Throughout March I tinkered with some Video Genie computers as part of the Retro Challenge.
You can see my initial post here. The idea was I would learn to use a couple of new diagnostic tools for digital hardware, and then use them to fix some of my old computers.
If you prefer watching videos, here’s a 21 minute video that covers most of what’s in the rest of this blog post. If you prefer words, read on!
Here’s a summary of what I did:
On day one I got my logic analyser working and learned how to use it. In the end I didn’t use it for any of these repairs, but I have it ready for the next ones!
On day two I dug out two faulty Video Genie computers. These are clones of the TRS-80 model 1, and I have a soft spot for them as the first computer my family owned was a Video Genie. There is a great site devoted to the Video Genie (aka System 80) by Terry Stewart.
The first Genie wouldn’t boot (it just displayed garbage characters on the screen, which shows the video board is working but the CPU hasn’t managed to clear the screen during the boot process). I did a bit of probing with my new logic probe, which indeed showed lack of CPU activity, but didn’t give me a clear idea of what was causing it.
I had a third working Genie 2 computer, so I swapped chips from the faulty computer into the working one to identify what was broken.
This showed the Z80 CPU was dead. Having replaced that there was still corruption of a few characters on the screen, indicating faulty screen memory. Through my knowledge of the video hardware, and analysing binary ASCII charts, I was able to narrow down which SRAM chips were faulty, and replace them. The computer now booted correctly!
I made a 32 minute video of this – the first 15 minutes talking about the Video Genies, and the rest showing my diagnostic process. Useful if you have similar faults!
On day three I looked at the other faulty machine. I very quickly identified another dead Z80 CPU, and so within minutes I had it booting. That was the only digital fault.
However I then tested the integrated cassette deck, and found the power supply to it wasn’t delivering enough voltage. I identified the analogue voltage regulator circuit on the board, and placed an order for pretty much every component in it!
I made a 21 minute video of that day’s repairs, but it’s probably not that interesting to other people!
On day four I tried to fix the voltage regulator circuit. I removed components from the circuit, tested them with my new component tester, and replaced the faulty ones. I thought I’d found the culprit (a transistor that no longer worked). But replacing it didn’t fix the problem. More work needed!
I made an 11 minute video of these repairs – possibly useful viewing for the process of using a component tester to identify bad components.
On day five I returned to the voltage regulator circuit, and decided to be more methodical and continue replacing everything. I found a power transistor to be the second faulty component, and replacing it fixed my problem :-) It seems it worked at low current, but when high current was called for it just shut down.
The cassette now had power, but the rewind function didn’t work. I tracked it down to a loose drive belt that had stretched over the years. I managed to measure the correct length, and order some new ones.
Here’s a 12 minutes video of me figuring out how the voltage regulator circuit worked, and diagnosing the mechanical issues in the cassette deck.
Day six was my last full day on the Retro Challenge as I’ve been very tight for time this year, and had to limit myself to a lightweight entry.
I fitted the new drives belts to the cassette decks in both machines, and then tested the cassettes.
One machine won’t record to tape at all, and the other machine puts quite a noisy signal down onto the tape. Neither will read back a tape.
I’ve looked at the schematics for the cassette circuits, and I think I understand how they work. I have a few ideas for further diagnosis – some I can just use my existing digital tools, but others I’d need an oscilloscope for.
I made an 11 minute video of the repairs.
Sadly I’m out of time this year, but something for next time! Who knows, I might even have saved up enough for an oscilloscope by then!
I set out to learn to use my logic analyser and logic probe – done!
I wanted to repair my Video Genie computers – well they both now boot, and all the digital electronics work, which I count as a success. There’s still minor issues in the peripherals which I need to revisit.
If I had enough time I wanted to use my new tools to look at one of the faulty Osborne 1 computers that had me stumped last year. (I repaired one, but the other wouldn’t boot, which I narrowed down to a fault somewhere in the disk interface.) But hey, I didn’t have enough time.
I still had fun though, which is what it’s all about :-)