Smoke from a Kaypro II. The fault and the fix

Kaypro IIIntroduction

Boys love their toys and I’m no exception. Some of my computers I regard as sheer treasures due to their fine external condition, cultural impact and just plain retro-coolness.

My Kaypro II fits the description above. While I love it, it doesn't seem to love me. First there was the drive issue, and now a couple of things happened which suddenly dumped a couple of un-looked for hardware projects on my lap. The first of these is the subject of this entry …

Holy smokes!

Here's what happened . A few weeks ago I was building a software library for the machine. I'd had the computer running for about 1/2 an hour when I heard a sudden pop, and smoke started to issue from the back. Alarming? Well, yes three years ago it would have been. However, after experiencing this first-hand in two other machines (an Osborne 1 and BBC micro) this time I barely raised an eyebrow. I knew exactly what this was. I simply left the Kaypro going and continued what I was doing.

Complacent? Probably, but I was fairly confident of the cause. These old computers of the early 80s all have RFI suppressor capacitors in the power supply to strip away any RF interference the machine might put into the AC lines. They are usually made of rice paper and old ones almost ALWAYS blow eventually. This failure is invariably accompanied by a pop and acrid smoke. Blown capacitors of this nature don’t affect operation of the machine at all, but it is good to fix them for the sake of other appliances on the same AC line whose performance may be degraded by the stray RF signals.

The Fix

The next day I removed the hood and checked out the power supply. Sure enough, one of the smaller RFI filters was blackened and burnt.

Kaypro II - blown PSU capacitors

Picture 1. Blown RF filter capacitor in the power supply unit (PSU)

Checking in the parts bin I found some spare filters left over from my Osborne fix. Only problem was they not the same capacitance; the Kaypro specs being 4.7nf and these ones being 10nf. However, the feeling from friends on the Vintage Computer Forums was that the ones I had would do fine. I not only replaced the one that had blown, but also the other two similar sized caps that had not. I felt while I had the PSU naked, I might as well do the lot, especially as I had the parts.

The larger 0.22uf one there is also likely to go at some stage. However, I had no spares on hand so that one was spared the indignity of replacement...for now (-:.

Kaypro II - Fixed capacitors in the PSU

Picture 2. Replaced RF filter capacitors

The caps were duly replaced (figure 2), the PSU was reattached, the computer reassembled and tested. Everything was fine....

Kaypro II from front

Picture 3. Kaypro all fixed!

I was happy again...At least for a few days….and then something else failed!

And this time a diagnosis and fix was a lot more challenging! See Part 2....


15th January, 2010

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