Wednesday, April 7, 2021

Zee C900 Computer Story

In 2003, a kind German fellow named Claus Schoenleber helped me get my hands on a pair of prototype Unix machines built by Commodore around 1984-85.  One was a server (the Model 1) and the other a workstation/client (the Model 2).  Because of the issues I had with them, it would take until very recently to get them working, and require the help of many, using teamwork only the internet could make possible.  That's the story for today.

The C= engineers referred to these machines internally as "Z Machine" for their Zilog Z8000 chipset, but would be marketed as the Commodore 900.

It was rough being a software guy who collected old hardware in the early 2000s.  My knowledge of basic troubleshooting was very weak, and the internet was still mostly a desert for a niche hobby like this.  As excited as I was about getting these machines, I quickly realized that their problems, and even their requirements and basic usage, were beyond my understanding.

Last year though, thanks to some encouragement from my friends at the Commodore International Historical Society and the Central Texas Commodore Users Group, I decided to apply all I'd learned in the intervening years and see what I could do with them.

The Model 1 (server) was the first one I tackled, as its power supply seemed functional.  At first, the issue was just figuring out what kind of video it generated, but experimenting got me around to this simple monochrome PC monitor, which gave me a first glimpse of the machine in action. Unfortunately, the hard drive clearly had issues, so this was as far as I got.

In 2020, I was able to chat with a fellow named Santo who had a Model 2 he had gotten working, so I decided to give that one a shot.   The power supply in that machine was non-functional, and the best efforts of my friends at the Central TX Commodore Users group to help me diagnose and repair it had progressed slowly.  

But then Dave from the Commodore International Historical Society, whose interest in these machines knew no bounds, offered to send me a near-identical power supply from a U.S. Commodore PC-10 to get it working.

That was only the beginning though.  You see, the Model 2 had a proprietary video standard to achieve its 1024x800 resolution, which means having a one-off monitor that didn't even have a known model number.  I also lacked a video cable, and its 220V requirement was certainly an inconvenience.  

Santo to the rescue though!  He had discovered a jumper in his own monitor that allowed it to accept 110V, and when my home-made video cable still failed to produce a useful signal and I discovered that one of the wires inside the monitor had become disconnected, he photographed his own board to help me figure out where it was supposed to go.

And walah!  We have video!

And this is where new problems began to surface:  the hard drive was unreadable, and the boot sequence reported memory errors.

The computer has 32 individually socketed 41256 ram chips, and the brilliant engineers at Commodore were kind enough to more or less report exactly which chip it was failing on the boot screen.  Well, sort of.  Either way, some experimental swapping with good replacement chips ended up fixing the memory issue.

The bad hard drive was a tougher problem.  But Santo to the rescue again.  He pointed me at the fine folks at who produce a remarkable board capable of both hosting a working MFM hard drive for imaging, as well as emulating one using locally stored image files.  Since my own hard drive was unreadable, I used a hard drive image he had taken of his own installation.

Of course, now there were new problems.  The X-Windows-Like GUI desktop software required a C900 mouse, which I did not have, and the floppy drive with its proprietary format appeared unable to format a new floppy or read known-good ones.  

And then even the keyboard started crapping out on me.  Luckily, the C900 can use a standard PC keyboard, which Claus had included in his shipment.  I also later discovered that the C900 keyboard was fine, and that it was a stuck key that was not visibly-stuck causing it to fail to boot. It was that ugly wide skinny stop/continue button in the upper-right.

After the keyboard, I tackled the mouse issue.  Mr Santo provided me detailed photos of his own mouse, so I went to the schematics and prepared to build one from scratch using, perhaps, an old Amiga mouse as a base.  To my great delight, however, I discovered that the pinouts for the two were identical.  The C900 mouse *IS* an Amiga mouse.  

Now I was in business.  I spent the better part of a month playing around with the demo software, using its Kermit terminal to do some wifi modeming, writing some little utilities in C, learning the cryptic keys of Emacs, and otherwise enjoying the machine.  You can see some videos of the demos here, and here

But eventually I wanted to revisit the Model 1/Server machine.  If you recall, it had a bad hard drive.  Well, I discovered the hard drive issue was merely intermittent, and I already had a good long-term solution when dealing with the Model 2, namely the MFM emulator.  And so...

18 years was a long time to wait to finally see these machines happy and functioning again, but definitely worth it.   2020 may have been the worst year ever, but for a shut-in Commodore collector: not so bad!  And definitely a team effort that only good hearts and the power of the internet could provide.

I hope you enjoyed my little journey.  Until next time...

Thursday, February 4, 2021

Best Week Ever!

Way back in 2002, I was lucky enough to lay my hands on a beautiful Commodore 256-80 computer, the top dog of the CBM-II line of business machines.  

It had issues almost from the beginning though.  

First off, it would only power up occasionally, and when it did, the power supply made a horrible screeching sound for as long as I used it.  

Second, it had been equipped with the 128K BASIC roms instead of the 256k roms, which means BASIC was using less than half of the available memory.

Lastly, my 8088 card, which I acquired from the same owner, absolutely refused to work inside this computer.  

I despaired, but remained thankful for what I had.

You might think the first thing I would concern myself with was fixing that screeching noise from the power supply.  However, I didn't.  I had been told it was probably due to slightly loose wiring in one of the transformers vibrating during normal usage, and that replacing the irreplaceable transformer was the only real fix.

I focused instead on the booting problem of the power supply.  For nearly 20 years, perhaps twice a year, the computer would stop booting.  I would then take it apart, remove the board shown above, and then try to figure out what was wrong.  Typically, after a few days of poking around, the computer would start booting again, and so I'd button it back up and move on to something else.

Along the way, I tried restoring the BASIC 256 roms by burning the images to some 68764 EPROMS I had, but the computer would only boot into the monitor, indicating a failure.  Did I have bad memory in the upper two banks?  Is that why the 128K roms had been put in there? <sigh>

I also made repeated attempts to get the 8088 board working, but the boot disks would never do more than hang the computer during load, indicating to me that the board was still failing or incompatible.

Recently, however, Dave McMurtrie acquired an identical 256-80 machine with a bad power supply, and managed to fix his by replacing basically every source-able component on the board.  This was inspiring to me.  Why couldn't I do that? If he can fix a dead power supply, surely I can fix an intermittently bad one.  

Why did I have to live with any of these problems?  

And so I got to work.

My first step was to replace every capacitor on the board except the two largest, which measured fine.  If I had any doubts about the need to replace the others, they were dispelled when I tried measuring one of the old capacitors on the left side of the board and it was reported as a diode.  I also found myself cleaning off tons of electrolytic fluid that had been leaking.  You hear all the time about how bad capacitors will bulge on top when they go bad, but these were all bulging out at the *bottom*. 

Dave had sent me some of his leftovers, so I also swapped in some fresh voltage regulators and transistors, and even replaced the choke, though I doubt they were bad.

When I was done, I tried it out, and to my delight, the computer was booting again.  But that wasn't the best part:  the screeching noise was GONE! It wasn't just loose wires in the transformer after all!  Something I wish I'd known 15 years ago, but better late than never and all that.

Flush with optimism from the power supply repair, I went back to looking at those BASIC 256 roms.  The 68764 burns I had made still verified as correct in my eprom programmer, but also still refused to boot in the computer.  So, for no reason, I made new ones, this time in some 68766 eproms, and guess what?  BAM!

I was on a roll now.  Nothing could stop me.  I pulled the 8088 daughterboard off the shelf and once again carefully installed it inside the computer.

I had two boot disks, which I'd tried interchangeably over the years:  an MS-DOS 1.25 boot disk, and a CP/M 86 boot disk.  I hooked up a drive and tried MS-DOS 1.25...

I almost fell over with joy, right then and there.  After 20 years: a stable quiet power supply, the correct version of BASIC, and the 8088 card working perfectly.  Perhaps the 8088 card had been too much for the power supply in the past?  Either way, it was all good now.

So, of course I tried the CP/M-86 boot disk and now it, too, came right up.  Looking at the directory on the disk, my eyes went straight to that first program. 

WordStar on my C128 had gotten me through college, so how could I resist another trip around the keyboard with it?

And with some finagling and some help from Steve Gray with the PET Emulator, I even managed to play some PETSCII robots:

This had been the best week ever!  

... of course, all of the above triumphs consumed perhaps 2-3 days.  You'll have to wait to find out how the week actually started.  If this didn't convince you I'd had a good week, the next installment will.