> -----Original Message-----
> From: Peter Coghlan via cctalk <[email protected]>
> Sent: 10 May 2023 12:04
> To: 'General Discussion: On-Topic and Off-Topic Posts'
> <[email protected]>
> Cc: Peter Coghlan <[email protected]>
> Subject: [cctalk] Re: Rainbow H7842 PSU Fault
>
>
> >
> > 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.
>
So, I did the following test. I used my bench PSU to apply +12V to the
regulator output and -12V to the actual -12V output. Under these conditions the
PWM operated correctly. I repeated the test on the good PSU and the result was
the same.
The good PSU drew 13mA on the -12V line and 92mA from the regulator side. The
bad PSU was 16mA and 86mA respectively. When I test only applying +12V to the
regulator output, the good PSU draws about 90mA and the bad one 140mA.
It is worth repeating that the PWM operates correctly on the good PSU when I
only apply +12V to the regulator, but not on the bad one. I guess there must be
some kind of short somewhere, but really not sure where it might be. It has to
be something that is pulling the -12V line slightly higher. The only place
where this seems like it could happen is around E1b I think. Perhaps there is a
path through the positive input to E1b to Vcc on the LM393 (Control Module
Sheet 1). Does that seem logical?
> 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?
The component nearest to the exploded transistor is the 10uF capacitor on the
output of the 12V regulator. There are some carbon deposits on it. I did a
cursory check for resistance and ESR and it seemed OK.
Regards
Rob
