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The set will work perfectly for perhaps a few hours, then the
cutout will operate. If this is reset, the set will again work
satisfactory for a while but will eventually trip. This can
happen several times in the course of a evening. A possible
clue is that R609, which is in series with the chopper transistor, tends to burn out. The tripping continues after replacing this resistor. The cutout itself has been replaced, and
everything else appears to be in order.
The cutout operates when the crowbar thyristor W621
fires, shorting R609 to chassis in the process. It seems
therefore that there is repeated crowbar action. Suspects in
this area are the crowbar thyristor W621, the sensing zener
diode W617, and C618 (100µF) which is present in this circuit to suppress transients. Alternatively, the h.t. supply
could be rising so that the trip operates. Disconnect the
feedback amplifier by removing R621: this should give a
small picture, with the h.t. supply falling to about 30V. If
the trip still operates, the chopper transistor is probably
faulty. If the h.t. is rising due to the monostable being held
on too long, check W615. Make sure the print around
W621 is clean – the electrolytics tend to leak.

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