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