Okay so here I am, still a novice when it comes to fixing vintage computers.  I don’t have a scope or even a logic probe, so I’m reduced to shotgunning the problems on this Digital Group Z80.  Which, usually does not end well.

That said, no harm trying.

Since the machine isn’t coughing up a prompt and is still erratic, I began to wonder if the CPU card was running at all.  First thing I checked was the Z80 CPU itself.  It tested perfectly in one of my Sinclairs, so it definitely wasn’t that.  Another possibility was a bad EPROM.  The EPROM on the Z80 contains the boot code.  It’s a programmable 1702 IC, and the programming window was exposed.  That opened the worrying possibility perhaps it had corrupted or erased.  There was nothing I could do about that though — Z80 EPROMs just aren’t for sale out there and even if I could find the correct code, I don’t have anything that can burn 1702s.

I looked at the crystal oscillator next, a 2.5MHZ unit.  It slides in and out of its little holder with relative ease.  I knew nothing about these so I tapped it gently and heard a ‘tinkling’ noise in there.

I don’t have any crystals lying around, but I did have a 2.6ish MHZ unit from an home built Apple II clone project I got with a Rev 4 Apple II case.  After receiving assurances from Marty that it wouldn’t hurt anything, I plugged it in.   This time I got a screen full of commas.  I cycled power again.. but now I was back to blocks.  Hm.

The crystal I had borrowed had been soldered to an old video coard and consequently the legs were shortened and possibly not making good contact.  So I extended the legs a bit with some paperclip pieces.  Now when I powered up, I consistently got the random character screen.

So that’s good.  We’re getting somewhere, albeit slowly.  Next up, checking the memory again..