27 Nov
2022
27 Nov
'22
4:06 a.m.
On 27 Nov 2022, at 10:21, Rob Jarratt
<robert.jarratt(a)ntlworld.com> 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.
I think that a good thing is to ramp up the input voltage slowly. Use a bench PSU to
supply the Vstart voltage and then use a variac with insulation transformer to feed the
rest if the supply. Use some small loads on the 5V and 12V outputs. Now you can safely
probe the PSU and monitor base and collector voltages of the main switch transistor and
see that everything looks fine. Check output voltages. Stay below 50VAC input and very
little harm can be done. Could be good idea to have an amp-meter inline with input AC from
the variac to find out if there is short somewhere.
If this works then try with full input. Remove any scope probes…
/Mattis
Regards
Rob
From: Rob Jarratt <robert.jarratt(a)ntlworld.com>
Sent: 24 November 2022 21:45
To: 'Mattis Lind' <mattislind(a)gmail.com>om>; rob(a)jarratt.me.uk; 'General
Discussion: On-Topic and Off-Topic Posts' <cctalk(a)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 <mattislind(a)gmail.com>
Sent: 22 November 2022 07:54
To: rob(a)jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts
<cctalk(a)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
lure/
/Mattis
Thanks
Rob
