25 Nov
2022
25 Nov
'22
1:44 a.m.
It is often possible to infer the component ratings needed from the other
components around them. A component in the base circuit of a transistor is
likely to experience lower currents and voltages than one in the collector
circuit.
In this case, we can see from Tony's diagram that there is a 2.7 Ohm resistor
in parallel with the diode. Suppose it is a 1W resistor. This means that
the from P = I squared R, the average current the resistor is likely to pass
is less than 1A. From Ohm's law, V/I = R, this means the average voltage
across the resistor is likely to be no more than 2.7 Volts.
It is possible that the peak current / voltage involved could be higher than
the average for short periods of time but we have plenty of margin for error
here so we don't need to think about that too much. A diode with a PIV of
200V should be fine here.
Regards,
Peter Coghlan
Tony Duell has reverse engineered the following schematic.
http://www.bitsavers.org/pdf/dec/rainbow/duell_schematics/psu.pdf
I will go with the 1000V as you suggest anyway.
Thanks
Rob
From: Mattis Lind <mattislind(a)gmail.com>
Sent: 25 November 2022 07:12
To: rob(a)jarratt.me.uk
Cc: 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
On 24 Nov 2022, at 22:45, Rob Jarratt <robert.jarratt(a)ntlworld.com
<mailto:robert.jarratt@ntlworld.com> > wrote:
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?
Given that I didn’t have a schematic and this is on the primary side I went
for the recommendation of 1000V. 200V may a bit low on the primary side
depending on the application of the diode. On the primary there can be
sustained voltages up to 400V and peaks that go even higher. Using a diode
with higher PIV almost never affects the operation as long as other
parameters stay the same. In this case the most important parameter is the
trr. It has to be a fast recovery diode. In this case the UF4007 is slightly
slower than the UF4004. But I doubt it has a big significance. Actually the
A114 is much slower. 200 ns.
:Mattis
Regards
Rob
From: Mattis Lind <mattislind(a)gmail.com <mailto:mattislind@gmail.com> >
Sent: 22 November 2022 07:54
To: rob(a)jarratt.me.uk <mailto:rob@jarratt.me.uk> ;
General Discussion: On-Topic and Off-Topic Posts <cctalk(a)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-fa…
<https://robs-old-computers.com/2022/11/20/dec-rainbow-h7842-power-supply-failure/>
/Mattis
Thanks
Rob
