[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Antonio Carlini via cctalk]

On 20/11/2022 21:03, Rob Jarratt wrote:

Thanks Antonio,

The location of the diode is arrowed on this picture: 
https://rjarratt.files.wordpress.com/2022/11/img_20221120_205802-arrowed.jpg

You can also see the heatsink where the transistor used to be.



This is mine: 
https://drive.google.com/file/d/1igM9AqrICX93t-KHH3N4gRCr1Z8H6tAY/view?usp=share_link, 
so as you can see, the pick-and-place machine decided to rotate the 
diode for almost maximum inconvenience!



I have two more I can open up and look at, but I cannot get to them 
tonight and I'm probably out tomorrow too. But I think I can get to 
those other two supplies on Wednesday. Hopefully at least one of them 
will be readable! Otherwise I can desolder the diode from one of the 
other two and hopefully find a useful marking.



Antonio


--
Antonio Carlini
anto...@acarlini.com

[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Mattis Lind via cctalk]

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
>
>