Problem will almost always be Q801 regulator IC.
If BEFORE shut down the B+ rises above 125VDC try that first.
If you have a variac run the set at abt 90-100 volts. for abt 5 seconds
& see if the horz output Q402 is getting hot. If so that points to the FBT
almost always.
73 Zeno
LFOD !