> > I will do all the suggested checks, but I won't be able to do this for a > number of days. However, I wanted to understand something in the meantime. > The conditions I am applying are (I think!) what would happen during > startup, and during startup the control board has to make the PWM run, > otherwise the main switching transistor won't operate and the transformer > won't operate to produce the -12V in the first place. So surely in the > startup condition I am applying (which is to supply Vstart from a bench PSU) > is valid? Where is the flaw in my reasoning here? >
The PSU also gives a kick start to the -12V line by applying some negative voltage probably around -15V to it via a 2k7 resistor (on sheet 1). If you are not providing this, perhaps this is why the -12V line is able to swing slightly positive and upset conditions in the -12V current sense circuit enough to prevent the PWM from starting? Even if you are providing this resistor limited -12V startup supply, could it be insufficient due to a faulty component leaking current from the -12V line to ground meaning that the -12V line does not become sufficiently negative to fulfil the startup conditions? Could the PWM be managing to start up and run for a cycle or two and then stop due to a fault in the PWM circuit somehow attempting to draw too much current from the -12V line? I don't see any connections from the -12V line to the PWM so this is probably not the case. Is this the same PSU whose chopper transistor exploded a while back? Could there be any carbon deposits remaining on the board or conductive remnants wedged under components etc causing leakage from the -12V line to ground? Regards, Peter.
