Normally the picture and sound are good. Now and again
however the contrast automatically increases, resulting in a
very black and unstable picture accompanied by hum. After
a minute or so the set returns to normal. The manual
contrast control operates correctly. The set is used on 625
First check the a.g.c. clamp diode D9, type M 1, by
substitution; then check the coupling capacitor C44
(120pF) between the two vision i.f. valves; finally check the
first of these valves (VS, EF183). Too high a setting of the
contrast control on this model can cause intermittent a.g.c.
lock -out. (ITT VC4 chassis.