I have finally fixed the H7878 PSU from my DECstation 5000 Model 240. When I last posted about it, the PSU was not working very well, it was unable to sustain any load. Following that post I asked for suggestions on the ClassicCmp mailing list.
The first suggestion was to see if there were any significant dips in the voltage across the input smoothing capacitors. But the voltage was rock solid around the 330V mark on the DM DC range of my Digital Multi-Meter.
The second suggestion was that perhaps something was shorted on the outputs, causing the overcurrent sensing to switch it off. Some suggestions talked of checking low value resistors for shorts, there are two 10ohm resistors, and one looks like it has a bit of physical damage, but they both measured fine. I also checked the capacitors across the 5V output, particularly the ones that I had not by then replaced, they seemed OK. Nevertheless I decided to replace the two big ones across the 5V output just in case, particularly the 3300uF one (can’t see a label to reference it) which measured a bit high on ESR, the 8200uF one also had a relatively high ESR, so it was worth replacing too. Replacing them failed to make a difference.
I then had a closer look at the riser board on the output section, as this is probably doing the regulation. I noticed that C407 (120uF 25V) was measuring high on ESR, and then I realised there were signs of leakage. There were also some marks (heat perhaps?) around C408 (120uF 16V). I decided they should be replaced.
Having removed C407 I realised it had definitely leaked. Reaching C408 was going to be hard, so I removed the whole riser. This would make it much easier to repair and clean, and I could see that there appeared to be some cleaning needed. Once I got the riser out I could see some shiny residues on the underside, I am not sure that they can be capacitor leaks, but they did look a bit like it.
I finished replacing C407 and C408 and cleaned everything up. This is the result:
I re-installed the riser and tested it again. I used the MicroVAX 2000 load board on the 5V output. This time all the fans turned immediately, the 5V output looked good, and there was no significant ripple. The 12.1V output measured 12.4V though, I am not sure if that is OK or not.
I partially reassembled the PSU and installed it in the machine. The machine ran fine, I ran the self tests a few times and left the machine running for around an hour or more and it stayed running. So I finished reassembling the PSU, re-soldering back the LED wires and finally reassembled it again.
I replaced quite a few capacitors in the end:
Here it is, working once more:
Rob's Old Computers 
Hi, I hope you can help me, I have a 5000/25 decstation the PSU is an HP7817-AA, connecting the power cable, the fan makes a half turn and stops. I took the PSU to a technician to repair it, he told me that the power supply works, but waits for a signal from the motherboard to activate. Do you know this? Should PS PSE work by itself?
THANK YOU
Hi, I hope you can help me, I have a 5000/25 decstation the PSU is an HP7817-AA, connecting the power cable, the fan makes a half turn and stops. I took the PSU to a technician to repair it, he told me that the power supply works, but waits for a signal from the motherboard to activate. Do you know this? Should PS PSE work by itself?
THANK YOU
I don’t know this power supply so I can’t say for sure. It is possible that the PSU receives some kind of signal from the system board, but I can’t say I know this in your specific case. It is also quite likely that the PSU is shutting down of its own accord because it is detecting an overvoltage or excessive current draw due to a shorted component somewhere.
Thanks for your answer, I think the power supply works the same way for the DEC STATION family, so I was asking you if your power supply, if it is not connected to the MB, still works?
Apologies for misunderstanding your question. On my DECstation 5000/240 it was not necessary to connect the PSU to the machine itself, but it did need an actual load in order to operate correctly. I usually pick an old hard disk drive that I don’t care about and plug that in, although I had a comment recently that they may not present enough load, so you may need to use a few hard disk drives, or some older ones, or just anything that you don’t care too much about.