I'm having a lot of problems with DEC power supplies. Newly failed ones are getting added to the end of the queue quicker than I am managing to pop fixed ones off the top of the queue :-(
The latest one to give trouble is a H7822 in a MicroVAX 3100. This machine apparantly never got deployed for the purpose it was purchased for so it has had very little use and it is clean and pretty much dust free inside, including in the power supply. The two SCSI buses only worked intermittently when I got the machine first many years ago. Now, they seem to have failed completely. A single common fault seems to be responsible for both failures but I haven't been able to find it so I use it by accessing disks across the network instead which is a pity because it has lots of room inside for disks. A couple of years ago, the two fans in the H7822 (which are the only fans in the machine) stopped working leading to overheating and shutdown due overtemperature if it was left on for too long. This turned out to be due to a faulty 12V zener diode in the fan control circuit. I replaced the zener diode and that seemed to fix this particular problem. More recently, the machine was working away nicely when it suddenly stopped. Powering it off and back on later didn't help. Months later when I finally got around to looking at it, it still failed to do anything noticable when powered on. I took out the power supply, opened it up, checked for blown fuses or anything else obvious, found nothing, put it back together, applied power (without a load) and the fans started spinning indicating it now seemed to be working. Thinking there might be a short on the system board, I put it back into the MicroVAX, connected it up and powered it on. The fans spun, all the diagnostic LEDs lit and stayed on but the green LED on the power supply did not light. The machine appeared to be held in reset. There's probably a voltage missing from the power supply I thought. While I was getting the meter to check this theory, the green LED came on, the diagnostic LEDs began counting down and the output of the POST appeared on the console terminal. While I was making notes so that I didn't forget what happened, I noticed the fans stopping making noise, the green LED and diagnostic LEDs had all gone out and the console was no longer responsive. It had failed again. I put the meter on the front disk drive connector to check the voltages from the power supply. It showed +5V and +12V as if everything was normal. I put it on the rear disk drive connector and both lines were varying around one or two volts. I plugged in an AUI tranceiver which happened to be to hand and the LEDs on it varied with the meter readings. I tried connecting a test lamp to the +5V and +12V lines on the front disk drive connector to load it a bit. They held steady while the LEDs on the AUI tranceiver continued to vary. It seems there are at least two independent regulation systems in this power supply. I left things as they were and went to make more notes. A few minutes later, I heard the fans starting up again. I looked at the machine and the green LED on the power supply was on, the diagnostic LEDs were counting down, output was starting to appear on the console and the LEDs on the AUI tranceiver were lighting normally. I've tried power cycling it several times since and it doesn't seem to want to fail now. Access to the H7822 is not great. There are two boards (each with permanently connected daughter boards), one mounted in the base and one mounted in the lid. The boards are connected together at one end by about twenty flexible wires and at the other end by two two core cables which can be unplugged to allow the two boards to be laid out flat end to end. I suppose I could make up extensions to the two two core cables to enable better access for testing. However, it doesn't look easy to devise a layout where everything can be cooled by the fans for testing over an extended period of time. At least the H7822 does not seem to suffer from the leaky capacitor problem like the H7821 does. I have another MicroVAX 3100 which looks like it should have a H7822 in it too but I'm not sure how this could be helpful given the symptoms. Any suggestions on how to further diagnose this nightmare? Regards, Peter Coghlan.
