Marconi TF 1313A Service manual

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Page 1

MANUAL INSTRUCTION

Page 2

NOTES AND CAUTIONS

ELECTRICAL SAFETY PRECAUTIONS

This equipment is protected in accordance with IEC Safety Class 1. It has been

designed and tested according to IEC Publication 348, 'Safety Requirements for Electronic

Measuring Apparatus', and has been supplied in a safe condition. The following pre-

cautions must be observed by the user to ensure safe operation and to retain the equip-

ment in a safe condition.

Defects and abnormal stresses

Whenever it is likely that protection has been impaired, for example as a result of

damage caused by severe conditions of transport or storage, the equipment shall be made

inoperative and be secured against any unintended operation.

Removal of covers

Removal of the covers is likely to expose live parts although reasonable precautions

have been taken in the design of the equipment to shield such parts. The equipment

shall be disconnected from the supply before carrying out any adjustment, replacement or maintenance and repair during which the equipment shall be opened. If any adjustment, maintenance or repair under voltage is inevitable it shall only be carried out by a

skilled person who is aware of the hazard involved.

Note that capacitors inside the equipment may still be charged when the equipment

has been disconnected from the supply. Before carrying out any work inside the equip-

ment, capacitors connected to high voltage points should be discharged; to discharge mains filter capacitors, if fitted, short together the L (live) and N (neutral) pins of

the mains plug.

Mains plug

The mains plug shall only be inserted in a socket outlet provided with a protective earth contacts The protective action shall not be negated by the use of an extension lead without protective conductor. Any interruption of the protective conductor inside or outside the equipment is likely to make the equipment dangerous.

Fuses

Note that there is a supply fuse in both the live and neutral wires of the supply

lead. If only one of these fuses should rupture, certain parts of the equipment could remain at supply potential.

To provide protection against breakdown of the supply lead, its connectors, and

filter where fitted, an external supply fuse (e.g. fitted in the connecting plug) should

be used in the live lead. The fuse should have a continuous rating not exceeding 6 A.

Make sure that only fuses with the required rated current and of the specified type are used for replacement. The use of mended fuses and the short-circuiting of fuse holders shall be avoided.

RADIO FREQUENCY INTERFERENCE

This equipment conforms with the requirements of lEC Directive 76/889 as to

limits of r.f. interference.

(2a)

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Section 1 GENERAL INFORMATION

Contents

1.1

1.2

1.3

Section 2 OPERATION

2.8

2.9

2.10

2.11

Section 3 TECHNICAL DESCRIPTION

3.1 Circuit summary

3.2 Measuring bridge a. c. supply

3.3 Measuring bridge d. c. supply

3.4 Measuring bridge circuits

3.5 Amplifier detector

3.6 Power supplies

3.7 D. C. Choke Adaptor Type TM 6113

Introduction Data summary Accessories

Installation Controls and connectors Preliminaries

Connecting the component Resistance measurement Inductance measurement Notes on inductlve loss balancing Using the D. C. Choke Adaptor Type TM Capacitance measurement Notes on capacitive loss balancing Use of external oscillator and detector

6113

9 10 10 12 16 17 18 21 21

23 24 24 24 25 26 26

Section 4 MAINTENANCE

4.1 General

4. 2 Removal from case

4.3 Access to components

4.4 Mains input adjustments

4.5 Working voltages

4.6 Replacement of valves

4. 7 Adjustment of preset components

4.8 Checking the loss balance diai calibration

4. 9 Access and adjustment to balance control assembly

Section 5 REPLACEABLE PARTS

Universal bridge D. C. choke adaptor

Section 6 CIRCUIT DIAGRAMS

Fig. 6.1 Power supply and oscillator Fig. 6.2 Bridge Fig. 6.3 Detector Fig. 6.4 D. C. choke adaptor

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41 53

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General information

1.1 INTRODUCTION

TF 1313A is a general-purpose impedance bridge with one tenth per cent measurement accuracy over a wide range of inductance, capacitance, and resistance values. It offers exceptional discrimination and resettability, wide-range loss balancing, and facilities for using an external oscillator and detector.

The single direct-reading dial used for L, C, and R measurements has coarse aiid fine concentric controls; the coarse one moves in 1%

steps and the fine gives continuously variable interpolation between steps. The variable and non-linear sensitivity of the detector facilitates easy measurement of completely unknown comp-

onents. An external audio oscillator can be plugged in where L and C measurements are required at frequencies other than the internal 1 and

10kc/s; the detector output is available externally to allow an oscilloscope or headphones to be used for balance indication.

This bridge can be used for precision evaluation of resistance, capacitance and inductance, for measuring the dissipation factor of capacitors and Q of inductors, and for quickly identifying unknown components. Measurements on high-loss components are made easier by the low D and Q ranges of the loss balance control and the relative independence between the adjustment of the main

and loss balance controls. Its high discrimination and resettability make it particularly suitable for comparative measurements such as checking the difference between an unknown and a standard

component. Measurements on inductors carrying d. c. can be made by using Adaptor TypeTM6113, which is available as an optional accessory, while an external d. c. supply can be connected direct for polarizing capacitors.

Fig. 1.1 0-1% Universal bridge TF 1313A

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1.2 DATA SUMMARY

Resistance measurement

General information

Range

Accuracy :

Residual resistance

Inductance measurement

Rangé :

Accuracy :

3 mΩ to 110 MΩ. in eight ranges with maxima of 11Ω, 110Ω,

1.1 kΩ, 11 kΩ, 110 kΩ, 1.1 MΩ, 11 MΩ, 110 MΩ.

Basic measurement error :

±0.1% of reading, or ±0. 015% of rangé maximum, whichever

is greater.

Rangé errors :

110 Ω to 1.1 MΩ ranges inclusive  basic errors only.

11 Ω and 11 MΩ ranges  basic errors, and additional ±0.1% of reading. 11 MΩ X 10 rangé - basic errors, and additional^±0.15% of

reading.

Less than 0.003 Ω.

0.1 µH to 110 H in seven ranges, with maxima of 110 μH,

1.1 mH, 11 mH, 110 mH, 1.1 H, 11 H, 110 H.

Basic measurement error at 1 kc/s :

±0.1% of reading, or ±0. 015% of rangé maximum whichever is

greater.

Residual inductance :

Capacitance measurement

Range :

Accuracy : (when D is not greater than 0.031)

Basic measurement error at 10 kc/s :

±0. 2% of reading, or ±0. 025% of range maximum, whichever is greater.

Range errors :

1.1 mH to 11 H ranges inclusive - basic errors only. 110 ΜH and 110 H ranges  basic errors, and additional ±0.1%

of reading.

Additional errors at Low Q :

± (0.1 xf/Q)%, where f is in kc/s.

Typically 0. 05 ΜH.

0.1 pF to 110 µF in seven ranges, with maxima of 110 pF, 1100 pF,

0.011 µF, 0.11 ΜF, 1.1 ΜF, 11 ΜF, 110 ΜF.

Basic measurement error at 1 kc/s :

±0.1% of reading, or ±0. 015% of range maximum, whichever

is greater. (When D is greater than 0. 031 additional error

typically

±0.3D2%).

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General information

Accuracy :

(when D is not greater

than 0.031)

- continued 

Residual capacitance

Temperature range

Temperature coefficient

Q and D measurement

Range :

Basic measurement error at 10 kc/s :

±0. 2% of reading, or ±0. 025% of range maximum, whichever

is greater. (When D is greater than 0.031 and less than 0. 31, additional

error is typically

±6D2%.)

Range errors ;

1100 pF to 11 ΜF ranges inclusive  basic errors only. 110 pF and 110 µF ranges  basic errors, and additional ±0.1%

of reading.

Less than 0. 05 pF.

18 C to 28 C for the stated accuracies.

Additional error of ±0.01% per degree C, for temperatures between 10°C and 18°C, and 28°C and 35°C.

Low Q range : Normal Q range Normal D range High D range :

1 kc/s

0 to 3

0.5 to 31

0.0005 to 0.031 0,005 to 3

10 kc/s 0 to 30 5 to 310

0.005 to 0.031 (limited) Not required

Accuracy :

Bridge sources and detector

Internal sources :

External oscillators :

Additional L and C errors Typically :

Normal Q : ±5% of reading, ±0. 5% of full scale. Normal D : ±5% of reading. Low Q and High D : ±10% of reading, ±3% of full scale.

Additional D or 1/Q error at 1 kc/s and below :

less than ±0. 0005 with correction supplied (on top of instrument), or less than ±0. 0015 without correction. (Above 1 kc/s multiply by f kc/s.)

1 kc/s and 10 kc/s oscillators for L and C measurement, accuracy ±2. 5%; output level, depending on loading, up to 750 mV.

D. C. supply for R measurement; less than, 100 mW component loading.

Frequency range: 20 c/s to 30 kc/s. Input level required : 3 to 20 V depending on frequency. (An external tuned detector is also necessary to achieve the quoted

measurement accuracies.)

Frequency 20 c/s 100 c/s 20 kc/s 30 kc/s

% error ±.05 ±.03 ±0.2 ±0.35

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Test terminal voltage at balance:

General information

CAPACITOR BIAS :

Up to 350 V d. c. may be applied for polarizing electrolytic

capacitors.

POWER SUPPLY

A. C. mains :

100 to 130 V and 200 to 250 V, 45 to 180, 275 to 300, 366 to 400 c/s.

25 VA.

DIMENSIONS & WEIGHT

1.3 ACCESSORIES Supplied

Three telephone plugs, type P40, for external oscillator and detector and bias jacks.

Available

D. C. Choke Adaptor, type TM 6113 ; enables d. c.

currents up to 200 mA from an external supply

to be passed through inductors under test at

1 kc/s in the range 100 mH to 100 H; fitted with test leads for attaching to bridge terminals. Errors introduced by the adaptor do not generally exceed 3% and may be eliminated by simple substitution methods.

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Operation

2.1 INSTALLATION

The 0.1% Universal Bridge is normally dispatched with its valves in position and with its mains input circuit adjusted ready for immediate use on 240 V within the basic supply frequency range 45 to 400 c/s. Note that harmonics of supply frequencies of 200, 250 and 333 c/s may give rise to interference which will obscure the

balance point.

If required, the instrument may be adjusted

for operation from other supplies wlthin the ran-

ges 100 to 130 and 200 to 250 V. To check, or alter the tappings on the mains transformer, refer to Section 4.4.

2.2 CONTROLS AND CONNECTIONS

(T) TEST TERMINALS. Connect the component

between the Hl and DET terminals.

(2) COMPONENT PLATFORM and ABRIDGED

OPERATING INSTRUCTIONS. Supports components, isolating them from chassis.

Fig. 2.1 Controls and connectors

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Operation

L C R switch.

surement. RANGE switch. Adjust to suit component.

If approximate value unknown, set BALANCE controls to half scale, and then select range

giving lowest meter reading.

BALANCE METER.

minimum reading.

SENSITIVITY control. Adjust initially to give half scale meter reading. As balance is approached increase sensitivity to give required discrimination.

COARSE BALANCE control. Set to half

scale while searching for correct range, and then adjust for minimum meter reading.

FINE BALANCE control. . Adjust for

minimum meter reading once minimum for COARSE BALANCE control has been foimd. Note that when the instrument has been idle for some time the protective oil film may, before dispersion, cause erratic balance, A number of brisk rotations will restore the contact surface.

Adjust for appropriate mea-

Adjust controls for

EXT D-Q terminals. Linked for measure-

ments at 1 kc/s to 10 kc/s; connect to external potentiometer, to give continuous coverage when using an external oscillator at frequencies below 1 kc/s.

BIAS jack. Connect external d. c. supply of up to 350 V for polarizing capacitors.

SUPPLY switch.

PILOT LAMP.

2.3 PRELIMINARIES

Having checked that the instrument is adjusted for the supply voltage it will use, proceed as follows :-

1. If necessary, mechanically zero the meter.

2. Connect the mains lead to the power supply.

3. Set the SUPPLY switch to ON; the pilot light should now glow.

LOSS BALANCE control. Adjust to obtain final balance, (minimum meter reading) in conjunction with FINE BALANCE control.

FINE D-Q control. Adjust for final balance

only when LOSS BALANCE control is too coarse.

1 kc/s - 10 kc/s switch. Select 1 kc/s except for low value inductors. LOSS BALANCE dial readings multiplied by 10 when this switch is at 10 kc/s. Inopera.tive for resistance measurements.

D-Q switch. Select range appropriate to component under test. Inoperative for resistance measurements.

EXT OSC. Connect an external oscillator with frequencies from 20 c/s to 35 kc/s.

EXT DET socket. Connect an external detector to monitor output, when using an external oscillator.

4. Before using the instrument, allow a short warm-up period. Two or three minutes is normally sufficient for most purposes.

If you are using this Bridge for the first

time, it will be helpful at this stage to get to know the recommended technique of reading the main dial, before making a measurement.

The coarse control moves the pointer of the outer dial in steps equal to one dial division; the fine control for the inner dial is continuously

variable, and the range 0 to 100 of the inner scale

is equivalent to one division of the outer scale. The coarse control is a switch with light indexing

for ease of operation; no attempt should be made to obtain a setting between the marks.

The value of a component is most easily

found by first noting the reading of the outer scale

followed by the reading of the inner scale, and then putting in the decimal point by referring to the figure displayed in the nearest scale window. Typical

examples are shown in Fig. 2.2.

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operation

2.4 CONNECTING THE COMPONENT

Connect the component between the HI and

DET terminals, using short leads where possible.

Small components can often be directly supported by the test terminals; those which can't be sup ported should be laid on the insulated component platform. Note the following precautions : 

(a) Impedance between the HI side of the comp

onent and earth, shunts the 'balance' arm of the bridge, and may result in an inaccurate reading.

(b) Stray pickup on the lead to the DET terminal

is applied to the input of the bridge detector and may cause difficulty in obtaining a clear balance indication. Using a screened lead, with the screen connected to the E terminal, helps to avoid this effect.

the component will shortcircuit the bridge and make measurement impossible.

(d) The bridge energizing voltages appearing at

the test terminals (see Data Summary) are too small to cause damage to the component

or shock to the operator.

(e) When measuring screened inductors, the

screen may be left disconnected, or connected to the E terminal.

2.5 RESISTANCE MEASUREMENT

Resistance can be measured at d. c. only. The 1 kc/s10 kc/s switch and the loss balance controls are inoperative.

Set the LCR switch to R (or Rx10 if the anti

cipated value is above 11 MΩ).

Fig. 2.2 L: Reading 3,782 mH

Reading

R: Reading 83,22 M Ω

0.006652ΜF

If the resistance value is completely unknown,

set the RANGE switch to its highest setting.

Set the COARSE and FINE BALANCE pointers

to mid scale.

Adjust the SENSITIVITY control to give a meter deflection of no more than half scale.

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Adjust the RANGE switch to give the lowest meter reading. If the resistance value is approximately known select the appropriate range.

Balance the bridge by adjusting first the

COARSE and finally the FINE BALANCE

Controls, to give a minimum meter def

lection, progressively advancing the

SENSITIVITY control as required.

Read the resistance value from the BAL ANCE scales as described in Section 2.3.

measuring Low wattage resistors. The general Circuit used for resistance measurement is shown in Fig. 2. 3. The bridge source resistance is 30 Ω and the first two range resistors are 1 Ω and

10 Ω. Using this Circuit to calculate the power dissipation in resistors under test it will be found that the maximum is 100 mW for resistors between 1050 n.

If a resistor of less than 100 mW rating is to be measured the power dissipation can be red uced by adding a resistor of known value in series with the test resistor. The value of such a res

Note : It may be observed that many composition

resistors show a continuous drift in value.

Measuring low-wattage resistors

On the lower ranges, the effect of the 4 V

source e. m. f. may have to be considered when

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SERIES RESISTOR

RANGE RESISTOR

TEST RESISTOR

Fig. 2.3 Circuit used in measuring low-wattage resistors

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Operation

istor should be no larger than is sufficient to reduce the dissipation to an acceptable level otherwise measurement discrimination will suffer. Alternatively, at the expense of accuracy, selecting a higher range than usual will have the desired effect.

Detector zero

The detector circuit used for resistance measurements is extremely sensitive and is sus ceptible to thermal e. m. f. 's especially on the 10 Ω, 10 MΩ and 100 MΩ ranges. Such an e. m. f.

will produce a residual meter deflection and an

internal preset control is provided to enable this to be offset. If the control requires resetting allow at least 20 minutes from switching on the Bridge for internal thermal stabilization and then remove the top plate as described in Section 4. 3.

The control is located behind the Balance switch assembly and is labelled RV11 SET ZERO. Set the RANGE switch and BALANCE controls to 10 MΩ, short circuit the DET and E terminals

with copper wire and adjust RV11 for minimum meter reading with maximum SENSITIVITY.

Residual and connection resistance

where this is considered to be significant the connections should be measured separately and allowed for in the result.

Further extraneous errors can be avoided

by not handling the resistor under test and making

sure that it is isolated from earth or any external supplies.

2.6 INDUCTANCE MEASUREMENT

When measuring inductance, the loss balance controls are used to balance out the resistive component of the inductor. The loss balance dial is directreading at 1 kc/s only; readings on either scale must be multiplied by 10 when measuring at

10 kc/s  further details are given in Section 2. 7. The main balance dial is direct reading at all frequencies.

Most audio and powerfrequency indiietors

may be measured at Ikc/s using the 030 Q range.

For small lowQ inductors such as r. f. coils it is

better to use 10 kc/s and the 03 Q range. Large inductors with a low selfresonant frequency, or small inductors with a saturable core need special precautions and these are described later in the section.

When measuring resistors near the lower limit of the range, the residual resistance (R0) should be deducted from the value obtained. Also note that the component connections can have a significant resistance.

An approximate value of R0 is given in the Data Simimary, but the actual value for a partic-

ular bridge may be found as follows : 

Short circuit the HI and DET terminals

with a thick copper strap (of 'zero' resistance). Set the LCR switch to R, and the

RANGE switch to its lowest setting.

Balance the bridge with the FINE BALANCE

control to as near zero on the meter as

possible. Read the residual resistance

from the FINE BALANCE scale; the '02' calibration is equivalent to 0.002 Ω.

Typical component connections have a

resistance of about 1 mΩ per inch (0.4 mΩ per cm).

This amounts of 0.1% per inch at 1 n. Therefore

Set the LCR switch to L.

Set the DQ switch to Q = 030.

Set the 1 kc/s 10 kc/s switch to 1 kc/s.

Set the LOSS BALANCE controlto about

10.

Set the FINE Q and the FINE and COARSE BALANCE controls to about midway.

Set the SENSITIVITY control to give a meter deflection of no more than half scale.

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Operation

Set the LCR switch to L.

Set the D-Q switch to Q = 0-3.

Set the 1 kc/s-10 kc/s switch to 10 kc/s.

Tum the LOSS BALANCE control fully

clockwise.

Set the FINE Q control mid-way.

Set the RANGE switch to its highest setting.

Set the COARSE BALANCE control to 0.

First measure the approximate inductance

as follows : -

Set the SENSITIVITY contrbl to give a meter deflection of no more than half scale.

Using the FINE BALANCE control only search for a null. If the best null is below

05 reduce the RANGE by one step. Once

an apparent null has been obtained on

scale adjust the LOSS BALANCE and FINE BALANCE controls suecessively to obtain a minimum meter reading.

If the inductor Q is extremely low, the balance indication will be rather flat.

Read the inductance value from the FINE BALANCE scale noting that its range is

1/10th of that indicated in the lower left

hand window. This will only be an approx-

imate value, although the sharper the balance indication the more accurate the value obtained.

To measure the coil accurately, proceed

as follows :-

' 8 ) Select the RANGE suitable for the approx-

imate value obtained above, usually two

(T) lower. Set the COARSE BALANCE pointer

to the value and re-balance the bridge with

(4) slight adjustment of the LOSS BALANCE

control to obtain the lowest meter reading.

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Operation

Offset the COARSE BALANCE control a step at a time (usually counter clockwise) re-balancing with the LOSS BALANCE control and advancing the SENSITIVITY control as necessary. Use the FINE Q control if the loss adjustment becomes critical.

It is important to progress to the lowest

possible reading, ultimately using the

FINE BALANCE and FINE Q controls to obtain a zero reading - or at least within one or two divisions - on the meter scale, with the SENSITIVITY control fully clock-

wise.

Measuring iron-cored inductors

To avoid errors in measurement, inductors

having a low self-resonant frequency should not be tested at 10 kc/s. If their self-resonant frequency is lower than about 5 kc/s, then test results evén

at 1 kc/s will be in error - inductance will appear high, and magnification low. It will be found that 1 henry is the practical upper limit for measurements at 10 kc/s.

These errors will increase rapidly as the

self-resonant frequency of the inductor approaches

the test frequency, and it is impossible to measure an inductor which resonates below 1 kc/s, since

it will exhibit capacitive reactance. This effective capacitance may be measured on the bridge, but it will appear to have a poor dissipation factor.

The inherent capacitance of the bridge at the test terminals is low - approximately 0. 05 pF - and will contribute little to lowering the self-resonant

frequency of the component.

Shunt capacitance however, due to the pos-

itioning of the components and arrangement of the

test leads, may lead to appreciable errors in measurement. Such errors, which will increase

with frequency, can be minimized by careful positionlng of the component, using short well-spaced

leads and,, whenever possible, supporting it on the insulated component platform.

Core saturation

If the inductor is of such a type that its

core is easily saturated, it is advisable to use

10 kc/s and to switch to the highest range that still covers the component value (for example by measuring a 10 mH inductor on the 10- to 100 mH rangé instead of the 0- to 10 mH range) thereby reducing the current through the inductor windings.

Note : Iron- or ferrite-cored inductors are part-

icularly liable to pick up stray radiation which, applied to the bridge via the DET terminál, will tend to mask the balance point indication. To check for this effect, insert an unconnected tele-

phoné plug into the EXT OSC jack in order to switch off the internal oscillator. Any reading on the méter is now due to stray pick-up, and the inductor should be turned or repositioned to minimize the effect.

ResiduaI inductance

When measuring inductance near the lower limit of the range, the residual inductance (Lo) should be deducted from the value obtained.

An approximate value of Lo is given in the Data Summary, but the actual value for a particular

bridge may be found as follows :-

Short-circuit the Hl and DET terminals

with a thick copper strap.

Set the LCR switch to L, and the RANGE switch to its lowest setting.

Switch to 10 kc/s; Q = 0-3; set the coarse BALANCE control to zero, and the fine control to '10 '.

Balance the bridge in the normal manner to obtain a balance as near zero, on the meter, as possible.

Read the residual inductance from the

FINE BALANCE scale; the '10' calibration is equivalent to 0.1 µH.

Screening cans on h. f. coils

These are usually best connected to the E

terminál but if already connected to the inductor then jóin to the DET terminál. 10 kc/s test frequency should be used as the shorted turn effect of an aluminlum or copper can rapidly reduces at lower frequencies.

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