It may be something to do with the CRT heater supply, which could indeed be
due to bad electrolytic caps, or bad solder joints, especially on the CRT
connector.

Dave,
I know about the dangers (I have a diploma in EE).
When you say it may be a bad solder/track, this does not explain
why the problem got progressively worse. A bad solder/track usually
causes an intermittant problem?




Dave