Bringing a Vic-20 back from the dead!

This is a (very) retrospective blog posting, as this repair was done about a year ago. I've already forgotten a lot of the details so I thought I'd better commit what I could remember to archive before it disappears entirely! There are not many photos in this entry. This repair was done before I learnt the value of documenting projects in images. Oh, well...

Figure 1. The Commodore Vic-20

The Vic-20 was arguably the first home colour computer. It was a great hit and the first micro my father ever bought (he was about 55 at the time).

I deemed the Vic-20 "a classic" so started to look for one for the collection. Already they were going for tidy prices on our local auction site and I missed out on a couple of models before finally winning a bid.

It was the second time I'd bought a computer on-line. I was about to learn a lesson.

The seller didn't actually say if the unit worked. He said "powers up and the power light comes on". I now know this doesn't mean much. It simply means power is getting to the circuit board and nothing else. However, I was keen to get one and assumed the computer was probably ok.

It wasn't.

Plugging it into a screen once it arrived revealed a screen border, but instead of showing the READY text it showed a bunch or graphic characters (sorry no photo). Pressing the keys produced more graphic characters or sometimes no characters. Hmmm...broken!

Advice from the Vintage Computer Forums pointed me a handy diagnostic and repair guide for the Vic-20. It wasn't definitive but it did limit the IC's that COULD be the problem to a mere 13! . Fine. But where would I get replacement IC's from? Maybe it would be better to just replace the board.

Again, the Vintage Computer Forums came to the rescue (or so I thought). with a member there sending me an old working board he had for a few bucks. However, when it arrived I discovered the VIC-20 actually came out in two versions, and the power supply plug was incompatible between the two. The replacement board was an early version and I had the other (later) version! Damn! However, the board DID give me a supply of ICs. The bad news is that nearly all were soldered into the board. Still, at least I had them.

With nothing else for it, I rolled up my sleeves and started to desolder and replace the ICs the guide above suggested might be faulty. I started with the character generator. No change. The next IC to try was a 74LS04 logic chip. Replacing this IC showed an improvement! I no longer had garbage characters but instead had real characters. But wait, the colours were all wrong! I obviously wasn't out of the woods yet.

Maybe it could be RAM? However there was more than one RAM IC. I replaced the first 2114 RAM IC. An improvement! The letters seemed better formed and more stable. The colours were still mixed up though? I replaced the other two hoping that things would improve even more. No difference.

Ok, time to take stock. The Vic-20 was better than it was but complete recovery was yet to be realised. Reading the diagnostic guide again, the colour interface logic chip CD4066 seemed to be a possible candidate. I replaced this, switched on and Volia! A perfect boot-up screen! FIXED!!

So, it seems three ICs had been damaged (see below). I don't know what had happened to this machine in the past but it couldn't have been pretty!

Figure 2. Arrows show ICs replaced. Red arrows show the position of faulty ones.

Anyway, it's all good now!

As well as not working, this VIC-20 was badly discoloured. That has been rectified and this little micro now it sits as a treasured piece in my collection.

Figure 3. Working Vic-20


20th February, 2009

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