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The sound behaves oddly on this set. After switching on,
the sound appears just after the vision on 625 lines but
takes six minutes to appear on 405 lines. When the set has
been operating for a while switching from 405 to 625 is
normal but on switching from 625 to 405 lines there is a
delay of about thirty seconds in the appearance of the
sound. I’ve changed the sound output valve and probed for
dry -joints, but nothing has come to light.
Fixed bias is applied to the first sound i.f. transistor on
625 lines, but on 405 lines the transistor is biased by an
amplified a.g.c. feed which comes from the cathode of the
triode section of the PCL82 audio valve. When this valve is
operating normally there should be 2.1 V across the triode’s
cathode bias resistor R2099. It’s anode load resistor 82101
should have a value of 100k S2, giving 135V at the anode.
Also check the cathode decoupler C2020 (25µF) and
R2090 (1MS2) via which the triode’s control grid is returned
to chassis on 405 lines.

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