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@Yamamaya42, great to hear that damper circuit failure causes similar symptoms! I'll check what I can in that area of the board. But almost everything has been replaced there, including the two couplates, except for two original carbon comp resistors that measure correctly in circuit.
@bandersen, I just checked and cleaned all 3 of those tube sockets with a tiny wire brush and DeOxit Gold inside, no difference. There are no obvious cracks in the sockets, and they don't wiggle.
@jr_tech, I watched carefully, and the Horiz Out tube (6DQ6B) heater DOES go out just as the picture starts fading -- the other two tubes continue to have lit heaters. Interesting!
Seems like a thermal issue. If I try startup a minute or two after picture loss it never works -- the Horiz Out filament does not light up (never noticed that before), and I usually hear some oscillator noise but no picture. But if I wait 5 to 10 minutes after shutdown it will usually work again, and I'll have picture for 3 to 5 mins, with a longer "off" time leading to a longer "on" time.
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