GENERAL SERVICING AND ALIGNMENT PROCEDURES


GENERAL SERVICING
----------------------------------

INTRODUCTION

1. The following tests will assist in checking the performance of the
receiver.

2. Component layout illustrations, fig.8 to fig.23 inclusive, give on
overall picture of the receiver sub-assemblies and chassis underside.

3. Removal of the main base cover will, without removal of further covers,
reveal the power and audio stages, and the 100 kc/s i.f. amplifier
chassis.

4. To gain acces to other stages, furher covers must be removed, they are
the second mixer (compartment 7) and the crystal oscillator/amplifier
and harmonic generator stages (compartments 4, 5 and 6) fig.15.


TEST EQUIPMENT REQUIREMENTS

5. The following test equipment will be required to carry out tests on the
receiver:-

(1) Telephone headset

(2) Output power meter

(3) Signal generator

(4) Multimeter



RECEIVER TESTS
----------------------------

RECEIVER OVERALL GAIN TESTS (C.W)

6. Perform a sensitivity test as follows:-

(1) set the controls on the receiver as follows:-

R.F.RANGE 2-4 Mc/s

MEGACYCLES 3

KILOCYCLES 500

R.F ATTENUATOR MIN.

SYSTEM switch MAN.

BANDWIDTH 3 kc/s

B.F.O. switch ON

B.F.O. NOTE 1 kc/s

R.F./I.F. GAIN fully-clockwise


(2) Set the controls of the output meter for 600 ohms impedance and
a range of 6mW. Connect the output meter across one of the 600
ohm 3mW windings and terminate the other windings with resistor
to match their marked impedance.

(3) Set the controls of the signal generators for a c.w output of
1µV at 3.5 Mc/s and an impedance of 75 ohms. Connect the output
of the signal generator to the antenna input of the receiver.

(4) Tune the receiver to the output frequency of the signal
generator and check that a reading of 3mW can be obtained within
the range of the A.F. GAIN control.

(5) Repeat the test with the B.F.O. switched off and a 30%
modulated signal at level of 3.5µV from the signal generator.


SIGNAL-TO-NOISE TEST

7. Perform a signal-to-noise test as follows:-

(1) Set the receiver controls as follows:-

R.F. RANGE 1-2 Mc/s

MEGACYCLES 1

KILOCYCLES 500

R.F. ATTENUATOR MIN.

SYSTEM switch MAN.

BANDWIDTH 3 kc/s

B.F.O. switch ON

B.F.O. NOTE 1 kc/s

A.F. GAIN fully-clockwise

SPEAKER OFF


(2) Set the controls of the output meter for an impedance oa 3 ohms
and a power range of 60mW and connection it to the 1W 3 ohm
output terminals of the receiver.

(3) Set the controls of the signal generator for a c.w output of
1µV at 1.5 Mc/s and an impedance of 75 ohms. Connect the output
of the signal generator to the receiver antenna input socket.

(4) Tune the receiver to the output frequency of the signal
generator and adjust the R.F./I.F. GAIN control a reading of
50mW on the output meter.

(5) Switch off the input signal and check that the output meter
does not reade more than 0.82mW.

(6) Repeat the test with a 30% modulated signal at a level of
3.5µV and the receiver B.F.O. switched off.

(7) Perform signal-to-noise ratio tests at 3.5, 6.5, 12.5 and
24.5 Mc/s.


A.V.C. TEST

8. Perform a test of the a.v.c. circuits as follows:-

(1) Set the controls of the receiver as follows:-

R.F. RANGE 2-4 Mc/s

MEGACYCLES 3

KILOCYCLES 500

R.F./I.F. GAIN fully-clockwise

R.F. ATTENUATOR MIN.

SYSTEM switch A.V.C.

A.V.C. switch SHORT

BANDWIDTH 3 kc/s

B.F.O. switch OFF


(2) Set the controls of the output meter for an impedance of 3 ohms
and power range of 200mW. Connect the meter to the 1W 3-ohm
terminals of the receiver.

(3) Set the controls of the signal generator for a 30% modulated
signal of 1µV at 3.5 Mc/s and an impedance of 75 ohms.
Connect the signal generator output to the receiver antenna
input socket.

(4) Tune the receiver to the output frequency of the signal
generator and adjust A.F. GAIN control until the output
indicates 10mW.

(5) Increase the output of the signal generator to 100mV (+100dB)
and check that the reading on the output meter does not exceed
50mW (+7dB on 10mW).


A.F. LEVEL METER TEST

9. Perform a test of the A.F. LEVEL meter calibration as follows:-

(1) Set the controls of the output meter for an impedance of 600
ohms and a power range of 120mW. Connect the meter across the
600-ohm 10mW output terminals.

(2) Set the controls of the signal generator for a 30% modulated
output of 3.5 Mc/s at an impedance of 75 ohms and a level of
5µV. Connect the signal generator to the antenna input socket
of the receiver.

(3) Tune the receiver to the output frequency of the signal
generator and adjust the A.F. LEVEL control until the meter
reads exactly 10mW. Check that the external output meter
reads within 1dB of 10mW.

NOTE: It is important that the A.F. LEVEL control is not turned towards its
maximum position unless the 10mW 600-ohm winding is suitably terminated
with a load.


NOISE FACTOR TEST

10. Perform a noise factor tests as follows:-

(1) Set the receiver controls as detailed below:-

R.F. RANGE 16-30 Mc/s

MEGACYCLES 29

KILOCYCLES 0

SYSTEM switch MAN.

R.F. ATTENUATOR MIN.

BANDWIDTH 3 kc/s

B.F.O. switch ON

LIMITER OFF

R.F./I.F. GAIN maximum gain position

B.F.O. NOTE ±1 kc/s

SPEAKER ON


(2) Set the noise generator RANGE switch to OFF.

(3) Connect the noise generator output to the receiver antenna
input socket.

(4) Set the controls of the output meter for an impedance of 3
ohms and a power range of 60mW and connect it to the 1W 3-ohm
output terminals of the receiver.

(5) Set the A.F. GAIN control for a convenient level and adjust the
MEGACYCLES tuning and R.F. TUNE controls for maximum noise in
the loudspeaker.

(6) Adjust the A.F. GAIN control to obtain a reading of
approximately 10mW on the output meter. Check that the
MEGACYCLES and R.F. TUNE controls are set for maximum output
and then reset the A.F. GAIN control for exactly 10mW.

(7) Set the noise generator RANGE switch to 0-10.

(8) Adjust the noise generator output level control until a reading
of 20mW is obtained on the output meter.

(9) The noise factor of the receiver is given by the noise
generator meter reading for the range in use.

(10) Perform noise factor tests at 1.5, 3, 6, 12 and 24 Mc/s, the
noise level should not exceed 7dB throughout the entire
frequency range.




ALIGNMENT PROCEDURES
------------------------------------------

INTRODUCTION

11. The receiver will, under normal conditions, maintain the factory
alignment over an extremely long period of time. Consequently
ALL POSSIBILITY OF OTHER CAUSES OF TROUBLE SHOULD BE
ELIMINATED BEFORE RE-ALIGNMENT IS CONSIDERED.

12. If it becomes necessary to re-align any part of the receiver, only a
very small angular adjustment of any trimmer should be necessary. The
signal generator must have a high degree of frequency resetting accuracy and
be very stable.

13. Unless otherwise stated, the front panel mounted meter is used as the
output indicator.



100 KC/S I.F. AMPLIFIER
--------------------------------------

FIRST AND SECOND I.F. AMPLIFIER

14. Remove the second v.f.o. valve V12. Set the SYSTEM switch to MAN, the
R.F./I.F. GAIN to MAX and the meter switch to R.F. LEVEL. Connect the
signal generator (100 kc/s c.w) via a 0.1µF capacitor to the grid of V16
(pin 1) Adjust C191 to obtain maximum indication on the meter. The output from
the generator required to produce 100µA deflection on the meter should be
approximately 320mV. Connect the signal generator via a 0.1µF capacitor to the
grid of V14 (pin 1) and connect a 4.7-kilohms damping resistor across L72.
Adjust C179 and C195B to give maximum indication.

15. Remove the 4.7-kilohms resistor from L72 and connect it across L73.
Adjust C171 for maximum indication. Remove the 4.7 kilohms resistor.
The signal generator output required to produce a 100µA deflection should be
approximately 800µV. Tune the signal generator through the passband and note
the `double peak` response. The peak separation should be approximately 9 kc/s
and be symmetrical about 100 kc/s. If the peak amplitudes differ, slight
re-adjustment of C195B will compensate for this. The 6dB bandwidth should be
approximately 14 kc/s.



100 KC/S (L-C) FILTER
----------------------------------

16. Remove the left hand gusset plate. Remove the 1.7 Mc/s crystal XL300
and set the controls as in 14 above. Connect the output signal
generator (100 kc/s) to socket SKT303. Remove the L-C filter can. Locatethe
two red free-ended leads connected at one end of the trimming capacitors C153
and C158 in the second and third sections of the filter and connect the free
ends to their respective 470k damping resistor R77 and R80 at the terminal post
ends. Replace the filter can. Set the bandwidth to 100 kc/s. Tune the signal
generator to give maximum indication on the front panel meter then switch to
1.2 kc/s. The frequency of this setting should be within ±100 c/s of 100 kc/s.
Adjust the trimming capacitors C162, C158, C153 and C147 in this order several
times until maximum output is obtained.

17. Remove the L-C filter can and disconnect the red leads from the
terminal post ends of the 470-kilohms resistors. Replace the filter
can. Set the controls of the signaal generator for an output of 200µV
approximately for 100µA on front panel meter. Check that the bandwidths agree
(approximately) with the folowing figures:-

-6dB -66dB Sensitivity for 100µA

3.0 kc/s 15 kc/s Less than 200µV
(Measured input
becomes reference
level)
100 c/s Less than 1.5 kc/s)
300 c/s Less than 2.0 kc/s)
1.0 kc/s 8 kc/s ) To be within 10dB of
) reference level
) measured on 3 kc/s
) position
7.0 kc/s 22 kc/s )
13.0 kc/s 35 kc/s )


CRYSTAL FILTER

18. Remove the 1.7 Mc/s crystal XL300 and set the controls as in 14. above.
Set the BANDWIDTH switch to 300 c/s. Connect the signal generator to
socket SKT303. Tune the signal generator slowly through the passband and
observe the crystal responses (f1 and f2). Care must be taken as the tuning of
these is very sharp. Retune the signal generator to the mean of f1 and f2 and
adjust C110 and C148 for maximum output. Reset the signal generator frequency
to 100 kc/s and adjust the output to produce a reading of 100µA. Set the
generator frequency to 101,025 c/s, increase the output by 66dB and adjust
the phasing control C199 to obtain minimum output (i.e. the point of recection
occurs). Increase the generator frequency slowly and ascertain that the meter
reading does not exceed 100µA. Slowly decrease the signal frequency until
100µA reading is obtained and check that the frequency is not greater than
100,900 c/s. Tune through the passband, adjusting the signal generator output
as necessary to avoid meter damage. Note the highest frequency at which a signal
generator output equal to that used at 101,025 c/s gives an output an output
of a 100µA. This frequency should not be less than 99,100 c/s.

19. Slowly decrease the signal frequency and ensure that the output does
not rise above 100µA. Decrease the generator output by 66dB and re-
check the frequency response within the passband, re-adjusting C110 and C148
if necessary. Set the signal generator frequency to 100 kc/s and adjust the
output for 100µA level. Increase the signal generator output by 6dB and check
the bandwidth for 100µA output. The bandwidth should be between 270 and 330 c/s
and the mid-position should not deviate from 100 kc/s by more than 25 c/s. The
sensitivity should be approximately 200µV for 100µA deflection.

20. Switch the BANDWIDTH control to 100 c/s. Repeat the procedure with
signal generator frequency settings of 100,925 c/s, 100,800 c/s and
99,200 c/s. Adjust the phasing capacitor C118 only. The 6dB bandwidth should
be between 80 and 120 c/s and the deviation from the mean less than 25 c/s.
For 100µA output, the input should be approximately 150µA.

21. Disconnect the signal generator and refit the 1.7 Mc/s crystal.


USE OF DIGITAL FREQUENCY METER

22. The alignment of the i.f. amplifier and in particular the crystal
filter involves the measurement of frequencies to far greater
accuracies than those normally obtainable from signal generators. A digital
frequency meter should therefore be employed. The equipment should be connected
to SK8 or SK9. The exact frequency passing through the circuit will be
displayed on the indicator panel. Should the level of output at any time during
the alignment procedure be insufficient to drive the frequency meter, the
signal generator output can be increased to obtain the frequency check but must
be restored to the lower value for level measurements. When such increases are
made, the meter on the receiver panel should be switched to A.F. LEVEL to avoid
damage.


SECOND V.F.O.
------------------------

MINOR CORRECTIONS

23. THe variable capacitor has been carefully adjusted and should not be
re-adjusted unless absolutely necessary. Minor corrections can be made
as follows:-

(1) Set the SYSTEM switch to CAL.

(2) Set the KILOCYCLES cursor in line with the MEGACYCLES
cursor (i.e. central)

(3) Ensure that the B.F.O. switch is OFF.

(4) Rotate the R.F./I.F. GAIN to MAX.

(5) Set the BANDWIDTH switch to 3 kc/s.

(6) Set the KILOCYCLES scale to zero (0 kc/s) and adjust the
capacitor C306 to give zero beat note in the loudspeaker.

(7) Set the KILOCYCLES scale tothat zero beat point which is
nearest to the 1000 kc/s position.

(8) Lock the drive sprocket.

(9) Adjust the position of the film scale to produce a correct
calibration.

NOTE: When moving film scale relative to the sprockets, grip both
sides of the film scale in order to create a loop which will
allow the film to slide round the drive sprocket; the drive
sprocket is on the left when facing the receiver and hence
movement of the film scale will have to be to the left.

(10) Repeat (6) to (9) until an adequate degree of accuracy is
obtained.

IMPORTANT NOTE: The tuning slug of L55 has been sealed by the
manufacturer and must not be touched under any
circumstances.


REPLACEMENT OF VARIABLE CAPACITOR

24. The procedure described below should not normally be carried out
unless the variable capacitor C310 is being replaced. Before electrical
adjustment, the following mechanical points should be verified:-

(1) Check that the fixed and movable vanes of C310 are fully
meshed.

(2) Check that the distance from the cursor to the extreme end
of the scale adjacent to the 1000 kc/s point, is approximately
7½-in. Should this distance vary appreciably from 7½-in.
Carefully lift the scale from the drive and move the scale
round the required position.

(3) Whenever the scale is replaced, endeavour to re-align by
adjusting the film to the correct position before trimming.

25. The procedure for electrical adjustment is carried out as follows:-

(1) Proceed as in 24. (1) above.

(2) Check the calibration of the v.f.o. at 100 kc/s intervals;
if the error exceeds 1 kc/s, adjust carefully the plates of
the rotor of the variable capacitor in order to correct the
calibration.


1.6 MC/S REJECTION FILTER

26. Disconnect lead to SK300A and apply 1.6 Mc/s signal generator output
to SKT300A. Set METER switch to S-METER and adjust core of L302 for
minimum meter deflection.


B.F.O.

27. Set the SYSTEM switch to the CHECK B.F.O. position. Switch the meter
switch to R.F. LEVEL. Switch the b.f.o. on and set the b.f.o.
frequency control knib to zero. Adjust C199 as necessary to obtain zero-beat.
Observe that the meter reads at least 100µA.

28. If the b.f.o. frequency control knob has been removed, adjust the
frequency capacitor for zero-beat with the identification mark on the
shaft uppermost. Replace the knob so that the pointer indicates zero.


37.5 MC/S FILTER AND AMPLIFIER

29. Remove the 1 Mc/s crystal, second mixer valve V9 and first v.f.o.
valve V5. Check that all the screening covers are in place. Connect
a suitable valve voltmeter, shunted 12pF, to TP3. Inject an accurate 37.5 Mc/s
signal at TP1. Ensure that the valve voltmeter and signal generator leads are
short to avoid regeneration. Adjust L50, C90, C81, C72, C63, C55, C45, C35,
C24, L28 and L33 in that order, several times, to obtain maximum output. The
input required to produce 1V should be approximately 2.5mV. The 6dB bandwidth
of the 37.5 Mc/s chain should lie between 229-300 kc/s. The bandwidth at 40dB
should not exceed 750 kc/s. The mean of the frequencies corresponding to the
6dB points should not deviate from 37.5 Mc/s by more than 20 kc/s and more
than 25 kc/s at 40dB banwidth.

30. C108 is adjusted to avoid interaction between the 37.5 and 40 Mc/s
filters and should not normally require further adjustment. Fit the
1 Mc/s crystal, the second mixer and the the first v.f.o. valve.


1 MC/S OSCILLATOR

31. Connect the valve voltmeter to the 1 Mc/s output plug PL2 and adjust
L2 for maximum output (2-3 volts). C2A may be adjusted to "pull" the
crystal to the correct frequency; howewer, adjustment of crystal frequency
should not be attempted unless a standard is available having an accuracy of
better than one part in 10\7.


SECOND MIXER DRIVE LEVEL

32. Remove the second mixer valve V9. Connect the valve voltmeter, shunted
to 12pF, to TP3. Tune trough each megacycle calibration point and check
that the level output lies between 2 and 10V. To equalize the drive at 28 and
29 Mc/s carefully adjust C7.


FIRST V.F.O. CALIBRATION

33. Slacken off the mechanical end-stop until it is inoperative. Set C76
to maximum capacity and ensure that the calibration mark at the zero
end of the MEGACYCLES dial coincides with the cursor. Tighten end-stop after
moving the scaale free from the stop. Check that the mechanical stops operate
before the capacitor end-stops become effective at both ends of the band.

34. To re-adjust the first v.f.o. calibration, a heterodyne wavemeter
should be employed. This is Coupled very loosely to V7 by placing its
input lead in the vicinity of the valve base. The 1 Mc/s crystal, V12 and V27
should be removed.

35. Set the wavemeter to 40.5 Mc/s and the MEGACYCLES dial to zero.
Adjust L36 for zero-beat. Change the wavemeter setting to 69.5 Mc/s
and the MEGACYCLES dial to 29. Adjust C77 fir zero-beat. Repeat adjustment as
necessary. Check the frequency calibration at 1 Mc/s intervals and ensure that
the megacycle positions are reasonably central on the scale markings. Remove
the first mixer valve V7 and connect the valve voltmeter, shunted 12pF, between
TP2 and the chassis. Check that the valve voltmeter indicates at least 1.5V
over the range. Refit the 1 Mc/s crystal, V12 and V27.


ANTENNA CIRCUIT

36. Remove the first v.f.o. valve V5 and the first mixer valve V7 and set
the receiver controls as follows:-

R.F. ATTENUATOR MIN.

R.F. RANGE MC/S 1-2 Mc/s

SYSTEM switch MAN.

R.F./I.F. GAIN MAX

37. Remove the screening cover from around C18A/B and connect a 1 kilohm
resistor across the secondary section (C18B rear section). Set the
R.F. TUNE control approximately 7/8ths of its travel in a clockwise direction.

38. Connect the valve voltmeter, shunted to 12pF, between TP2 and chassis.
Connect the output of the signal generator to the aerial input socket.
Set the generator for a frequency of 1 Mc/s.

39. Remove the top core From the transformer L8 and adjust the primary core
for a maximum deflection on the valve voltmeter. (The position of this
core should be such that it tunes at a point nearest the bottom of the
transformer).

40. Remove the 1 kilohm resistor from the secondary section and connect it
across the primary section of C18.

41. Refit top score (secondary) and adjust it for a maximum deflection on
the valve voltmeter.

42. Remove the 1 kilohm resistor from the primary of C18.

43. Reset the signal generator requency to 2 Mc/s and adjust the R.F. TUNE
control (C18) for maximum output on the valve voltmeter then adjust the
trimmer capacitor C233 for a maximum deflection on the valve voltmeter also
check for symmetrical response.

44. Repeat the above procedure for the R.F. RANGE switch settings and
frequencies listed in Table 1 below. Check that the maximum voltage
input to give 0.5 volts output is as shown in Table 2 below.


TABLE 1
-------------

R.F. RANGE INDUCTOR ALIGNMENT TRIMMER ALIGNMENT
FREQUENCY FREQUENCY
2 - 4 L7 2 Mc/s C234 4 Mc/s
4 - 8 L6 4 Mc/s C235 8 Mc/s
8 - 16 L5 8 Mc/s C236 16 Mc/s
16 - 30 L4 13 Mc/s (C18 at C237 30 Mc/s
max.)

TABLE 2
--------------

R.F RANGE L.F. H.F.

1 - 2 Mc/s 7mV 7mV
2 - 4 Mc/s 10mV 10mV
4 - 8 Mc/s 12mV 16mV
8 - 16 Mc/s 22mV 26mV
16 - 30 Mc/s 22mV 30mV



CRYSTAL CALIBRATOR

46. Should no output be obtained from this unit when the SYSTEM switch is
in the CAL position and the KILOCYCLES scale set at a 100 kc/s check
point, or if spurious responses are obtained over the kilocycles range, proceed
as follows:-

Set the KILOCYCLES scale to a 100 kc/s point and check the
tuning of L70 by carefully rotating the core a half-turn
either side of the setting. If the signal does not appear,
restore the core to its original setting and repeat the check
with L75. If the signal is heard, the cores of L70 and L75
should be set to centre of the range of adjustment over which
a clean signal is produced.


47. Should a major fault be suspected, or if L70 or L75 have been inad-
vertently misaligned, it will be necessary to remove the unit and make
up an extension cable so that the unit may be operated outside the receiver.
The crystal calibrator may be aligned as follows:-

Remove V13 and connect the valve voltmeter probe to grid 3
(pin 7). Inject a 900 kc/s c.w. signal, from the signal
generator, at the grid of V15 (pin 1) and adjust L75 for
maximum output. Disconnect the valve voltmeter and the
signal generator, replace V13 and remove V15. Connect the
signal generator to grid 1 (pin 1) of V13 and the valve
voltmeter to the grid 1 connection (pin 1) of V15. Set the
signal generator to 100 kc/s c.w. and adjust L70 for maximum
indication on the valve voltmeter. Disconnect the valve
voltmeter and the generator. Fit V15. Connect the coaxial
connector to PL2 on the receiver.


48. The output should be approximately 0.2V measured between pin 6 of the
octal plug and earth.


40 MC/S FILTER

49. This filter is over-coupled and cannot be readily aligned without a
40 Mc/s sweep oscillator. Re-adjustment therefore should not be
attempted unless the specially designed test equipment and factory-type
alignment jigs are available.


1.6 MC/S BAND-PASS FILTER

50. To carry out alignment of this filter, the mixer chassis must first
be removed. After the removal of the chassis, turn the receiver on
to its side and reconnect, from the underside, the two leads (6.3V and 200V
h.t.) to their respective pins.

51. Remove all the valves on the chassis except the third mixer V25.
Connect a suitable valve voltmeter, shunted to 7pF, to pin 7 of V26.
Inject an accurate 1.6 Mc/s signal at socket SKT301 (pink). A large input
from the generator should be used initially and reduced as necessary
throughout the alignment. Adjust cores L306 and L309 in the first i.f.
transformer and cores L313, L314 in the second i.f. transformer for a maximum
reading on the valve voltmeter.

52. Check the gain of the i.f. amplifier as follows:-

(1) Connect the signal generator to SKT301 of V25. For an input
of 125mV, an output of not less than 500mV should appear at
pin 7 of V26.

(2) Check that the 13 kc/s bandwidth is obtained with not more
than 2dB fall in output and that the response curve is
reasonably symmetrical.

(3) Refit valves and mixer chassis.


2 - 3 MC/S BAND-PASS FILTER

53. This filter is pre-aligned and should not require further adjustment.
If the performance of the receiver has deteriorated and the filter is
suspected, it should be returned to the factory to re-adjustment.


1.7 MC/S OSCILLATOR/AMPLIFIER

54. With a valve voltmeter connected to SKT306, adjust the core of L330
for maximum meter indication.


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