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The trouble with this set is occasional field jitter. With the
contrast turned up high the fault is present almost
continually, but with the contrast turned down, giving a
dark picture, the fault hardly ever occurs. The jitter is
downwards, about fin. deep, and starts on a change of
Since the fault is affected by the contrast control setting,
it could well be in the a.g.c. circuit, which can be responsible
for this effect. The main suspects here are the
reservoir/smoothing electrolytics C2041 (40pF) and C2040
(also 40µF). Another suspect is C2047 (12.5µF) which
decouples the screen grid of the luminance output valve,
also the black -level clamp transistor T2146 in its control
grid circuit. Any of the valves in the field timebase can
cause this trouble – there are three, the ECC81
multivibrator (V4002), the cathode -follower V4003a (half a
PCC85) and the PL508 field output valve. Another
common cause of the fault in the field timebase is when
R4092 (33k52), one of the anode load resistors in the
multivibrator circuit, changes value.

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