DECCA 70 SERIES CHASSIS

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The problem is incorrect field lock at switch on. Repeated
channel changes will produce correct field lock, alterna-
tively retuning a channel will bring the picture to the
correct position. When a channel change is made however
the picture is split. The field timebase and If. i.c.s have
been replaced with no improvement and the voltages in
these areas are roughly correct. The a.g.c. action seems to
be correct.
This sort of problem is generally due to faulty
electrolytics. The suspects in this case are C307 (10µF) in
the field sync pulse integrator stage and the two
electrolytics 1C11 (1µF) and 1C12 (33µF) associated with
the a.g.c. circuitry. An oscilloscope to check the quality of
the field sync pulses at the base of the video emitter –
follower 1Tr1 would be invaluable.

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