PYE 697 CHASSIS

With the contrast control at the setting which is correct for
the picture, there’s a dark band across the screen whenever
words such as the BBC -1 caption appear – the band is the
height of the lettering. Even with no lettering, there are
bands in varying shades across the screen, depending on
picture content. The fault can be cleared by reducing the
contrast control setting, but the picture then lacks contrast.
I’ve replaced the colour -difference and luminance output
valves, also several components in the beam limiter circuit –
I had to use two 25µF electrolytics back-to-back to replace
the 12.5µF non -polarised electrolytic C201 in this circuit
incidentally. The TBA480 intercarrier sound i.c. was
replaced because of a sound fault, and the tuner because of
a noisy picture. After all this work, I’d appreciate help with
the remaining fault.
There seem to be two possibilities, either a defective c.r t.
or incorrect frequency response in the luminance output
stage. It would be worth checking all the decoupling
capacitors in the luminance output stage – C351
(0.0047µF) in the cathode circuit, C353 (4µF) and C354
(101iF) in the screen grid circuit and C352 (321LF) in the
anode circuit.