FAULT DIAGNOSIS


INTRODUCTION

1. The following notes and test procedures enable the faulty section of
the receiver to be determined with the minimum of delay. Unless other-
wise stated the meter on the front panel is used for measuring purposes. This
is set to R.F. LEVEL and the reference figure is 100µA for all sensitivity
tests.

2. Since the audio stages of the receiver are conventional and accessible,
normal practice will serve to trace any fault which may occur in this
section.


TEST EQUIPMENT REQUIREMENTS

3. The following test equipment will be required:

(1) Valve Voltmeter.

(2) 12pF trimmer capacitor.

(3) Signal generator.


NOTE: The input capacitance of the valve voltmeter must be padded to 12pF
by the trimmer or alternatively by a fixed capacitor. Before the value
or the trimmer or the fixed capacitor can be selected, the input
capacitance of the valve voltmeter must be know. If the trimmer is used
this should be connected across a capacitance bridge and set to the
required value.


FAULT DIAGNOSIS

4. Set the controls on the front panel as follows:-

A.F. GAIN set to max.

R.F./I.F. GAIN set to max.

B.F.O. switch to off.

LIMITER switch to OFF.

SYSTEM switch to MAN.

5. Remove the valve V12 and crystals XL1 and XL300, and connect the
output of the signal generator to socket SKT303.

6. Set the BANDWIDTH control to 100 c/s and tune the signal generator for
maximum indication on the meter at 100 kc/s. Switch through the
bandwidth positions. The sensitivity should be approximately as follows:-

3 kc/s less than 800µV for a deflection of 100µA
100 c/s )
300 c/s )
1.2 kc/s ) To be within 10dB of sensitivity measured on 3 kc/s
) position
6.5 kc/s )
13.0 kc/s )

7. In the event of the figure above not being realized, the renewal of one
or more of the following valves will probably effect on improvement.

V26 fourth mixer
V14 first i.f. amplifier
V16 second i.f. amplifier
V27 1.7 Mc/s oscillator/amplifier

8. Set BANDWIDTH control to 3 kc/s. Refit the 1.7 Mc/s crystal XL300.
Connect the output of the signal generator to socket SKT301 (pink)
and tune the generator to a frequency of 1.6 Mc/s. The sensitivity should be
better than 75µV for 100µA.

9. In the event of the figure above not being realized the renewal of
V25 will probably effect on improvement.

10. Refit the valve V12 and set the KC/S scale to 500. Connect the output
of the generatorto socket SKT11 and set generator to a frequency of
2.5 Mc/s approximately and tune for maximum deflection on the meter. The
sensitivity should be better than 250µV for 100µA.

11. Set the KC/S scale to 0 kc/s and 1000 kc/s and the signal generator
to 3 and 2 Mc/s respectively. The sensitivity should not vary from
250µV by more than 3dB.

12. The maximum difference between check point should not exceed 3dB. The
renewal of V11, or V9 will probably effect an improvement if this
figure is not met, providing that the conditions outlined in previous
paragraphs have been achieved.

13. refit the crystal XL1 and check the 1 Mc/s output (SKT2 on top of the
main chassis) with the valve voltmeter to ensure that there is at least
2V output.


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