New TRS-80 Model III Owner with Questions

[rockman_x_2002]

So a friend of mine sent me a TRS-80 Model III that her dad owned, along with a handful of software. It's a 48k model with two disk drives, so it was the high-end Model III that Radio Shack sold.

 

It arrived a few days ago, and I've been checking it out to make sure everything works, and for the most part, it appears to be in pretty good condition. It's seen little use over the years, and has spent a large amount of its time in storage. Unfortunately, it does seem to have developed some issues over the years, and I wanted to check with the community to get some advice on what I need to do to repair these things, and if I need parts, where I can get those. Here are the issues I've found thus far:

 

1. The brightness and contrast pots are both seized up. My guess is due to oxidation over the years, possibly from being in storage and not really being used a lot.
2. Neither of the two disk drives seem to be getting any power. The computer will turn on and boots up with the "CASS?" message in the upper-left corner. The disk drives do not spin when switched on. I'm suspecting the power board that controls the disk drives, and my guess is one or more capacitors have failed. I did a quick glance at the board last night and didn't see any that looked like they had been physically burned or anything like that, but I can supply photos later tonight if that would be of help.
3. One of the disk drives has a broken latch. The latch can still operate and will still hold the disk down, but one of the plastic bits has broken off, and sometimes when you set the latch, the drive mechanism inside pops loose and goes back up. It's a minor issue, and shouldn't affect anything so long as I slowly set the latch and make sure it's secured down. This is the least of the issues I've found, and one I can easily fix later.

The most pressing of the issues, of course, is the power board, because the brightness and contrast are already set at a comfortable level. It's just that at some point I'd like to fix that as well so I can make adjustments as needed later.

[Diecrusher]

Hi and welcome!

 

I have been down a similar route as yourself. I started with a non-working Model III and have now worked into a couple more of those plus a number of Model IV's, Model I's and Model 100's. Don't even get me started on the Tandy's! Beware these old computers are addicting.

 

Now, on to your issues. I am by no means an expert but have been wearing out google to try to find the same answers. This board is rather quite but you will find some really helpful folk occasionally.

 

I don't guarantee my processes will work for you or that they are the overall excepted and recommended methods but they worked for me. Your mileage may vary. DISCLAIMER: Please be careful with the power supply and don't poke around in there with it plugged in unless you know exactly what you are doing! Repeat, I am no expert but please realize that potentially lethal voltages are in there plugged in or not! I assume no responsibility as I am a novice at this stuff too!

 

Now that I made that clear......

 

1) For the brightness and contrast pots. I removed them or at least removed the screws so that I could move them out of the case over the edge so that I could flush them with a trickle of Isopropyl while rotating the pot. Let dry before powering the machine back up. This seemed to clean the contacts just fine and then I checked values with my multimeter to verify that they were ok.

 

2) A known problem with the power supply are the Film Capacitors that seem to fail often with a smoke display. I replaced them with higher quality polypropylene/polyester ones that I got from Mouser. I replaced all of the electrolytic caps also since I was in there with high quality, high-temperature ones even though, honestly, they were probably all still okay but I wanted to make it bulletproof for the future.

 

3) I actually found the stock disk-drives to be relatively easy to disassemble and clean. Just pay attention to the order that you take things apart and don't mess with any of the alignment adjustment screws. It is also helpful to have a manual for the disk drive you are working on. Google is your friend here, do a search for "trs-80 disk drive manual" for example and that should lead you to what you need. You can also check the "Internet Archive" for manuals for the Model III.

 

4) Make sure that your disk drive ribbon cables are hooked up properly also.

 

I was in the same position as you awhile ago and found it hard to find much help. Made a few posts here but didn't get much response so hopefully I can lend a little assistance.

Edited January 8, 2016 by Diecrusher

[+sm3]

Hi, there are 3 capacitors that usually need to be replaced on the power supplies of the Model 3 & 4. Check out this video

 

He is a fellow from Australia that repairs and sells parts for the TRS-80. The non-working drives could also be a bad FDC or possible the ribbon cable that connects the PCBs.

 

You could also email Ian directly for advice http://ianmav.customer.netspace.net.au/trs80/

[Turbo-Torch]

If the Cassette prompt is showing up, there is an issue with the disk controller board. Most likely the flex cable is corroded or has open traces. They were junk back in the day and I was always disassembling the computer to reseat the RS232 and disk controller cables.

Eventually the fingers inside the connector on the motherboard started to break off, so last year I hard soldered a ribbon cable from the motherboard to the disk controller. I doubt I'll ever see an issue with that again. There is also a smaller jumper flex cable on the motherboard that I had to do the same to.

 

Buy a can of Deoxit and use it on the cable connections, screen control pots and any socketed ICs that you can remove.

Get those power supply X capacitors replaced ASAP. They will go out with a bang and you don't need to risk of further component damage or a fire.

[rockman_x_2002]

If the Cassette prompt is showing up, there is an issue with the disk controller board. Most likely the flex cable is corroded or has open traces. They were junk back in the day and I was always disassembling the computer to reseat the RS232 and disk controller cables.

Eventually the fingers inside the connector on the motherboard started to break off, so last year I hard soldered a ribbon cable from the motherboard to the disk controller. I doubt I'll ever see an issue with that again. There is also a smaller jumper flex cable on the motherboard that I had to do the same to.

 

Buy a can of Deoxit and use it on the cable connections, screen control pots and any socketed ICs that you can remove.

Get those power supply X capacitors replaced ASAP. They will go out with a bang and you don't need to risk of further component damage or a fire.

 

That is correct, I'm only seeing the "CASS?" prompt at startup. The reason I suspect the power board and not the actual disk controller is because neither disk drive shows any activity of any kind. Were it the disk controller, but not the power board, then I would've expected the disk drives would both spin for a moment on power on, and their LEDs would light up for a moment as well. None of that is happening, which I why I thought the issue might be with the power board itself.

 

As to the X caps, I had researched that and knew that those were prone to blowing out, especially with age. My aim initially was simply to test and make sure the TRS-80 turns on at all, and to gauge about how functional it is (and as it turns out, it's mostly alright). My next step was to replace the X caps on both the disk and main power boards, because it's very evident that the ones present are indeed the original ones. So good, bad, or indifferent, the old ones are coming out and new ones are going in their place for pure safety reasons. After I do this, I'll test the machine again, and if the disk drives still don't function, then that should pinpoint the disk controller as you've said, and I will investigate that further. Also, I will grab some Deoxit and give that a try on the locked up display control pots and whatever else I can find that needs some cleaning work done. Thank you.

 

Also, sm3, I did watch the video you posted, and it's very helpful, thank you. The boards present in my TRS-80 Model III are a bit different from the one in the video, of course, but I have found a guide that shows the X caps and what you need to replace them with and all that. So I think between that guide and the video you gave me, that should be plenty enough to help me complete this little project. I've not done a whole lot of soldering work, but it does appear to be fairly simple, and I believe it's something I should be able to do without too much trouble. Worst case, I know someone here at work in IT that solders stuff all the time, and I'm sure he could help get me going if I see that I can't get things to work out.

 

Edit: Forgot to mention, too, sm3. The Model III my friend sent me contained a couple of pennies, just like the guy in the video showed. She said she put them in there as a little kid, believing the computer to be a penny bank years ago (fortunately, this was after the computer had been sat in a corner and not powered on for some length of time, and in fact, wasn't powered on in almost 20 years until I got it and plugged it in a couple of days ago). So I suppose depositing money inside TRS-80s must be a fairly common thing.

Edited January 8, 2016 by rockman_x_2002

[+sm3]

Great! I really hope you can get it working again. I have a Model 4 myself, but I really love the look of the III and 4.

 

Just remember to be very careful when you pull the case apart so you don't catch the back of the picture tube on the housing for the PCBs. But sounds like you have already pulled yours off, so you are probably aware of that.

 

I think the older Model IIIs came with 2 power supplies if they had the disk drives, but I don't claim to be a TRS-80 expert.

 

Good luck.

[Turbo-Torch]

It sounds like your floppy drive power supply is completely dead as it also powers the controller board.

Your system is acting like a standard cassette based system. With a working controller, you'd need to hold down BREAK while turning on the power to get the Cass prompt and go into BASIC. Fix the power supply and you'll probably be 100% functional.

 

Controllers and power supplies can vary as there were a number of aftermarket kits. I installed an aftermarket J&M drive kit in mine back in the mid 80s as it was less than ½ the price of Radio Shack's. Also, if you need to replace a drive, install a double sided for twice the storage.

Edited January 9, 2016 by Turbo-Torch

[rockman_x_2002]

As a followup, here's a picture of one of my power boards. This is the disk one, which has the capacitor that I believe to be bad, and I've circled it:

 

 

That's the only X-capacitor that I've found on this board, and the other main board is exactly like this, although its capacitor does look to be in somewhat better shape for the time being. Again, I intend to replace those two capacitors with two of these: http://www.mouser.com/Search/ProductDetail.aspx?R=PME271M610MR30virtualkey64620000virtualkey80-PME271M610MR30

 

Now I tried cleaning the two potentiometers, and while one of them did loosen up a bit (though not enough to be useable), the other is completely seized up and wouldn't budge, even with some force. Therefore, since I'm going to have to replace one of these, and since I've already de-soldered both of them, I figure I might as well save myself trouble later down the road and replace both with new parts.

 

The question is, what do I need to order for these? I know that one of them is a 500 ohm, and the other is a 500k ohm, but there are no other indicators on the housing of these pots. Also, both pots have a D-type shaft which also matches the adjustment wheel's center hole. I want to make sure to get the right kind of potentiometers to replace these with so as not to damage the computer itself, so I'm looking for advice there. It would be great if I could get something from Mouser, since I'm already ordering the caps from them, and that way I could just get everything shipped to me in a single package.

[Turbo-Torch]

The tech ref manual doesn't go into too much detail on the monitor.

In the illustrated parts section, it looks like the pair is listed as item 120 Knob, Thumbwheel.

Manufacturer part # 8719112 Radio Shack part # AK4298.

I don't see any resistance ratings.

[rockman_x_2002]

As a follow-up to this, I took a few pictures of the original pots that came out of the machine, front and back. In the picture with the two pots and adjusters pointed toward the camera, the one on top is the 500Ω one (with red coloring on the back), and the bottom is the 500k (silver backing).

 

 

Both are manufactured by Centralab USA. Hopefully some of that information may help determine the right power rating I need to look for, and where to find a suitable replacement for these.

[Diecrusher]

Again, I intend to replace those two capacitors with two of these: http://www.mouser.com/Search/ProductDetail.aspx?R=PME271M610MR30virtualkey64620000virtualkey80-PME271M610MR30

 

I would not recommend replacing the film cap with the same type as the paper ones are junk but would replace with a higher quality polypropylene/polyester one that in all probability will last for the life of the power supply.

 

I miss-spoke earlier when I said Mouser as I actually ordered from DigiKey. Here is the part that I got but you'll want to verify for yourself. https://www.digikey.com/product-detail/en/ECQ-UAAF104K/P14786-ND/2674018

 

 

 

As stated before, you would be well served to download a manual for your machine from the 'Internet Archive'/'Wayback Machine', as you can find a full schematic and parts list which lists all of this and more.

Edited January 12, 2016 by Diecrusher

[rockman_x_2002]

I would not recommend replacing the film cap with the same type as the paper ones are junk but would replace with a higher quality polypropylene/polyester one that in all probability will last for the life of the power supply.

 

I miss-spoke earlier when I said Mouser as I actually ordered from DigiKey. Here is the part that I got but you'll want to verify for yourself. https://www.digikey.com/product-detail/en/ECQ-UAAF104K/P14786-ND/2674018

 

 

 

As stated before, you would be well served to download a manual for your machine from the 'Internet Archive'/'Wayback Machine', as you can find a full schematic and parts list which lists all of this and more.

 

Thanks for the link. I had already placed an order with Mouser for the two caps as well as the two pots, but I will go ahead and order a couple of these as well tonight, so hopefully they'll get here around the same timeframe.

 

I also spoke with an electrician friend of ours who has taught electrical engineering at the college level for some number of years. I explained the situation to him, showed him the two original pots that I took out of the machine, and he said that he would imagine their power rating around be 1 or 2W, given the application of them, and the fact that the monitor seems to be getting its power from the +12V, 2.02A line from the power supply. His advice was to find a similarly-sized pot with same shaft size and type (if possible), rated at 2W if I could find it just to be safe, and to make sure I got the resistance correct. I couldn't find pots with the flatted shaft like the originals are, but I did find similar length with the specs needed for everything else (they're Honeywells). These are as close to the existing ones as I could find on Mouser's website, so hopefully I'll be able to get these to work:

 

http://www.mouser.com/search/ProductDetail.aspx?R=0virtualkey0virtualkey380C3500

http://www.mouser.com/search/ProductDetail.aspx?R=0virtualkey0virtualkey380C3500K

 

I can go ahead and install the pots into the machine first and test those, since the power supply board is still hooked up, and it's working fine for the moment. That'll at least give me a chance to get those installed and that part of the project done with. Then once the X caps get here, I can put those in as well and hopefully that'll get everything up and running smoothly again.

[Turbo-Torch]

The caps you linked to are fine. That's the exact part # I used and my original made it 34 years before letting out the smoke.

[rockman_x_2002]

The caps you linked to are fine. That's the exact part # I used and my original made it 34 years before letting out the smoke.

 

Awesome. If I can get that kind of lifespan out of it, that'll be plenty of time for me to enjoy the thing, and besides that, after that length of time I would imagine it would probably have developed a whole host of other random issues well before a new cap gives out on it. I did order the second set of X Caps from Digi-key, so those should be arriving right about the same day as the others come in. I'll just store the extra caps in safe keeping for spares just in case I ever do need them.

 

My goals are actually quite simple: A fully working TRS-80 Model III that isn't on fire.

[Diecrusher]

The caps you linked to are fine. That's the exact part # I used and my original made it 34 years before letting out the smoke.

Just offering my opinion and trying to help out.

[rockman_x_2002]

Well, as luck would have it, all the replacement parts from both Mouser and Digi-key arrived on the same day. So far, the only repair I've had time to do is replacing the two pots,and that went well. The brightness and contrast are adjustable as they should be, and everything appears to be working as it should.

 

So my next step will be replacing the two caps, which will probably be sometime tomorrow. Since everything came in at once, I'll use the polypropylene/polyester ones for this job, and I'll keep the other two caps for spares just in case I ever do need them, either for this one or if I ever come across another TRS-80 or something else that uses these.

[rockman_x_2002]

Quick update. I was able to replace both of the X capacitors on the two power supply boards, and that went well and I had no problems with that part of the project.

 

Unfortunately, the disk drive power board still does not seem to be supplying any power output to the drives. I checked the molex connectors that provide 12V and -12V to the disk drives, and both of those have zero output, so there must be something else wrong with that particular power board. I suppose the next thing would either be to find out what component (or components) on this board has malfunctioned and try to repair those as well, or see into possibly just finding a known-working replacement board from somewhere else. Just wondering what you might recommend at this point.

[Turbo-Torch]

Should be simple enough to diagnose with a multi-meter. Do you have a large transformer that sits under the bottom drive? That would be the first place I'd checking for primary and secondary voltage. I can't recall if the oem has that transformer or not.

[rockman_x_2002]

It's been a while since I posted an update but I thought I'd bring everybody up to date. I've been fairly busy so I haven't been able to really work on the TRS-80 that much to get the disk drives going until the last day or two. During that time, however, I did find someone who posted for sale on eBay a set of fully-working power supply boards and a full wiring harness out of a Model 4, which I purchased. Thankfully, it does appear that the X-caps have already been replaced, so I didn't have to go through doing that. Installed one of the new power boards to replace the old non-functioning one for the disk drives, fired the system up, and everything comes up and works just fine, it seems. So it looks like I now have a fully working TRS-80 Model III.

 

So, my next step is to build or buy a cassette cable and lay hands on a cassette recorder that will be compatible with the system. I saw that Walmart still carries these older-style cassette recorder with all the appropriate ports (ear/speaker, mic, and aux), so I'm guessing that will work so far as the cassette recorder part is concerned (though if someone has better knowledge on that please let me know). As for the cable, I've found some schematics online and it appears the cable is actually fairly simple to build. In fact, it's somewhat similar to what I would also need for my TI-99/4a, so if I go through the trouble making the cable, I might be inclined to do so, and build an adapter that lets me convert the cable from one machine to the other for convenience.

 

So the next question I have regards getting software onto the thing. Cassette software I figure would be simple (just use the aforementioned cassette cable on a PC sound card to output data to the system), but disk software is another matter. Is there a recommended way to get disk images that work in a TRS-80 emulator on my PC onto actual diskettes or, failing that, a device that runs those images directly on the TRS-80? Something similar to the 1541 Ultimate II cartridge for the Commodore 64, for example, or even as simple as an X1541 cable so I can write to an actual disk is what I had in mind, but I have no idea if such an animal exists.

[+sm3]

Hi, I'm glad you were able to get your Model III working 100%, that is great news!

 

I'm sure you've seen these 2 items, but I'll share them here anyway. I'll say up front, these are not super cheap.

 

First, there is the FreHD: http://ianmav.customer.netspace.net.au/trs80/emulator

 

For FreHD, Ian sends along 2 (1 for boot 1 for backup) floppies that you boot from to start up the FreHD and then once it boots, you can remove the disk from the real drive. He sells (or you can burn your own) a ROM that will allow you to boot without a floppy, but I have no experience with that. You can place disk images from the internet onto the SD card and copy them back to real floppies.

 

Second, there is the M3SE http://bartlettlabs.com/

The creator of this item sells them on eBay along with the Model 1 version (MISE) http://www.ebay.com/itm/222018785069 (you may be able to get a few bucks off if you buy direct from the seller outside of eBay)

 

I don't have the Model III/4 version, so I can't speak about that, but the Model 1 version boots without a real floppy in the drive and then you can also copy disks back to the real floppy drives if you wish.

 

Maybe someone else can recommend cheaper options.

[Turbo-Torch]

Cassette cables show up all the time on ebay pretty cheap and one guy sells newly made cables for $12.00. Not sure how well a regular tape deck would work. I'm still using my original CTR-80A and even with that, the III was always picky. Even setting the deck too close to the computer would result in corrupt read/writes.

 

As far as transferring files, you'll need a good copy of TRSDOS, LDOS or NEWDOS 80 and a terminal program. I prefer LDOS as it's simple and takes advantage of my double sided drives.

I then attach the III to an older normal PC using the RS232 and a null modem adapter. On the III I use XTERM and on the PC I use TELIX in host mode...been using both of these since the mid 80s.

I then use the III to "log in" to the PC and download all the files I want just like it's a BBS. 4800 baud is the max and if you have issues just drop to 2400.

 

Keep in mind you'll need something that can still run an MSDOS program and has a real RS232. Might be possible to use a USB adapter and some sort of shell on a newer PC.

[Justin Payne]

Thanks for the info. I also have one of those film caps that popped and one those knobs is stuck so I'll use the suggestions posted here and replace caps and clean pots.