TRS-80 CoCo 2 - Keyboard Problem That You Won't Believe
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- Опубліковано 26 сер 2024
- It's #septandy 2022 ... except it isn't. I didn't get the video out in time. So here is a belated treat! I try to fix a TRS80 colour / colour computer 2 (Tandy colour computer 2) in need of much TLC.
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Warning: I am an amateur attempting repairs. In these videos I try to fix or work on junk that I usually buy on eBay. This is NOT a how-to video. The processes in the video may not be the best way, the correct way or even safe. Please do not copy anything shown in this video. If you do so it is entirely at your own risk.
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I remember the absolute struggle I had getting mine to play software with a cassette deck. About 40 years ago….thanks for the memories
Thanks josh, so happy to be bring back fond memories (or frustrating ones LoL). Thanks for taking the time to comment. Sorry for the delay in responding
Seeing you tapping all the keys keeps making me giggle. I understand the frustration.
Thanks! It was quite a challenge... really odd fault. Thanks so much for feeding back. Glad that you enjoyed the video
This exact same model (CoCo 2, 16K) was my first home computer ever. Brings back some good memories!
Awesome! Glad I was able to bring back some happy memories. I have a real soft spot for these machines. Great for developing my electronic repair skills, easy to repair using off the shelf parts and with a processor that is so flexible and easy to program. Thanks for your comments. Glad you enjoyed the video
I had a CoCo 2 with worse yellowing... A week or so in the sun will brighten it right up!
I removed the motherboard and power supply and wrapped the keyboard in cellophane, and just set it outside each morning... Amazing results!
Edit: lol obviously I was still watching when posting. Congrats!
@neb6 in my case the sun reduced the yellowing by several shades. It was quite impressive!
I didn't video the process but I have photo documentation.
Nice work Roger, good to see you back 👌👍👍
Thanks Vince!
Oh, I had one of these as a kid.
Hi Andrew. I had a zx spectrum probably around the same time but was a bit jealous of computers like this that looked like computers with proper keyboards. I've come to really appreciate the coco easy to repair with no custom chips and a great processor. I hope to bring another video on it at some point. Thanks for you comment glad I could bring back some memories
Great to see you back Roger. Interesting as always. Thanks
Cheers Richard. Sorry that is been a while. Thanks for your continued support. Really appreciate the comments. Thanks so much
Wow, I'm amazed how far the CPU could execute the ROM code without a functional R/W line. I would assume the flashing cursor would involve a write. Nice work tracking that down.
Thanks Stephen... bizarre isn't it! I guess the SAM does a lot of the heavy lifting in the architecture. Not sure. The CPU behaved quite differently in the Dragon. Still booted but crashed as soon as a key was pressed. Thanks for sticking with the vid to the end and for taking time to comment. Really appreciate it. Sorry its been so long.
That reminds me of the time our arcade museum's Q*Bert's sound board started acting up. The sounds were weird in a "the wrong sound is selected, probably a wire is broken" kind of way. Also whenever Q*Bert would start talking he never would stop until another sound effect using the SC-01 is played.
And of course, when you turn on the machine, it says "Hello, I'm turned on".
I changed all the chips in the sound line circuitry, made an LED debug cap I could piggyback onto the input latch to see if all sound lines get toggled, even made some kind of harness where you could manually select all the sounds. I eventually gave up.
Then came the day when I had to test a 6502 CPU from a different machine and I used Q*Bert's speech board for that. All the sounds were correct. I put the original 6502 in my Commodore 8032SK - garbage screen. Another case of a broken CPU working well enough not to be suspicious.
And just on this Friday evening I was working on an Atari Sprint 2, I removed one of the audio amps because it was very quiet and extremely hot. When I put it back, the machine stopped working. Long story short, the 6502's clock input became high impedance and I had to stack two identical logic ICs on top of the one creating the clock signal to get the clock signal back and the CPU works just fine!
(I didn't have a spare 6502, but I had a bunch of 74xx32s used to create the CPU clock. With zero, there were just little spikes on the clock. With one piggyback, high was at around half a volt. With two piggybacks, it was a good TTL signal with high > 4V)
@@RetroTechRepair recently I’ve seen two instances on this happening. I think that the 6809E finally overheated and failed in this odd way. Weird.
That troubleshooting part was fascinating :) I bet you put a lot of work into that. It's always a joy to watch and there is a lot to learn from it.
Awe thanks so much Norrectio that really means a lot. The troubleshooting was a bit of a challenge took me a long time in reality and along time to edit down to 20 min! I also checked all the connections against the schematic and checked the supply voltages to the chips but I forgot to include that! Anyhow glad that you enjoyed it and thanks so much for your kind words.
You’re back!
Hi Clarance yes I am and hopefully for a while, still slow to her vids out but hopefully better than before. Thank you for your patience.
Very enjoyable video. I wouldn’t have guessed the cpu was at fault! I have one of these, and what you have is the best experience in the coco2 being ntsc. The coco generates colours by “tricking” the colour decoder in your tv. These are called artefact colours, and do not happen on pal systems. So now like me, you can see the games the way they looked in the us, and not the restricted way we got to see them in pal land!
Thanks telemedic. You may like the second video in this series. It looks at some of the very things you discussed. Great comments
Nice work Roger
Thanks so much. Really appreciate you sticking with me and taking the time to comment. Hope you enjoyed the vid.
Thanks for the upload. Well done for figuring out the fault, and yes, I did enjoy the video.
Awesome! Thanks Ghost Retro. So pleased that toh enjoyed the video
Also the sound problem will be be in the modulator, as us vhf tv frequencies do not line up with European ones. I modified my modulator for composite, the details were online and worked like a charm. Good luck
Thanks! You might like the second video in this series too!
Great persistence there, well done 👍🏻
Good to see a new video.
Thanks so much. I am really grateful for the feedback. Sorry its been so long.
Roger goes the distance with these repairs. If you haven’t already; check out his laser disk repair video. It was a rollercoaster.
This looks like a good candidate for a fluke 9010 processor emulator
Yes it probably would be. I keep buying bits of test kit as I try to grow the channel. Maybe one day I'll be able to stretch to one. Thanks so much for your comments! Very much appreciated
That’s nuts!
That's what I thought. I'm guessing the cpu would have failed to run other stuff if in could have typed a command into it. Anyhow I'm very appreciative of your comments glad that you enjoyed the vid. Thanks for taking the time to comment.
Great fault finding. I was sure you where going to buy that expensive chip in the end. Glad I was wrong
Yeah.... the mc6821 and mc6822 are not 100% compatible I posted in the coco Facebook group for advice and everyone who commented dismissed my suggestion the 6821 would work assuming that to be the reason. Hope they watch the video. Thanks for your comment. Very much appreciate it
Been waiting forever for another video. I love your content. Thank you!
Then you have been very patient. THANKS! I might do a channel update to explain at some point. Thanks so much for the feedback and continued support. Hope that you enjoyed the video
@@RetroTechRepair The video was great. I love this retro repair and you have a great way of troubleshooting and presenting it. Quality content for sure. Keep up the great work and know that you have fans out here always checking in for more content.
Thanks Brian thas genuinely lovely to hear. No other way to put it. Thank you
Fab video as always - nice work with the diagnostics! I was wondering, when checking for bad chips (when they are soldered rather than socketed) could you not place a replacement chip over the top so that the pins of the new chip touched the pins of the soldered chip? I am sure I've seen another repair channel where the presenter did that to diagnose arcade board issues...
Thanks MrVectrex. I think the piggybacking chips technique only works on some chips (eg, some RAM) as the chip needs to be of a type that has a high impedece state (tri-state logic high,low,high-z). If a "normal" logic chip fails the chip may drag a line high or low rather than go Into the high-impedence state that would be needed for pigging backing to work. Nice thinking though. Thanks for your comments (on this and my other vids).
Well, at least the 6809E can be replaced with the 6309E
Oooh that's an interesting idea. Thanks for the suggestion. I didn't actually didn't know ow about that processor. Thanks so much for your comment
@@RetroTechRepair It offers a slight speed boost (almost imperceptible) and additional operations that new software is designed for taking advantage of. Canadian Retro recently did a video about whether it was a worthwhile upgrade and found a few reasons why it was. It’s easy to do in the CoCo 1 & 2 because they are usually socketed.
I seem to remember there being a relatively easy composite mod also. I think several Tandy/CoCo enthusiasts channels have done this.
Lastly, if you really want to go crazy, there’s a keyboard membrane swap that provides micro mechanical switches giving a more tactile feel.
@@jeffreyphipps1507 Thanks. Great info! Will definitely look into this. Thanks again.
@@jeffreyphipps1507 actually, with optimized code there is a significant speed boost.
Without it, it's basically the stock CPU.
As a former Coco 1 and 3 user it was intriguing that the 6809e worked as much as it did. I vaguely recall a writer in Rainbow magazine stating the 6821 and 6822 are not directly compatible despite the similarity. NTSC has 4.5Mhz sound but UK TVs use 6.5mhz so they can’t tune in the sound. You can get sound via the cassette as the bit banger sound is paralleled with the 6 bit DAC which is used to generate the cassette record signal. Sounds like a UK modulator or an AV mod is on the cards. And, one advantage of an NTSC CoCo is the artefact colours work rather than black and white on the PAL version. I used to have a lot of fun finding the value written to &hFF22 and change it so PMODE 4 games would use PMODE 3 instead. Usually the assembly code is LDA # xx then STA FF22. Those were the days
Thanks Wenlocktvdx... awesome info what great comment! I never thought about the audio sub-carrier freq. I have yet to edit up the second part to this vid so I will mention it then! Yes the artefact colours are great... really so much better than the PAL system. I honestly think that's what did for the Dragon 32. Artefacts were always the intention of the chipset, but only ever worked on NTSC systems leaving the PAL systems with a very limited pallet. Shame really the processor is a joy program compared to the 6502 and easier than even the z80. Lovely system. Anyway... glad I could bring back some fond memories. Tell or your friends... uptake has been a bit low on this one :). Thanks again for the very informative comments.
Trying to recall more about the PIAs. The 6822 could drive more load than the 6821 so it sounds like totem pole output versus open collector. The NTSC modulator has only 4mhz video bandwidth versus 6mhz with a UK modulator. Australia is in between with 5.5mhz sound and 5 MHz video bandwidth. Artifact colour was tried in PAL but it needed a change of pattern in the middle of the screen to have the same colour all the way across. Very few even used it. I used mine for things like slow scan TV thanks to Graficom, weather fax and even radioteletype (RTTY)
Great video. Found it while searching for fixes to my keyboard issue. Found my old TRS-80 I got around 1985 when I was 12 years old. First hurdle was getting it connected to a computer monitor. Did so by going through a VCR and an RCA to HDMI adapter.
Issue now is that the keyboard spits out garbage. I press "C" I maybe get "C", but I press "F" and getting something like "52DX". I'm going to get some contact cleaner and a soft brush and clean what I can, but most of this is over my head. Any suggestions?
Hi Brian, good thinking on the VCR. I can't say for sure what the keyboard problem is, mine has a membrane keyboard, not mechanical switches so I am not sure if contact cleaner would help. IF IT WERE MINE I would probably not apply contact leaner but look at either a membrane repair or replacement. My keyboard looks like the one shown here: www.leadedsolder.com/2020/02/02/coco-keyboard-membrane-repair-electric-paint.html
I would probably also check the connections at the main PCB, shorting the appropriate ones together to see if the problem replicated (fault on main PCB) or did not (Fault on keyboard). None of this is advice, just what I might do if it were mine. Anything you did would be at your own risk. Thanks for your question. Good luck with the repair and thanks for your comment.
@@RetroTechRepair thanks much for the info. I’ll certainly look into it.
I have a Sinclair Spectrum +3 with zero sound output and I have no idea why. It's not a TV problem
Hi there... I have a spectrum to repair but it's a rubber keyboard one. The +3 used an AY sound chip. Although I would like to get one someday. I have no experience and can't offer any help with your repair. Sorry
is there any way i can run a Dragon 32 rom inside of Return of coco?? the older coco emulator from 2000 or 2001? thanks......................
Hi @ms-ex8em, I am sorry but I don't know. I love these machines but mainly I am using original hardware (I have some Dragon 32 vids on my channel too). Sorry I cannot help but thanks for your questions. Hole that you enjoyed the video
Trying. To. Fix Trs. 80 color. Computer 2. This. Repair. Might
Thanks Kenny
How much do these weigh?
I have to admit i haven't weighed it... interesting question though. 🤔 if I remember I'll pop it on the scales. Hope you enjoyed the video.
@@RetroTechRepair yes :).