TRS 80 Model 100 Repair Part 1
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- Опубліковано 22 сер 2024
- Description
Follow along this Part 1 of this hard-fought battle to bring an old TRS-80 Model 100 laptop back from the brink. Not being familiar with this machine the diagnosis was lengthy but with the help of the good folks on the Model 100 mailing list and Saleae logic analyzer it did spring back to life.
In Part 2 we’ll finish the re-cap and replace the old NiCad memory backup battery with a modern Super Capacitor.
If you have questions are comments, please leave them below. Be sure to subscribe, and click the ‘Thumbs Up’ button if you would.
Links
VCFED post: www.vcfed.org/forum/showthread.php?57850-TRS-80-Model-100-Repair-Suggestions
Club 100: www.club100.org/
Club 100 mailing list: bitchin100.com/...
Wikipedia page: en.wikipedia.o...
M100 ROM disassembly: www.club100.org...
Credits
Wikipedia picture By Napoli Roma - Own work, Public Domain, commons.wikime...
By Hustvedt - Own work, CC BY-SA 3.0, commons.wikime...
Chiptune Thing No. 3 "Victory" by Steven O’Brien / stevenobrien
Creative Commons - Attribution 3.0 Unported- CC BY 3.0
creativecommons....
Music provided by Music for Creators • Chiptune Thing No .3 V...
Hi Jeffrey thank you so much for sharing this video. I bought a non working model 100 from eBay. It had the same black screen error. But thanks to this video i managed to fix mine by removing RAM number 0 and replace it with a working one from another socket. Greetings from Denmark.
Hi. I'm glad the video was helpful. Enjoy using your 'new' M100 :)
Just found this. Watching you diagnose and fix yours was very interesting. I still have my M100. I also bought a few extra on Ebay when they went obsolete. Got them for $50 each thinking they would be sources of parts one day. I have pulled all the NiCads and I have 2 packs of replacement capacitors but I'm a bit fearful of getting in there. Maybe I'll practice on one before I do my original. Thing is I don't really know what to do with my 100 and all the stuff that goes with it in this age of smart phones and Apple M1 chips. My son shakes his head in bewilderment. I'm afraid everything will be in an estate sale and he will get $10 for it 🙁 I have a few ROM chips but my favorite is Lucid ... a spreadsheet ROM.
This was the first M100 I had worked on, so the repair process was a bit lengthy. Luckily now a friend of mine has made a M100 test harness/ROM set that can diagnose a faulty memory module in just a few minutes.
I was going to suggest that it would be RAM, but there's absolute value in approaching it the way you did - just to show how using the proper logical techniques you can follow through and work out the exact cause. Brilliant video =D I've been working on a Dragon 32 btw - and as you said it uses pretty much the same architecture. I was in the process of creating my own diagnostic ROM to test the RAM, and also to look at making it run as a CoCo as well as a Dragon 32 with a ROM and keyboard mod, but that might be a later video after I get the initial Dragon 32 repair up.
If nothing else I learned a lot about how the system operates. Given that knowledge another repair like this will be much easier. There is also the chance when you pull a part that you ruin what might be a good part or damage the board. The way this PCB was laid out it was somewhat difficult to rework, preheating the PCB was the trick that helped out the most. I cut through all the work it took to get that RAM module out as it would be boring to watch me working on it for 5-6 minutes :) Thanks!
Great addition to the M100 community.
Thanks!
Thanks for a great video. I just finished working on my M102. It was just missing the Tandy badge on the case. I picked up an M102 off eBay with a good case, but was littered with battery damage on the motherboard. So swapping the front of the case? No big deal. Not so fast. The caps lock key-cap fell off my M102 and the end of the base that snaps into the mechanism broke off and stayed below the lock. I tried all the extra key caps I had before I finally realized why no key cap would snap into the mechanism. So swap keyboards with the new M102? Unfortunately the keyboard in the new M102 was less than responsive and many mechanisms were rusty. So, ended up using the caps-lock mechanism which I removed from a parts M100 keyboard. More trouble than I anticipated when all I had planned to do was swap the front case.
Your video helped so much to educate and relax. Thanks.
It is like archeology sometimes taking some old piece of equipment apart, trying to figure out how they put it together and what. It is invaluable that we can all share what we learn so easily. Without the help of the M100 mailing list I would have been lost. early on, chasing my tail with that 'odd' data latch artifact.
Ok after decoding the data bus for a full day, I found that I got into the cold boot routine up to 7E08H. You can get here by triggering on the 3rd instance of (CDH) opcode fetch from a cold boot . Once there, something fails before I reach 7E1BH which is first instance of op code (AFH). There is a lot of ram copying happening there, so my guess is that I am also suffering from bad ram. This is an 8k machine. Fun learning experience. I have a few 6264 ram chips so I think I'm going to try to etch some ram module pcb's this weekend.
It is copying a small chunk of code from ROM to RAM so it can run as self-modifying code. The RAM failure I had caused it to overwrite the stack which causes it to try to RETURN to some random location in memory.
@@HeyBirt Thanks Birt!
Great work. Thanks for sharing.
Thanks! I have more Model 100/102 videos in the works including a look at the DVI (Disk Video Interface)!
Thanks for the great video!
Thanks Bob. I have some more M0del 100/102 videos to do. I managed to pick up a DVI (Disk Video Unit) which I want to get working as well.
Hey Jeff, great video - but I do have some quick questions if you have a chance-
1. Are the capacitors radial design, any certain place you got them?
2. Do you repair model 100s as sort of on the side gig? I am trying to repair one of my extra units, but shaky hands with the solder don't mix haha.
Thanks!
Hi Vinny, in the description below there is a link to color coded map with part numbers from Mouser. I do some repairs for others. You can email me at birt_j at soigeneris dot com
Thanks for this video! I decided to delve into old electronics and got a Model 100 recently and I've got the black screen of death as well. I've got completely identical symptoms (except my caps aren't leaking yet and the battery has some corrosion but holds charge) so I suspect the RAM must be my issue as well.
Do you have a datasheet (or even a napkin drawing) of which pins you hooked the logic analyzer up to for the data latch ram/rom dump? Also any tips on the inevitable desoldering of the RAM? I've done a decent bit of soldering and it seems desoldering anything with more than 2 pins requires a proper desoldering station or the patience of a saint.
I did not make any notes on which specific pins I tested. Basically, I just connected to the data lines after the demux. Have you done a full reset, Off the top of my head I forget the key sequence but you hold down a couple of keys and press reset that tells the system to try and copy the bits of code from ROM to RAM that are used in the start up process.
@@HeyBirt Yep, tried full reset (CTRL + PAUSE + RESET and CTRL + BREAK + RESET since I saw people saying both) many times. Both normal reset and full reset are unresponsive.
I actually had it working for about 15 minutes after receiving it, then it glitched out (garbage on screen) and the problems started. At first I was getting garbage data on the screen and RESET was causing it to change, but eventually it just got to the black screen and stopped responding to anything I do.
Just once after black screening, it managed to get back to having garbage on screen and somehow drop me a BASIC shell but eventually it went back to the black screen and it's been like that since
@@Anachronism91 Sometimes if the memory battery is bad it can prevent the computer from booting properly. In fact if the battery is dead you may have to let the machine set overnight to charge it up enough to get things to work properly.
If you still have the original memory battery it will need replaced anyhow. You can desolder it or clip the leads from the top side to remove it and then test the computer again. Without the memory battery in there (or without a properly functioning one) you will need to do a hard reset every time you power up. I have the memory batteries (newer NiMh type) on my website.
You should also check the power supply voltages. As I recall there are two separate 5V rails and a -5V rail for the LCD.
Hey Birt! I desoldered the battery but no change. I’m still getting the correct voltage going into all ICs and I still have the black screen. Battery definitely needs to go anyway since there’s some corrosion crystals on it. Thankfully it hasn’t started eating away the PCB
@@Anachronism91 Hmmm, if you have any optional RAM modules remove those and reseat any socketed chips. The intermittent nature of the problem bothers me as it sounds like a possible bad connection, etc.
Please tell me where to find the ROM source... I've googled for hours and even asked on the vintage computer forum and no luck finding it.
I got it from the club100.org member files section. There is a lot of stuff there but hard to find. I added the direct link to the video description.
@@HeyBirt Thanks!! I'm battling the exact same issue. My logic analyzer shows the cpu start to boot and then stop. Sometimes it goes into an infinite loop.
@@helipilot727 This can happen with a RAM or ROM issue. I have had broken traces also cause this sort of failure. Check every pin of the ROM and RAM (remove all but soldered in RAM modules) to make sure that all signals are getting through. This can be tricky as when it hangs it might stop trying to use some higher address so those lines look dead.
Also note that the flux used on the M100 will turn slightly conductive and cause all sort of strange issues. It is also a huge plain to clean off.
@@HeyBirt Thanks!!
@@helipilot727 In some of my later videos I use a test harness that a friend of mine developed. It makes it easy to find a bad RAM module, etc. I show it in this video here: ua-cam.com/video/cwMzl_Rqqw0/v-deo.html
He built a few extra and I may still have one extra test harness on hand if you are interested.
I just replaced the keyboard today :)
That get it fixed up then? Keep the old KB for parts :)
@@HeyBirt Oh yes, the broken one I bought off of ebay had a wall power supply, the expanded RAM option with the 3 chips, plus Microsoft Multiplan spreadsheet program on a chip, and the working keyboard I was after... The M100 I fixed doesn't have the expanded RAM option... :(
I tried to get the M100 from ebay to power up but no go, no red light, no screen, nothing... I'll mess around with it when I have the time. The Multiplan chip I popped into my M102 :)
I attempted to follow your tutorial for repair but only made it worse? Can you repair it for me and I'll pay for it and the shipping. :(
I tried to replace the capacitors but I ended up de-soldering the wrong pegs. I could have sworn after checking multiple times with the desk light that the marker was the right place! HELP!!!(ASSUMING I CAN"T FIND a local electronics repair shop!)
Well, I tested it and its working .... Maybe, this is my first time soldering and I found out I can remelt the solder of the pegs back into place. Maybe, I'm too careful and overthinking.
update: i stayed up late, and found out that resoldering the pegs fixed the problem but not the capacitors. First time soldering(with practice run) and only got one capacitor soldered right. Its not broken.
Travis Wells - Glad you sorted it out. It can be hard to figure out which leads are which on the back of a PCB, that is for sure. I found on the Model 100 that the big ground plane traces made it harder to heat things up enough to properly desolder. I used a small heat gun to preheat the PCB in that are (to maybe 150F) which made the job easier.
I like to recommend practicing desolder and soldering on an old junk PCB that you do not care about. Then once you are comfortable with the process and your equipment move on to your actual project.
Jeffrey Birt
Well, my impatience for looking at details can be the cause. I did manage to fix that annoying flickering screen problem with those 10 uf 16 v capacitors. I intend replacing everything on that board that needs fixing.
I managed to clean out the board with rubbing alcohol with a bulb squeezer used for ears(like the ones for babies). Of course with the memory switch off .
I used a shot vac to dry out the defiant power switch and the rest aired out the night after.
I've seen accessories for this machine and intend in trying to run a basic text mode linux kernel with some hardware modifications.
Instead of using casette tapes, I'll think I use sdcards. Some research into custom roms might make it feasible to run it.
Hardware wise, maybe not possible.
The best place for Model 100 info is Club 100: www.club100.org/ there is also a facebook group.