After swapping, like, dozens of RAMs and not getting anywhere new and exciting, I’m wondering if anything else is amiss with my system.  I look at the CPU board, then the I/O board.  It’s a quick glance and I almost miss it.. but I just catch a glimpse of a potential problem — one of my 74100s on the board has a pin bent out.  Pulling it and straightening the pin, I reinsert into the socket, and then reinstall the card.  For kicks I hit the power button, at this point not really expecting anything.  But then this happens:

OMG

Not quite all there, but there all the same: the ‘famed’ Initialize message.  Still a lot of screen garbage but man, am I excited to see that prompt.  For me, it’s like finding the lost city of Atlantis!  I know now that there’s a good chance my EPROM is okay and that the basics are working.  At this stage I could well try loading a tape, but I’d like to clean up the garbage on the screen first, which I’m now certain is just more bad video RAMs.

Since I don’t have a working machine that uses 2102s, I’m forced to resort to removing an IC, subbing in an IC from my spare board, and then powering up to check the result.  If something changes, I set aside the IC I pulled and carry on.  If nothing, I pull the next IC and insert the apparently working IC from the last socket into its place.  This is tedious and probably hard on the power supply and capacitors, but, slowly but surely by process of elimination, the screen gets better.

And then at last, with one more RAM chip swap..

I’ve hit paydirt!  Yahoo!!  And all I can do is look at that beautiful prompt in astonishment.  Against all odds, I have raised this old girl from the dead!  It’s almost like traveling in time!

It’s tempting now to try to load something, but I realize the video board might actually make a really useful diagnostic tool.  Bad RAM chips will immediately distort the video in some way, so this is an opportunity to start swapping in all the chips I have knowing I have 8 known good, and start weeding out the bad.  It’ll take hours to do them all, 3-4 RAMs at a time gambling most will be okay, but it’ll be worth it in the long run — that will give me confidence the machine is in fact running right and capable of loading something.  Stay tuned!

Okay so at this point we’ve got things moving along.  At the suggestion of DG Guru Marty, I decided to reduce the number of potential problem areas by removing the extra RAM boards and focusing on just getting the basic system running.  That meant removing and resoldering the jumper wires in the center of the CPU board, which are used to tell the system how much RAM there is.

When I did that initially nothing major changed.  However, I then began to pull the newest 2102 RAMs from one of the now removed expansion boards.  As I did that, I started to get changes in the character patterns on the screen.  I noticed too that the more I hit the Reset button, the more of those characters became / marks.  So something is happening.

Marty reminded me of the video RAMs.  He was a bit doubtful I was going to get much further absent the availability of major diagnostic tools, but it was worth a shot.  Borrowing more 2102s from my newer RAM board, I began swapping out the 8 video RAM chips.  When I powered up, something interesting happened:

Note from the video what’s on screen.  No reset button being pressed, but things are changing.  Man!  I’m beginning to feel that frantic sense of excitement.  I’ve nothing to support it, but I feel like momentum is on my side!

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..

There are three boards (not including the motherboard) that are required to have a functioning Digital Group system: the CPU board, the TV/Cassette Board, and the I/O board.  A memory board is not required as there is already 2K of memory on the CPU card.

Being that the I/O card’s job is to move things around between the other cards, not having one means no system.  Luckily as mentioned in the previous post, the seller realized their mistake in leaving mine out and were willing to send the replacement without fuss.  Phew.  Here’s a pic of it after getting it running:

I was of course hopeful that the system would just magically work when I plugged it in.  Nope.  Instead I got the dreaded Screen ‘O Blocks.  Now what?

One thing I’ve learned to do with vintage computers is Read The Freaking Manual.  At least, when a manual is available.  Thanks to Bryan’s awesome Byte Collector site, I was able to figure out that the card had one of its 74100N ICs in the wrong port.. over on the right rather than left.  When I put that in, suddenly I had a different screen:

Hey.. it’s a result.  I bet that’s the first time this thing has coughed up recognizable characters in eons!

It took a long time to get to me, courtesy of Canada Customs, which just had to get their few bucks on a 40 year old computer.  I was pleasantly surprised to see only one casualty upon opening the box — the case fan shroud.  It had broken in transit and the fan was just lying on the case bottom.  Digging around in the box I found all the pieces to reassemble it and tucked those away.

The case is impressive.  Solid aluminum sheet, and I mean solid.  Like, stop bullets solid.

No lid either. This particular case was called the ‘Basic Box’.  It was offered by DG for hobbyists who didn’t want to shell out for the ‘Cadillac’ options – the fancier case, keyboard and so on with DG badges, but who did want something nicer than a plywood slab to mount their computer into.  The Basic Box appeared in Flyer #10(thanks to Bryan of byte-collector for scanning and making these available!):

An interesting feature of 70s hobbyist computers — they often have full on AC plug ins at the back.  Thinking about our babyproofed world here in 2016 — it’s amazing an open case with live AC outlets was considered safe!

The card cage is really neat.  It’s on a hinge that allows it to swing out to the left, so you can access the bottom.  The motherboard on DG systems does not make connections between slots via traces, they are made via wires the user installs!

Swinging the cards out, the thing that makes a hobbyist PC really special becomes very apparent — all those little handwritten ‘notes to self’.

And check out this keyboard?  It’s obviously stolen from something teletypish.  It and has this neat little row of LEDs poking through perfboard that I think correspond to the ASCII codes it generates when pressing keys.

Now, here’s the thing I never get with my machines.. software!

And to my surprise, there is a lot more than what was pictured in the auction.  Not just the OS and a few BASIC tapes but also Maxi BASIC, an Assembler and a bunch of game tapes.  Yay!

Okay so…. do I fire it up?  Many serious hobbyists would shrink from firing up an unknown quantity like this on 40 year old power supplies.  I knew from the auction this unit didn’t work, but all the same, you never know what else you could make worse if said power supplies go nuclear or things are accidentally shorted or broken.

My attitude however: ah, screw it.  Let’s see what happens.  Here we go:

Woohoo!  I know, doesn’t look like much, but from reading this screen of cursors is a good thing.  It means the ‘TV/Cassette’ video board (apparently a 64 character unit) is alive!   Of course, this screen also means the CPU board isn’t doing anything yet.  Some further reading reveals why:  a typical DG system requires 3 boards to operate: CPU, I/O and TV/Cassette board.  Looking inside, it appears I have a CPU card, the TV card and three large RAM boards.  The I/O board is nowhere to be seen!  I triple check the box, but no dice.  And these are fairly big cards — kind of hard to lose.  There is one on Ebay (for $123.00), but I decide to look back at the auction to see if something went astray.  Sure enough, there is a picture of the card there – separate of the computer.  I contact the seller and after a couple of days they acknowledge – indeed, a card is missing.  They apologize for the error and ship it out.  We will have to wait before we can do any more diagnosing on this old girl.  Shucks! 🙂