuit,

shutting

down the resultant

high

voltage.

FROM

PIN©

OF

T602

/ HEATER

\

\

pulse

;

>

R615

D603

-w

M—

D605

R618

—

^

VA-

R619?

±ZZC617

R6175

-^

77T

Figure

6. High

Voltage

Shut-Down

System

Circuit

R616

^

-AW-(V)

C607

7.

Power

Supply (Figs.

7

—

10)

The

entire

monitor

circuitry

is

protected

by

a

4.0A

fuse

(F701),

located

on the Main Chassis

Board (PCB-A)

that

is connected

to

the

hot side of

the

AC

line

input.

The

secondary

circuit

is

protected

by

a

1.5A

fuse

(F702),

mounted on

PCB-A.

AC

Line voltage

is applied

through

the Line

filter (L702)

to

the

power

ON-OFF

SWITCH

(SW701).

With SW701 set

in the ON position, AC

voltage

is ap-

plied

through

a

Bridge

Rectifier

circuit.

A

conventional type

Automatic Degaussing

circuit,

consisting

of

a

positive coefficient thermistor (PR701

)

in

series

with

the

degaussing coil assembly

(L701

),

is

used.

The

AC

input voltage

is rectified

by

the

Bridge

Rectifi-

er

circuit

(D701

,

D702,

D703,

D704) and then

applied

to the

Regulator

circuit.

A

switching

type

Regulator

cir-

cuit,

utilizing

a

silicon

controlled rectifier

(SCR701),

is

used to

maintain

a constant DC

voltage level

regard-

less

of

fluctuations \n the AC

input

voltage.

The negative

horizontal

pulses

that

are

produced

at

ter-

minal

@of

the

Horizontal Output

transformer

(T602)

are converted

into

a saw

tooth wave by

R713

and

C71 1 which

is

applied

to

the Phase

Detector

circuit

in

the

IC701 (Power Regulator

integrated

circuit)

via

pin

(5)

.

Output from the

Phase

Detector

circuit is

ap-

plied

internally

to

the

SCR

Drive stage

where

it is

am-

plified and

then

utilized as

the

SCR

gate

(timing)

pulse.

Gate

pulse timing is

determined by the

action

of the

Error Amplifier

stage

in

IC701

which is

controlled by

the setting of

the

+116V Adjust

control

(R707).

Filtered DC voltage

derived from the

pulses

produced

at terminals

@

and

@

of

T602

is

used to

turn

OFF

SCR701. If

the AC

input

voltage increases,

the

turn-

on time

of

SCR

701 decreases.

And

if the

AC

input

voltage decreases;

the turn-on

time

of SCR701

in-

creases.

The

+

5.

1

V,

+

1 2V

and

+

1

8V

supplies

are

produced

as

a

result

of rectification

through

D71

3

of the

pulses

developed

at

terminal @of

T602.

R631

D717

F701

L702

SW701

D701

~

D704

£

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FILTER

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AC

120V

60Hz

F701

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AC120V

w

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60Hz

BRIDGE

RECTIFIER

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IC701

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OUTPUT

XFMR.

REGULATOR

CIRCUIT

D714

T701

Power

Trans

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Recti-

fier

D708

Q701

R716

D713

-+12V

Regu-

lator

R401

+116V

+18V

+12V

+5.1 V

R481

Figure

7.

Block

Diagram

of

Power

Supply

PR701

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I

L701

ADG

O

C

33

33

m

Figure

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ADG

Circuit

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REFERENCE

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r

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Figure

9.

Block

Diagram

of

Regulator

Circuit

10

AC120V

60Hz

^C716

/*

+

-

C714

+145V

D717

Figure

10.

Power

Supply

Schematic

DISASSEMBLY

INSTRUCTIONS

Remove

the six

screws

(T) retaining the

rear

cabinet.

Remove

the rear cabinet.

(Figure 1 1A)

Note:

The CRT

must

be

discharged. Refer to

the

high

voltage discharge

procedure

on

page

4.

(1)

Remove the

CRT's

second

anode

cap(2)from

the CRT.

(2)

Remove

the

PCB--B

(CRT

PCS)

lead of

the CRT

grounding

strap.

(Figure

11B)

(3)

Remove the

PCB-B.

(4)

Loosen

the

wire

holder

on

the Flyback

trans-

bracket

and disconnect the connector K.

(Figure 11B)

(5)

Disconnect

the

connectors NA and M on the

PCB-A

(main

PCB-A)

(6)

Loosen

the

wire

holder

fixing

power switch

lead,

CRT

ground lead, RGB

output lead,

Degaussing lead

and

LED

lead. (Figure

1 1B)

(7)

Remove

the

PCB-A

(main PCB)

from the

front

cabinet.

3.

Remove

screw

(5)

for PCB-C the LED and

also

PCB-C

from the

front cabinet.

(Figure 1 1B)

Note: When

servicing,

be

sufficiently careful with

the

control door

because

it

may detach

from

the

cabinet if

it

touches

the

surface

while

the

set

is

inclined toward the front.

Figure

11A

Figure

11B

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