Jul 282012

Saw this the other day on eBay and it was going for a sensible price with sensible delivery costs so I took a shot and won it.
On power up we got this

Never seen this fault before and as everything was soldered in I didn’t want to go down the route of swapping everything out.
I could not use the cartridge port or type anything in but I did have what looked like a flashing cursor. I also found, when I pressed the CONTROL key the screen flashed up with a nice multicoloured garbage screen which I have seen before with a dodgy PLA.
Whipped this out and burned a new PLS100 chip.

Now everything is back to normal (or is it?)

Whilst running some diagnostics I saw there was a fault on one of the CIA chips that displayed “INTERRUPT”yet everything seemed to be working.
Running a different set of diagnostics I found that the RESTORE key did not function. As the restore key generates an NMI this was probably what I was seeing.
I don’t have any spare CIA chips so this will have to stay as it is for now.

UPDATE: Thanks to a comment left by Rocky I revisited this today armed with schematics. Found a completely knackered 556 timer chip that just so happens to control the NMI from the RESTORE key. Replacing this fixed all my problems.
Lesson learnt. Don’t assume you always know what your talking about!

Thanks to Rocky for making me look at this again

 Posted by at 7:12 pm

  5 Responses to “Commodore 64 ‘breadbin’ repair log #6”

  1. Could be the rubber feet under the key worn out, not necesserily CIA Chip. If you feel like, open up the keyboard and spray some Contact Cleaner there…

    • Already tried a known working keyboard but your right, I shouldn’t assume that a CIA is at fault. I’ll do some further probing and see what I find.

      • Well, you indirectly spurred me on to look for another reason and I found it (see update). Cheers mate

  2. also, i think you should include the repairs where you pull your hair out, kick the cat, and still cant fix anything. Reading your blog a newbie gets a sense that this is easy. He thinks he can get a $20 soldering Iron and start whacking out PCB boards.Some jobs are easy,some are difficult and some are Impossible no matter how good you are.

    • I encourage anyone to have a go.
      When I started I knew nothing, I tought myself and read what others had done.
      I did start with a crappy cheap soldering iron and made many mistakes but I learnt from them. If I had read a bunch of posts where others had failed then I probably wouldn’t have tried.

      I’ve always tried to help people when I can with this stuff and as a result I know I’ve set a few down the same path I took.

      The main reason I started my own blog was because the forums that were about at the time where elitist and not friendly to new people with a lot of questions. That’s changed a bit now and forums are more tolerant of people with questions.

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