DECstation 5000 Model 240 PSU Failing Under Moderate Load

The H7878 PSU in my recently acquired DECstation 5000 Model 240 had a shaky start and then stopped working altogether. I found a problem with the input rectifier smoothing capacitors, at least one of them anyway. It was a real job to remove them because they had been epoxied on. I had to cut the first one, somehow I was able to remove the second intact. The one I cut did not have any liquid electrolyte, although the core was damp with it, I am not sure if that is normal or not.

Notice the cardboard disc that was underneath the capacitors. It seemed clean in both cases, so I am not sure if there was leakage or not. The card discs seem to be there to isolate the base of the capacitors from some contacts on the board, so I kept them. The replacement capacitors were slightly smaller so you can see the cardboard discs now.

PSU With New Smoothing Capacitors From Above

After I put it back together I first tried turning it on with no load. The fans twitched once. I assumed that as there was no load the PSU was shutting down immediately. So I put it back in the machine, but nothing happened when I powered it on, the fans did not move at all. I later realised this was probably because the PSU cannot handle the load.

I took the whole thing out again and checked it over. The rectifier voltage across the newly replaced capacitors was looking good. I then measured the outputs of the main transformer. The three outputs seemed to be producing 50VAC, 1VAC and 8VAC. So the inverter and the main transformer seemed to be working fine.

I then realised that the PSU was actually producing voltages on its outputs. I put a 10W 15ohm resistor across the main 5V output and got a voltage of about 5-6V. The ripple looked like it might be high though. So I replaced as many of the output stage capacitors as I could easily reach. These were:

Label Value Voltage
C21 220uF 25V
C36 2.2uF 50V
C26 (see below) 10uF 25V
C24 330uF 16V
C40 120uF 25V
C41 120uF 25V

C26 was a bit different. It was a small purple capacitor made Sanyo, it was marked simply “10 25”. Since it is on the 12V output I guessed this meant that the “25” referred to the voltage. Below are some pictures of this capacitor:

After replacing all these I found that the behaviour was not a lot different, although ripple seemed to be much improved on the +5V output. The 12V fans twitch for a bit and then one of them will start running. The +12V output starts at around 1.5V and then after about 5 seconds or so it rises to +6V, whereupon one of the fans will start running, the +12V output then varies between +5V and +7V, sometimes dropping as low as 4V or rising as high as +8V. This happens when I load the +5V output with a 10W 15ohm resistor, no load for the -12V output, and the fans loading the +12V output.

If I put a bit more load on the other outputs then the +12Voutput won’t rise above 1V or so and the fans don’t even twitch. I tried a 2ohm load on the +5V output and that kills the +12V output, even though the +5V output is rated at 40A. Similarly I tried a 66ohm load on the -12V output and that too kills the +12V output. The -12V output is rated at 0.2A, so I am close to the rated limit in that case.

Basically it seems that the PSU simply cannot cope with much load.

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