H7826 Power Supply Repaired

After a spell of concentrating on other things I have finally got round to looking again at the H7826 power supply from my TURBOchannel Extender. When I left it back at the beginning of the year I was waiting for parts for the 5019572 daughter board which controls the switching transistors. I posted the details here, including a schematic of the daughter board.

Once I had replaced the parts on the 5019572 daughter board I bench tested it in isolation. I had to make sure that Vcc was above the ON threshold which is between 15V and 17V. The board drew what appeared to be a reasonable 20mA and the 555 started oscillating. So I reinstalled the daughter board.

With the 5019572 daughter board back in place I found that the 555 was not oscillating. This was because its Vcc was 0V. It looked like there might be a blip on Vcc when applying power, but I don’t have a DSO so I couldn’t be sure. Checking Vcc for the daughter card as a whole I found that it went up to about 15V on applying power, but then it almost immediately dropped to 0V. After removing power it stayed at 0V for a bit before jumping back up and finally decaying slowly.

This told me that the daughter board was either receiving a shutdown signal or it was sensing an overcurrent. A bit more probing showed that it was the shutdown signal. I traced this to another daughter board on the secondary side, labelled 5019574.

I removed the 5019574 daughter board and reverse engineered its schematic, which is reproduced below:



I tested the board’s current draw on the bench at 5V and it looked OK. While I was at it I also reverse engineered the schematic for the 5019576 daughter board, which is right next to it, as they are both involved in monitoring for shutdown conditions. This schematic is reproduced below:


None of the parts on these two daughter boards appeared to be bad. I did replace the LM393 on the 5019576 daughter board because during testing it had appeared to get hot, but I later realised I had made a mistake with how I had connected up the board, so I may have replaced it unnecessarily. However the PSU still would not work, the fans would twitch on applying power and that was all. One of the electrolytic capacitors on the 5019574 board had measured a bit high on ESR, and it was also looking a bit brown as if it had been subjected to a lot of heat, so I replaced it. But still the PSU continued not to work.

Then I realised that I could not trace one of the tracks connecting the 5019574 daughter board to anything on the main board. The track was visible but did not seem to be electrically connected to anything. I noticed corrosion on the track (this power supply is not in the best condition). I asked a couple of friends who have this power supply to confirm where this track went. Once I knew where it was supposed to go, I soldered a wire to the back of the main board to replace the broken track.

H7826 Repair of Corroded Track

Wire to replace a corroded track

This seemed to do the trick! I connected a load module from a MicroVAX 2000 and now the fans spin and there are voltages on the outputs. I checked the ripple on the 5V and 12V outputs and that was good. The LED does not seem to work, but the PSU powers up a hard disk and a PMAD-A card installed in the TURBOchannel Extender, so I think it is all working.

Now I need to find a cable to connect the TURBOchannel Extender to the DECstation 5000/240. I have found a 100-pin SCSI cable which fits but I don’t know if the pinout is correct so I dare not try it.

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3 Responses to H7826 Power Supply Repaired

  1. Pingback: DECstation 5000/240 With the TURBOchannel Extender | Rob's Old Computers

  2. S. Kraus says:

    Hi! Thanks for your post!
    I have a Decstation with a partially broken 7826 PSU as well. My fans are not spinning, but is seams I have 5V and 12V, but both the 7912 and the LM337 do not get any input power.
    Do you maybe have an advice?
    I am as well looking for the pinout of the connector that goes to the board. Do you maybe have this?
    Thanks a lot

    • rjarratt says:

      I don’t have the PSU in front of me and cannot locate the parts you mention in my schematics. Have you looked at my schematics? I have not worked out the pinout for the connector, sorry.

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