26 Nov
2022
26 Nov
'22
10:47 a.m.
Just trying to decide what to replace the failed diode with, and looking at the UF400x
series, as suggested by Mattis. It seems to me that as long as the PIV is 200V or higher
it should be fine from that point of view, the switching speed is never higher than 70ns,
while the original A114x (assuming it *is* an A114x) has a switching speed of 200ns
(possibly even 200us from the datasheet).
However, I am wondering about the forward voltage drop. The datasheets suggest that the
A114x parts have a 1.3V forward voltage drop. I have a spare H7842 that was working (until
I messed it up today, another story), so I tested the diode in that, its forward voltage
appears to be 0.5V, using a little tester I have. The UF400x have ratings of either 1.0V
or 1.7V.
How sensitive is the circuit going to be to the forward voltage on the diode? Given that
the forward voltage of the suggested replacement is higher, would it slow down the speed
with which the transistor is switched off too much and cause it to be overloaded and
fail?
Thanks
Rob
-----Original Message-----
From: Peter Coghlan via cctalk <cctalk(a)classiccmp.org>
Sent: 25 November 2022 09:44
To: General Discussion: On-Topic and Off-Topic Posts
<cctalk(a)classiccmp.org>
Cc: Peter Coghlan <cctalk(a)beyondthepale.ie>
Subject: [cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power
Supply
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.ht
ml
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-
supp
ly-failure/
<https://robs-old-computers.com/2022/11/20/dec-rainbow-h7842-power-
sup
> ply-failure/>
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