My second PDP-8/M is just about back in the land of the living. I followed largely the same process as with the restoration of my first 8/M. Luckily this examples case wasn’t corroded and so I didn’t bother respraying and left it in original condition.

The main sticking point with getting this machine back was a failed +5V and +15V rail on the PSU. Being no expert on PSUs and having been mildly ‘warning’ zapped by it when I decided to see if anything obvious was failed, I had a hunt for a company to perform the repair.

I opted for Electronic Repair Centre Limited after some internet searches. I was very impressed with their service, which was fast and very well priced. I thoroughly recommend them to anyone looking to get a power supply repaired.

After getting the PSU back, I reassembled the machine and with a bit of CPU board swapping (3 of the originals turned out to be failed), I now have a mostly working machine. The one gotcha I currently have is that on entering the DEC accumulator counting test, the accumulator counts at full bore and cannot be seen neatly increasing on the console. My suspicion is that the line time clock might be the cause of this.

The investigation continues…

The PDP-8/M uses a H740 switch mode power supply that provides +15V @ 1A, +5V @ 17A, -15V @ 5A together with LTC-L, PWR OK L, PWK OK EN, AC LO L, and DC LO L signals (it should be noted that not all of these signals are used within the PDP-8M). This power supply is also used within the 5.25″ PDP-11/05 and PDP-11/10.

To rebuild, I stripped the supply down and gave it a gentle clean with electrical cleaning fluid and a clean cloth. A paint brush was used to dislodge any dust in hard to reach places. The electrolytic capacitors were then removed and reformed.

The power supply was then reassembled and plugged back into its mains connector in the chassis. Power was then applied for short periods of 1 second, 5 seconds, 10 seconds, 30 seconds, 1 minute, 5 minutes, 10 minutes.

After each application of power, I carefully touched the cases of the capacitors to confirm there was no build up of heat. Care should be taken as these beasts hold a lot of charge for a long time, believe me you really don’t want to find out how much it hurts to have one discharge through you.

Once I reached 1 minute, I also tested the +5V, +15V and -15V DC ouptuts to confirm all was well. Luckily for me it was.

The final step was to hook up an oscilloscope and to check each of the power rails to check that they were stable and to confirm all of the control signals were working correctly. The line time clock (LTC-L) is the only signal of note when checking with an scope as this should give a 50HZ (or 60HZ in US) square wave.

Having passed all checks, the power supply was reinstalled into the chassis and connected back upto the OMNIBUS backplane. The consists of 4 connections, -15V, +5V, +15V and the PWR OK L signal (spade marked DV2)

Useful Links:

• H740 Schematics (PDF)
• Article on reforming capacitors