it’s not the tube! i have replaced with a known good one, and it is the same result…
also replaced all relevant transistors, except mpsa42, nothing.
i suspect the high voltage generator, maybe c6, c10, c11 or L1: if i heat them with a hair dryer, in several seconds the voltage drops to 300v and the duty cycle goes up to 90%, and stays there, even after the temperature goes back to normal (20c).
on the other (working) unit, the duty cycle goes up quickly to 90% when heated, but the voltage stays around 360v. and after it cooled down to room temperature, the duty cycle goes down to normal (60-70%).
.
