Thanks for your suggestions. I did the oscillator part first.
1. reducing the -5500 cathode voltage to -4400 V has no effect. Oscillation still stops at about 2 1/2 minutes
2. +120V and +30V (derived from the secondary of T710) are all good. I lifted R596 to reduce the 120V load but see no effect.
As I mentioned in my reply to Chuck, disconnecting the CRT has a very positive effect and oscillation doesn't stop anymore. I'm going to look at the DC restorer circuit with the CRT disconnected next.
Question though. You mention that CR769 doesn't clamp if the voltage never goes below 10V. I understand. But the voltage at R768 (on the side of C768) definitely goes below 10V. So something on the other side of R768 (which is the nexus of CR769, CR776 and R777) prevents that signal from going below 60V, correct?
Could this be related to the voltage shift at C768? C768 is pure AC centered around 0 when R768 is lifted but biased to 100V when R768 is connected?
JdR
jrseattle