On 2022/11/27 1:21 a.m., Rob Jarratt via cctalk wrote:
I have done a little more probing around. I have found that the 7812 regulator
that drives Vstart on sheet 1 of Tony Duell’s schematic is shorted, so I will
have to replace this too. I have not found anything else that looks obviously
suspicious. I can’t test the output rectifiers for shorts without desoldering
them, which I would rather avoid. I guess the next step is to replace the
broken parts and use the light bulb current-limiter method to power on the PSU.
Regards
Rob
My experience with blown 7812s is that there was a surge on the
unregulated side that went over the maximum input rating for the device.
You may want to add a Transient Suppression Diode on the input if this
is a future possibility or a suitable line protection MOV on the input
just after the line fuse.
John :-#)#
From: Rob Jarratt <robert.jarr...@ntlworld.com>
Sent: 24 November 2022 21:45
To: 'Mattis Lind' <mattisl...@gmail.com>; r...@jarratt.me.uk; 'General Discussion:
On-Topic and Off-Topic Posts' <cctalk@classiccmp.org>
Subject: RE: [cctalk] Identifying a Failed Diode in a Rainbow H7842 Power Supply
Thanks for the suggestion Mattis. The UF4007 has a PIV of 1000V, I had a
suggestion that the PIV should be 200V. Not sure what rating I should be going
for here?
Regards
Rob
From: Mattis Lind <mattisl...@gmail.com <mailto:mattisl...@gmail.com> >
Sent: 22 November 2022 07:54
To: r...@jarratt.me.uk <mailto:r...@jarratt.me.uk> ; General Discussion: On-Topic and
Off-Topic Posts <cctalk@classiccmp.org <mailto:cctalk@classiccmp.org> >
Subject: Re: [cctalk] Identifying a Failed Diode in a Rainbow H7842 Power Supply
Hello Rob!
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.
DEC used a lot of A114x diodes in their PSUs. They looked exactly like that
one. Those are fast recovery diodes.
https://pdf2.alldatasheet.com/datasheet-pdf/view/7563180/2074/A114F.html
I would replace it with a UF4007 or something similar.
https://www.mouser.se/datasheet/2/849/uf4001-2578577.pdf
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?
Also check all other semiconductors. Also on the outputs. If there is a 1 ohm
fusible resistor in the base drive circuit check that one as well. In the VT100
PSUs it happens that it blows.
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-supply-fai
<https://robs-old-computers.com/2022/11/20/dec-rainbow-h7842-power-supply-failure/>
lure/
/Mattis
Thanks
Rob
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