Classic Computer Magazine Archive COMPUTE! ISSUE 20 / JANUARY 1982 / PAGE 150

PET Repairs For The Amateur

Louis F Sander Pittsburgh, PA

My small keyboard PET has had several awful-looking symptoms over the past year, and each time I dreaded the size of the possible repair bill. But each time I was able to cure the problem myself, with no need for knowledge of digital electronics. Based on first-hand experience, and on many notes compared with others, here is what to look for when your PET is acting strange: loose connections, period.

Loose connections are probably the most frequent source of trouble in PET-like electronic equipment, and they are often the easiest to find and fix. You'll learn how I found mine, after a few words on safety. First, never look for trouble with your PET plugged in. Under normal circumstances, all lethal voltages are kept away from PET's main circuit board and other exposed parts, but when trouble comes, circumstances aren't normal. So always pull the plug when you're troubleshooting. Also, always take pains to avoid static electricity when you're poking around inside your PET. Tiny sparks that you can't see or feel can ruin some of the IC's in there, so don't take any chances. The best precaution is to ground yourself by touching bare metal on the cabinet whenever you touch an IC or the circuit board; it may look silly, but it's safe. Now for my war stories:

My first trouble was erratic operation. From time to time, I'd get a screen full of garbage, and my cassette motor would run and run. It looked like my reset button was locked down, but I knew it wasn't. On the advice of somebody who knew, I looked for an IC that was loose in its socket. When I found it, the trouble went away. With time and the flexing caused by neat, IC's all tend to walk out of their sockets. If you have symptoms of trouble, check this first. Open your PET and, with one hand touching the cabinet, firmly press down on both ends of every socketed IC, and walk them back into place. You'll be surprised how many are loose. Don't worry too much about flexing the printed circuit board itself — it can withstand a bit of bending.

My second problem came from a bad power connector. I'd lose everything on my screen, right in the middle of something important. At other times, I'd power up and not be able to get anything on the screen at all. When I found a hot power connector, I knew the cause was found. The power connector attaches your main circuit board to the wires coming from the large transformer and electrolytic capacitor at the left rear of PET's base. If you are having problems, especially ones that crop up after some length of'on' time, run your machine for an hour or so, then feel the power connector. If it's noticeably hot, it is a candidate for replacement. I replaced both ends of my connector with Radio Shack 274-226 and 274-236, for under $3.00 total. If you're not an experienced electronics person, turn this job over to an expert — it's easy, but the new connectors are far from exact replacements.

My biggest and most mysterious problem was caused by a dirty contact on the connector between the main board and tape drive #1. For several months, I'd get strange screen messages and frequent system crashes whenever I tried to load a program that was other than the first one on a cassette. I'd say LOAD "RINKYDINK," the tape would start to move, and then I'd get some horribly misspelled version of PILLEGAL QUANTITY ERROR, sometimes before and sometimes after the PET had FOUND the programs preceding RINKYDINK. It got so bad that I gave up on ever being able to put more than one program on a tape. I could tell that the problem was associated with the unrecorded gaps between programs, but that's as far as it went.

I found the problem one day as I connected an audio amp to the tape READ line. The recorder was running a totally blank tape, and the noise on the READ line was tremendous. I accidentally jiggled the wire going from the recorder to the main board, and the noise stopped completely. Later I found that a poor ground contact on the PC board connector was allowing motor noise to get into the signal circuits, and that PET was trying to read the noise as data. No wonder it got an ILEGAL QUANIY ERRR! Two minutes with superfine sandpaper cured the problem, and now I can read through a whole C-60 with no system lockups. Keep your connectors clean.

By the way, I've had one minor problem unrelated to bad connections: My PET likes to read tapes a lot better without any amplifier connected to CB2. I don't know just why, but the machine definitely works better with nothing connected back there. So now I disconnect the amplifier whenever I'm through with a program that uses sound. I guess this really is another loose connection problem, but one of a different sort — in this one, loosest is best. But take it from one who knows more about it than he wants to — loose connections are common in your PET, and you can usually fix them yourself.