ITT VC51 CHASSIS
The sound is o.k. but the picture sometimes doesn’t appear,
due to lack of line drive (line output valve overheating). The
difficulty is that when a voltage reading is taken at pin 1
(triode anode) of the PCF802 line oscillator valve the line
timebase suddenly starts to run, the picture then appearing.
When the fault is present there seems to be no voltage at
pin 6 (pentode anode) of the PCF802 – there should be
130V here. The 47E2 load resistor then has no voltage at
one side, but when the test probe ‘is applied to the other side
the line timebase suddenly starts up, with a picture a few
seconds later. The load resistor has been replaced without
alterating the situation and I’m now stumped.
This problem often causes confusion. It’s almost always
due to the polystyrene feedback capacitor C121 (800pF) in
the line oscillator stage. Replace with an 820pF silver mica
type. On very rare occasions the electrolytic capacitor that
smooths the supply to the line oscillator can cause the same
trouble. This is C110 (16µF).