PCB Examination

The machine powered on, but then displayed random charaters all over the screen - briefly. After a second or two, all went dark. The "garbage" screen is a common PET problem, but I later learned that it was normal for this to occur just after the computer is switched on. The blank screen was however a worry. Sometimes reseating the socketed chips is enough to get things back in shape. No such luck this time, so I whipped out the board for a closer look. I certainly wasn't the first to do so!

The underside of the board had a few not-so-neat repairs. It looks like a track had been cut in several places whilst diagnosing a problem with the rectifier diodes and a voltage regulator, both of which have been replaced. There are also a few IC replacements, given away by their sockets and a fair coating of soldering rosin on the pins. I figured I'd better tidy these bits up before trying to diagnose the problem further.

PET's lid lifts up just like a car bonnet.

Bottom of the board had a few bits like this...

Someone had had a good dig at some of the tracks.

Voltage regulator seemed to be soldered with thin air.

The rectifier diodes were particularly bad. I wondered what the big blobs of solder were for, until I removed them and found they were covering a broken PCB trace. The diode's legs were joined together to bridge the gap!

New rectifier diodes were begging to be resoldered.

Same area, partially resoldered and cleaned up ready for track repair.

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