Ohmster wrote:
I have no idea of what caused the problem. Faulty IC I would imagine, but I
did indeed enjoy reading the story. You did a good job, Bob.

Might be, but what causes a IC to work flawlessly for 20 years and then
suddenly fail?

I think (but cant really remember exactly) that the problem appeared
while adjusting focus and that i heared some click (probably one of the
many relays in there).

I already checked if the signal goes anywhere close to a high voltage
wire (that could spark to it if moisture was present) but didnt turn out
to be so.
I found the +18kV-wire being rather close to unrelated wires however and
tied it away from them just to be safe.

Btw, thats how it looks now:
http://www.hobbyelektronik.org/robert/1980_ok.jpg