10 May
2023
10 May
'23
11:03 a.m.
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.