Thanks for the analysis. There doesn't seem to be anything further back from
the diode, unless you mean further back behind the inductor?
I probably do. I guess the base drive for the main switching transistor
comes from another smaller transistor via the inductor and the diode which
failed? Or maybe the diode is in parallel across the drive voltage? Anyway,
it is this smaller transistor providing the drive that I'm suggesting
checking next as it may have got a belt of anything up to three hundred and
something volts DC when the main switching transistor shorted out an instant
before it exploded. If that one has failed too, check further back to
whatever is driving it.
I will check on the output side as you suggest.
The rectifiers on the output side may be tedious to figure out and test
due in part to the very low resistances of the transformer windings
feeding them unfortunately, especially if there are double rectifiers
(maybe in packages which look suspiciously like transistors) connected
to either side of centre tapped transformer windings.
The circuit breaker did pop out when it failed. The onboard fuse is intact.
If you mean the house breaker as opposed to a breaker in the power supply,
I would guess it was probably rated at 25A or more and given the
destruction of the transistor, I would think the instantaneous current
was way higher than that. Perhaps the fuse is a time delay one. It
seems likely things could have been worse if the breaker wasn't able
to cut the power as soon as it did and you had to wait for the fuse
to blow :-(
The house RCD triggered and cut the power to the whole house when the
I guess that the transistor exploding produced a momentary cloud of
ionised gas which allowed arcing/tracking from the live transistor
terminals to something earthed (power supply case maybe?) which
triggered the RCD. Or maybe the heatsink the switching transistor
is mounted on is earthed and the insulating washer between the
heatsink and the transistor failed causing it's destruction and
the heatsink then became the path to earth that triggered the RCD?
Do you, or anyone else, have an idea what the diode could be so that I can
find a replacement. Like I said, it seems to be marked D610, and there are
some other ones that look to be the same elsewhere in the PSU.
Sorry, I have no idea what the diode is. Hopefully Antonio will be able
to help with that. Or if your spare power supply is the same design as
the failed one, can you look at the diode in that one?
It has dawned on me that another mechanism for the switching transistor
to have failed so spectacularly is for it to have been switched on hard
and held on for more than an instant by something driving it resulting
in it effectively shorting out the rectified mains supply through the
primary of the chopper transformer.
When you next try powering it up, it might be good to use the old
(filament) light bulb in series with the mains supply trick in case
whatever caused the initial problem is still lurking in there. This
can be useful for locating shorted output rectifiers too. You might
see a slight voltage rise from the outputs with working rectifiers when
power is applied through the light bulb and a lesser or no voltage rise
on an output which has a shorted rectifier or other issue that needs
closer inspection. It also means there is no need to hide behind the
sofa when you switch it on :-)
From: Peter Coghlan via cctalk <cctalk(a)classiccmp.org>
Sent: 20 November 2022 18:50
To: General Discussion: On-Topic and Off-Topic Posts
Cc: Peter Coghlan <cctalk(a)beyondthepale.ie>
Subject: [cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power
I'm only guessing here. I think the sequence may have been that the main
switching transistor failed first as it would be under more stress than a
in the base circuit. If the transistor shorted
E-B-C then the HT would
connected to the circuitry at it's base which
would be compelely unable to
cope with voltages and currents involved. This probably resulted in the
failure of the diode. I think it may be worth looking at the components
further back the drive chain from the diode.
The inductor could be ok unless it is a very frail little thing but small
semiconductor components and/or resistors further
back may not have
fared as well as it.
It might also be worthwhile checking for shorted rectifiers on the output
in case this was the cause of the stress on the
However, the power supply might have an overcurrent trip to reduce the
possibility of this sort of damage. If there is an overcurrent trip or
trip, this may have been reset after the power
supply was powered off for
while and when it was powered on again, the
already damaged transistor
could have been teed up to fail more spectacularly? Like I said, just
Were there no fuses failed or cutouts cut out? Does it look like there
have been? I would think a shorted switching
transistor should have caused
some safety device to operate. Or is it the case of the old adage that
faster acting transistor managed to sacrifice
itself in time to protect
> blow fuse from blowing?
> Peter Coghlan.
> > The H7842 PSU in my Rainbow failed yesterday. At first the machine
> > just powered down and there was a slight burning smell, I wasn't next
> > to the machine when this happened, so I didn't see or hear anything to
> > tell me where the problem might be. Not being sure if there was a
> > short in the machine or a problem in the PSU, I disconnected the fans,
> > FDD and HDD and, probably foolishly, I applied power again to see if the
> machine would work.
> > At this point there was a bang and a flash in the PSU.
> > On opening up the H7842 power supply I found that one of the
> > transistors had completely disintegrated. It looks to be the main
> > switching transistor, here is a picture of it:
> > https://rjarratt.files.wordpress.com/2022/11/img_20221120_165850.jpg
> > I have identified a source for this transistor, but if anyone can
> > suggest a modern replacement that would be useful too. However, that
> > is not my main problem.
> > Given that before the transistor blew up there had clearly been
> > another failure somewhere else, I tried to find the original failure.
> > There were no obviously damaged parts, so I just probed around near
> > the transistor for any parts that were open circuit or short circuit.
> > I found a diode connected to the base of the transistor that appeared
> > to be short circuit. So, I decided to lift one end to check it. As I
> > de-soldered one of the leads, the diode broke in two. So clearly the
> > diode was either damaged by the failure of the transistor, or it was the
> cause of the failure. This is the diode:
> > https://rjarratt.files.wordpress.com/2022/11/img_20221120_165913.jpg
> > I can't quite make out the markings on the diode to know what to
> > replace it with. I think it says "D610". Would that be the right
> > designation? If so, can anyone suggest a suitable replacement please?
> > The diode seems to connect an inductor to the base of the switching
> > transistor and the collector of the transistor is connected to a
> > transformer. Should I be looking for other failed parts? Not sure if
> > the diode failed first and then caused the transistor to fail? Or if
> > something else has failed which caused these parts to fail?
> > I do know that there are no shorts in the Rainbow itself, because I
> > have a spare PSU that still works fine in the same machine.
> > I blogged this here (it repeats most of that I have said above):
> > https://robs-old-computers.com/2022/11/20/dec-rainbow-h7842-power-
> > ly-fai
> > lure/
> > Thanks
> > Rob