ITT VC51 CHASSIS

< Back

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).

Table of Contents

Leave a comment

I have read the privacy policy and accept
Read our privacy policy

This form collects your name, email and content so that we can keep track of the comments placed on the website. For more info check our privacy policy where you will get more info on where, how and why we store your data.

This site uses Akismet to reduce spam. Learn how your comment data is processed.