> -----Original Message-----
> From: Peter Coghlan via cctalk <cctalk@classiccmp.org>
> Sent: 20 May 2023 09:20
> To: 'General Discussion: On-Topic and Off-Topic Posts'
> <cctalk@classiccmp.org>
> Cc: Peter Coghlan <cct...@beyondthepale.ie>
> Subject: [cctalk] Re: Rainbow H7842 PSU Fault
>
>
>
> Ok, it looks like there is not a severe leak from the -12V line to ground
> then.
>
> I am puzzled by the extra current draw on Vstart by the bad PSU but I'm not
> sure that tracking this down would lead us to the real problem.
>
> On the other hand, did you mention at one point that Vstart was varying?
> If this is the case, the reason for this would probably need to be found and
> fixed independent of whether it leads to finding the main problem as this is
> supposed to be a stable supply.
>
> I don't think there is likely to be any serious leakage via E1b because the
> link
> to the -12V line is via a 75K resistor which would limit any leakage current
> to
> roughly 160uA. Of course this applies if the resistor really is 75K and
> doesn't
> have carbon deposits bridging the tracks and connections around it to
> somewhere else.
>
> I would suggest looking carefully at the resistors around E3d to make sure
> they have the correct values, especially the 360K resistor and making sure
> there is no debris etc around these components that could be bridging any
> connections associated with them to somewhere else, also that no
> connections have been severed. Problems here could be leading to E3d
> falsely triggering when there is no real overload.
>
> It might be useful to check the voltages and resistor values in the -12V
> regulator and compare with same in the good power supply, especially the
> voltage across the zener diode.
>
> > >
> > > 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.
> >
>
> This capacitor is probably there to ensure the 7812 doesn't oscillate.
> Looking
> at Vstart with an oscilloscope should confirm that this is not an issue. If
> it
> doesn't have excessive leakage current and has approximately the correct
> capacitance, it is probably ok. However, if there is gunk trapped underneath
> it around the leads, this might account for the extra current draw on Vstart.
>
> The explosion could have had other bad effects. Maybe E3 got damaged by a
> surge in its power supply when the transistor blew up? Maybe the -12V
> rectifier was affected? It is probably not as robust as the rectifiers for
> the
> other lines and the chopper transistor shorting would have likely caused a
> big current pulse in the chopper transformer primary, leading in turn to
> surges at it's secondaries. Also the diode in parallel with the 51R sense
> resistor might be suspect.
>
> I'm not sure how to test these components comprehensively without trying
> replacements for them.
>
> If the 7812 was damaged at the time of the explosion, other components
> powered from Vstart could have experienced surges as well. Maybe stuff on
> the input side of the 7812 too?
>
This evening I went to check Vstart for any oscillation. However, all of a
sudden, the current draw is down to 85mA and PWM has started working. I am at a
loss to explain it. I wondered if there might be a dry joint, but I have tried
a few light taps and shakes and it continues to work. Perhaps your idea of some
debris causing a short might explain it, otherwise I just don't know.
I am thinking I may put it back together and test with a light bulb in series.
Regards
Rob
