Repairing a Dick Smith System 80 / Video Genie/ PMC-80 Power Supply Unit

The issue

Yesterday I fixed some erratic disk-based behaviour in one of my Dick Smith System 80s. The problem turned out to be a faulty PSU. I worked around the problem by swapping out the bad PSU with a good one from a "spare-parts" unit.

The "spare-parts" System-80, although a little scruffy, actually works. It nagged me that I hadn't really "fixed" the problem, just transferred it from a tidy machine to a not-so-tidy one. I decided therefore to go the whole way and actually repair the flaky part.

The repair

I consulted the System 80 Technical manual. This had a troubleshooting section which dealt specifically with the PSU. It didn't describe the problem I had with the system rebooting and locking up with drives attached, but it did mention a wavy screen. In fact the image was a little wavy with this faulty power supply. I didn't think it was TOO bad, but it certainly moved around more than the video on the other computers. The machines with good power supplies had a rock-solid screen display.

The manual gave two suggestions. Replace a couple of diodes and/or the large 22000uf capacitor. I picked up the diodes no problem but couldn't find a large 22000uf capacitor in my local hobby electronics store. I guess they are just not that common anymore. The power supply also contained a 2200uf capacitor. Although it wasn't mentioned as a source of the wavy screen symptom I got hold of a replacement for this too. It never hurts to replace old caps and I might as well drop it in while the case was open. Figure 1 shows the open PSU, and the components that were replaced.

Figure 1. An open Dick Smith System 80 PSU showing the components replaced

Result

In my mind I was sure it was the large 22000uf capactor at fault. Too bad I couldn't source one locally, but I felt I had nothing to lose in trying the new diodes and smaller capacitor. After replacing the components, I reinserted the PSU then measured the voltages on the main board. To my surprise readings were now 5V instead of four. The screen now showed no signs of waviness at all! I hooked up drives to the machine with this PSU and everything was now fine!

The fix was a complete success!

The fault must have been with either one of the diodes or the smaller 2200uf 25v capacitor I replaced. Anyway, it's all good now. Three good System 80 PSUs and three working System 80s! Sweet!

Tez
30th June, 2010