I had the same problem with a Hitachi set. After some hours of
searching, it turned out to be a leaky capacitor between the collector
and the emitter of the H.O.Transistor (.39 mfd at 1.6 KV). It caused the
emitter voltage to rise slowly and then turn on the shutdown circuit.

Thomas Abell wrote:
Does anyone recall anything concerning a shutdown problem after replacing a
Zenith flyback with a generic? I have a SL2784BT using a 9-1268 sweep board
that will shutdown after about 20 seconds. Turning the set back on causes
the cycle to repeat. I used a generic fly from Tritronics. Thanks!