TRS-80 Model 1 repair and lower case text mod (With the diagnostic ROM)
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- Опубликовано: 15 сен 2024
- #septandy #lowercasemod
On the bench today we have a TRS-80 Model 1 that is having some strange issues. On a recent second channel video, I was doing a quick check-up of the machine and it suddenly started malfunctioning where the video output would go away.
In this video, I try to identify the fix the problem and at the same time also do a lower-case text upgrade to the Model 1.
Part 0: • TRS-80 Model 1 test an...
Part 1: This part!
Part 2: Coming Soon
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What I love about Adrian's videos is that you can go along with his diagnostic trail and make your own guesses - bad capacitor, dry joint, leaky battery, crappy power supply...
Yes, and he does it so well that the more you watch his videos the better you get at diagnosing the fault correctly!
Meaning hours of watching these videos has actually taught us something useful... Can't say that for many RUclips channels!
I was thinking cracked/dry solder joint lol
Don't worry about burning us out on the TRS-80 content, you have a large chunk of viewers who are big TRS-80/Tandy fans and we love to see these video's and test ROM development. Here's to many more in the future!!
About a month after the TRS-80 hit the market, my father and I went in together to purchase one. That's because it took me a month to convince the old man to do it. (At age 17, I spent my life savings!). It was $640 (including cassette player) in 1977, which is equivalent to $3,123 today and it had 4K of RAM and a 4K tiny BASIC on ROM, and it was AWESOME! Soon, we were writing games for Micro Pro International which advertised in one of the Magazines (probably Byte). Micro Pro was a desk in the corner of a barn on a pig farm in south Georgia, but it was Awesome! My father and I wrote Games Pack I, and later upgraded it to have voice with the Speech Synthesizer, and then a Monopoly game with good AI that would draw the board, throw and display the dice, do all the banking, handle all the chance and community chest cards, and even play against you. One of the games in the pack was Battleship that had an AI that played a very good, challenging game. It didn't win all the time, but enough to keep you interested. One day a kid called to ask how we got the AI so good with only 16K of RAM. I told him we worked very hard on it and we were really proud of how it turned out, and of course, our algorithm is proprietary, so I couldn't share anything more about it. The truth was, with only 16K it was impossible to have a decent AI. The only way we could make it a good opponent was to have it cheat. It secretly took up to 5 guesses on your position before declaring a miss. (ʘ_ʘ) I'm sure it happened a lot, especially in those days, but we found a good balance, and everyone loved it, and that's what mattered. All good wishes!
I'm guessing the regulator output transistor has a short between emitter and base where the driver transistor is taking all the load.
Eventually it gets hot enough where the 5v gets messed up that screws up the video ICs. Could also be affecting the character video output. Let's not forget that also ends up missing and Z5,6 don't affect character video output. At least not where we're looking.
Yes! was going to write something similar.
I think a complete short wouldn't generate 5V, but I'd say it may be a problem with Q4 rather than Q3.
I don't think the heat can get all the way across the board BUT it CAN make the voltage too jittery.
This could also explain the HSYNC jitter. *Check +5V with a scope!*. Check VCC on the chip pins.
If not a short the PNP transistor, Q4, could actually not work at all, leaving the 'regulator drive', Q3, to act like a voltage follower with the 68 ohm resistor on its collector to be the only current limiter.
@@billraty14 or if the regulator transistor is OC from B to C or SC B to E then the B to E junction will supply the driver transistor all the power it needs not limited by the 68R?
I did repair those machine in the past. Normally that 5 volt regulating transistor on its large heatsink got quite warm during normal operation - not really hot, just warm. The fact that it does not get warm at all means it is broken, I indeed in suspect a short between base and emitter - that would explain what is happening here.
Te Big transistor from the regulator is bad.
I always enjoy these repair videos, I feel like I'm finally getting to learn about the machines I always wanted as a kid but never got a chance to own. It's incredible to see the parts of a machine broken down into sections performing tasks, and you do a wonderful job of explaining things.
Like some of the others here, my first computer was the TRS-80. In mid seventies as a teenager I read many magazines, that being how one found out about new technology. So I read Popular Electronics, Creative Computing, and Byte among others. In these magazines I read about the all of the new microprocessors coming out and an saw new microcomputers that were being developed with these microprocessors. I dreamed of having my own, but most of them were kits designed for the advanced hobbyist. Most of them didn't have full keyboards or displays, the base models didn't have any means of data storage, they had to be programmed in machine code. There were all kinds of add-ons and upgrades developed to address these issues but all of them took money. Video display terminals, audio cassette storage options like the Kansas City standard. People were adapting surplus teletypes, and papertape readers and punchers. The bottom like was the cost of entry was too high for a young, poor teenager. But then I read about some new computers coming out that were all in one solutions the Commodore Pet 2001, and the Radio Shack TRS-80. The base model of both of these came complete with monitor, keyboard, cassette data storage, and built in BASIC for only $600. The Apple II was also coming out, but it cost more even without a display or cassette.
Since I had a good summer job by this time, I started saving my money, determined to get a real computer. It pretty much had to be the TRS-80 as I lived in a small town, and there were no computer dealers that I knew of within 100 miles. But there were at least a couple of Radio Shacks. I went to my usual Radio Shack and paid money to reserve a TRS-80, I don't remember now exactly how much it was, I think around $100 down, but I got the first computer they received at that store. There was virtually no software available at launch, I think a Black Jack game, and something else. But I had plenty of magazines with BASIC program listings, and I bought a book called 101 BASIC Games. After going through all of the lessons that were in the excellent users manual that came with my computer, I had started learning BASIC, and translating many of the typed listings from magazines, and 101 BASIC Games, taught me a lot more about programming.
It’s been said, but: I really appreciate all of the additional work you are putting into dividing the screen among multiple sources. Makes the already-good content more engaging.
You need not worry about burning me out on TRS-80 repairs as your troubleshooting and analysis are universal needs with retro repairs. Great video once again.
Keep the TRS-80 content coming! You do such a good job, Adrian. The fact you keep the warts and all content makes this such an interesting process. If you cut things down to just 'successes' it would lose so much. The Model 1 Level 2 16K was my first ever computer I encountered - my school teacher bought one with his own money and brought it into the school - in 1979. So, am particularly interested in what you do with this machine!
I'm very happy to say that I have my own TRS-80 up and running after replacing the RAM, 2x 74LS logic IC's, the 2x VRM IC's, the smaller of the 2 power transistors (Q6), the keyboard ribbon and the power on LED. Also, I didn't realise and got caught out by the fact that the video out is not wired the same way as my standard composite/audio cable. Most have pin 2 as GND, the TRS-80 doesn't use pin 2, so I had to connect pin 2 on the TRS to GND in order for the composite output to work with my cable. Looking forward to the next video BTW :)
Q4 is dead and Q3 drive all the current, that's why it's overheating and fall in protective mode, it's a very common fault in the trs80.
Good spot. Old PNP power transistors tend to be much more prone to failures as well. An MJ2955 should work as a replacement for the 2N6594... likewise a BD139 should work as a replacement for the MJE29. Interesting that they used a Sziklai pair arrangement !
@@jaycee1980 looking at the schematic I see the output from the 723 regulator is 5.8V and as you noted this is connected to a Sziklai pair which doesn’t suffer from two Vbe drops like a typical Darlington, I think the fault mode with a NPN Darlington pair could be s/c Q4 C-E and unregulated DC output to main board… Well spotted on the Sziklai front though :)
What keeps bringing me back to these videos is that Adrian is giving us a master class in digital and analog circuit troubleshooting, something I did for a living early in my career, but gave up for the "easier" job of software engineering. His thorough knowledge of circuit sub-systems (RAM, CPU, timing, video logic, power regulation, etc) and his broad experience in many different computer models and eras makes his troubleshooting presentations a joy to watch and a valuable educational tool. Kudos to Adrian for these excellent videos!
Thanks! It's very funny to say thuogh, as truly I am a mere here. Five years ago I'm not sure I would have been able to fix any of this stuff as I had only ever lightly dabbled in this -- I just kind of dove in head first and found this super fun, so now I'm just applied logical reasoning to repair work. That has always been one of my biggest strengths and luckily it helps me with other things, not just repairing computers.
I went into software because it took me most of a lifetime to get the self-discipline to not just throw some parts together and be mystified why they didn't work. With software you can (usually) go back to the previous version that worked and start putting back one thing at a time. With hardware, it's not so easy to go back to yesterday's version.
@@8bitwiz_ That's why I was complementing Adrian on his diagnostic skills. Debugging a live circuit just by looking at a schematic and poking leads with a logic probe is not easy AT ALL! Though I must say, that tiny oscilloscope was just plain sexy. Back in my day they were 50 lbs and required wheels to move around. Yikes!
The power supply failure, as others have talked about, is affecting some chip outputs, which was shown in the video (the double tier highs) and you ignored. The TO-220 package will always get hot, even when the power supply is working correctly, but the TO-3 package should be hot as well. This could be the source of the whole problem, so it needs to be fixed first. What happens when a Raspberry Pi doesn't have enough power?
This video issue only occurs when assembled and the video connector is plugged in. That is where you should be focusing, as it is most likely the load placed on the video out (75 ohms) that is causing the issue.
That tiny circuit board is a hardware work-around for a software bug in the ROM, in regards to the cassette interface. It basically replaces the software reset of a flip-flop with a hardware clock divider, as the software in the ROM resets the flip-flop too early. Back in '79 I just burned my own ROMs with a fix for the cassette, and another fix for the keyboard debounce. With the circuit board installed, you won't be able to use any of the TSR programs that allow you to use 1000, or 1500 baud.
First point - my thoughts exactly. Second point - probably yes but not necessarily.
It could be that covering Q3 with the keyboard makes it overheat much faster than when it's in the open.
It could also be that there is a crack and intermittent connection which only happens when keyboard is sitting on top (although mismatched in-out signal on 7404 probably disproves this hypothesis)
Wow. Now that is some specific insights. It will be fascinating to see if you are correct. But it does highlight that the knowledge you guys have maybe lost some day if it weren't for these videos.
Loved the accidental "glitch art" text effects when the 8th video RAM bit wasn't yet connected properly. And thanks for not cutting out your frustration about the hard-to-reproduce fault, it's very relatable.
I always appreciate the detail, Adrian! Besides, it's unrealistic to assume a repair is going to be successful the first time, especially when one is facing multiple faults (as I suspect you are). 👍
Any component level repair is appreciated. This is what I love, the bonus is that vintage technology gets repaired and a chance to live again. I wish I had the ability to make videos of the repairs I do. Sadly I don’t. But thanks to Adrian and other creators like shango066 , we have some superb diagnostics videos.
FYI. I have been watching more BECAUSE you are covering the TRS-80's.
Get yourself a can of freezer,that will help with intermittant faults,as a retired electronics engineer i can say freezer is a god send when intermittant faults come up.Love the videos,Thank you.
I cut my computing teeth on a Coco 1, so I love seeing the TRS-80 stuff on here! And this couldn't be more timely as I have a Model I sitting on my bench right now in need of repairs and will be using your diagnostic ROM very soon. I'm incredibly grateful that you and the others have created that! Once the Model I is working again I need to work on the monitor a little (tho I suspect the only problem there may be a dodgy pot for the vertical hold) and an expansion interface I picked up a couple months ago of unknown condition.
when Adrian said "bridge rectifier" i almost screamed HE SAID THE THING! then i remembered im not watching electroboom..
Either way, great content Adrian, keep up the great work, love these kinda videos.
I am really interested in HOW you think and in the way YOU solve problems! This adventure is riveting! Excellent!
I knew as soon as you said you were going to put it all back in the case, it was going to rear it's ugly head again! How did I know? Because it happens to me quite frequently. It's the very reason why I won't put the final screws in anything I repair until I've tested it one last time! Great stuff man...
According to an old copy of Popular Electronics I have a PDF of, in 1976 a 2102 memory was about $3 or $15.60 in todays money, so yes, there was a big incentive not to use more than necessary!
From an OLD original TRS-80 owner and modifier from back in the day. I can tell you right off that the issue is a dry joint, I had a few in my TRS-80 back in the day and I can tell you that the little board on top of the keyboard is a speed changing option, I forget which port you poke with a 1 or a 0 to turn the speed up from 1.77MHz to 3.54Mhz. But doing that effects everything including trying to read from tapes. So what we used to do is load the program at the slow speed, Change the speed and then save it out again at high speed. I also built the LNW80 interface for my TRS-80 which then expanded the memory to 48K added a serial interface Floppy interface and of course the parallel port. There was also a book I had that showed projects you could build for your TRS-80 to give it better graphics. (Ahh google is my friend..."The Custom TRS-80 and Other Mysteries" was my inspiration and from there I became a digital Electronics engineer. Needless to say I didn't need special Roms to debug systems. I could look at the issue and know where to go and what to do :) But having a certificate in "Digital electronics" helped back then too. The Custom TRS-80 book I referenced has a graphical interface / modification section that does have a wiring error. But that is discovered when you compare the original TRS-80 circuit to theirs. I wire wrapped the graphics interface and ...yeah fun. The stories I can tell you about my days back then...
I was thinking solder joint too.
I would love a series on the basics of using an oscilloscope especially when it comes to troubleshooting this kind of stuff.
My Model 1 currently has no problems, but watching these will be a huge help when/if mine fails.
I understand like 4% of everything Adrian talks about but I still watch every video regardless lol
Although we had a Dick Smith System 80 instead of the Model 1, I’ll always have the soft spot for the Tandy. They’re a pretty simple machine (both in design and functionality) but that’s part of the charm.
My Model 1 is an unmodified 16k Level 1 machine, so functionality is even more limited, but I decided to leave it as is, so hopefully I’ll pick up a Level 2 machine one day.
A lot of people seem to forget that the TRS80 out sold the competition for a good few years, post-1977 before getting overtaken by Apple in 1982/83. So although everyone is an Apple/Commodore fan, more coverage of the little Tandy is great to see.
Not everyone. Showing some Atari love.
Great video Adrian, it sure brings back memories. I immediately thought of the HPOS and VPOS pots as problems (been there, done that).
I had a couple of TRS-80 Model I way back in the day. Sadly, the power supply for the Model I was absolute garbage, and was the main cause of lockups and crashes. I ended up putting an external switching power supplies (Tandy figured that out for the Model III) on them and removed all of the stock power regulation circuitry (also in the expansion interface), and put on the gold edge connector mod, and the machine was rock solid after that. I used one of them to run a BBS for about 8 years ('82 to '90).
They both had the lowercase mod (with descenders, so you don't have flying g, p and q, I think it was a different CG ROM P/N), speed-up mods for overclocking to 3.54Mhz, I ended up replacing the CPUs with a Z80A.
I built a number of expansion projects, some from magazines and books such as Micro80, and the Wizard series, and after that started designing them myself. One of the fun things I designed and built was a programmable character generator so it was possible to write custom character maps on the fly (fancy fonts), or using the 0-31 positions for other character graphics. It was basically a small SRAM chip that you could electronically switch-in in place of the standard CG ROM, with read/write buffers and I/O decode for addressing it.
All of the experience scored me one of my first jobs working on Z80 assembly on embedded systems.
This brought back memories! The TRS-80 Model I was my first computer when I was a kid, 40 years ago. I completely remember moding it for lower case by bending out the IC pins to piggy back it and cutting the trace to get lower case. I was so nervous and worried that I was going to ruin my beloved Model I. And I was so proud of myself when it worked! Thanks for this!
I really like being along for the adventure and learning with you. Your presentation is top notch. It’s nice to see a variety of hardware too, but don’t worry if any adventure takes longer than expected…
Adrian, you said you hope you're not burning us out on TRS-80 content - not at all! I really like these deeper more difficult diagnosis videos - seeing all the things you try is very interesting. Keep em coming!
Very interesting video. Unfortunately I wasn't able to have/see a TRS80 because I'm living in Italy. Great job with the diagnostic ROM!
There is no need to ever apologize. We all admire your persistence! But this was a case of trusting your nose! :) It has to be that regulator getting too hot.
I for one, am enjoying the TRS-80 content. I have a couple of CoCo2s that I need to give some love soon, so keepit coming.
Interesting to see that you also use "poke"-commands on the TRS-80. I used "poke"-commands on my C64 back in the 80s.
I love trs-80, please keep making trs-80!
always enjoy watching your repair videos they are highly informative for troubleshooting older hardware
I had one of those computers back in the day. A great book for customizing the computer is "Custom TRS-80 and Other Mysteries" by Dennis Bathory-Kitz.
Got that book 40 years ago and was fascinated. Missed a chance to meet Dennis Kitsz when he came into my workplace in the mid 1980’s
Oh wow!
I learnt how to program BASIC and then Z80 machine code on one of them.
Such fun times!
Love your enthusiasm its fun fixing things and not giving up. I myself repaired all for 50 years. Liked my s100 z80 I built even wirewraped my cpu board and had the first memory mapped Video Mio from IMSAI . love when it all works.
Not burning me out. TRS-80s (Mod 1/3) are my thing and there aren't channels covering them.
This is a good journey which you are taking us on, it will be worth it to see the machine working properly in the end.
I remember modding a couple of these in my teens adding the extra piggybacked ram chip, also added a little toggle switch to flip the effects of the mod on and off, the long armed toggle switch fitted near the 5pin sockets.
I love this TRS-80 tangent.
On older machines I honestly think about replacing the voltage regulation with more modern switching power supplies. The new power supplies are much better than the old powers supplies and will be less likely to go "un-regulated" and blow components that are either hard to replace or impossible to source.
actually i'd say opposite, far more to go wrong with switching regulators, more efficient maybe, but more 'reliable' hmm ...dunno, if you're worried about overvoltage, fit a power zener across its output, such as with a 7805 or discrete 5v circuit, fit a 1N5339 5v6 5 watt zener across its output, .. ideally you want one 5v3 or 5v4 but no such voltage available ,, and ensure theres a fuse in the dc input somewhere ... i've never ...yet...had a 7805 or similar 'short through' or over voltage tolerance, go 'noisy'/poor regulation, yes....
I've worked on TRS-80s for a few years now. They're way trickier to debug than the other stuff I've worked on (Commodore, Atari, TI-99/4A, Apple). I think it's because they use so few custom chips. They're really an engineer's machine, because they're loaded with tons of glue chips that cooperate to produce a working machine. No custom chips to speak of. In one sense that makes them easier to debug (no black boxes), but because there are *so many* chips used to achieve various things, it can be tricky figuring out which one or ones are acting up.
At least if a glue logic chip fails you can just replace it.
If a C64 VIC, PLA or especially a SID fails you can only source it from a dead C64.
There are now FPGA replacements for some of the chips, but they are expensive and have their own issues.
I would temporarily replace the regulated 5v supply with a known good bench PSU and then diagnose the video issue. That will show whether the jittery video/intermittent HSYNC is due to a dodgy power supply or a dodgy logic device. Then look at the 5v regulator circuitry.
OK, its a holiday, ready for part 2 :-)
Took me 7 months to finally get my Model 1 back to perfect condition. So many areas that can go bad. But one of my favorite machines now with 48K maxed out expansion interface, lower case mod, and Level 1 / Level 2 ROM toggle. Regarding the lowercase mod, I don't recall that experience even on an older ROM version unless it's a strange character generator. By default with cassette BASIC the lowercase is not accessible for typing, even after installing the hardware mod, but should still render the uppercase character set as intended. There is a ML driver that can be ran before entering BASIC to gain the option to use the final bit and support lowercase characters when typing.
14:40 You need to get yourself an IR camera, you can get them that attach to your phone. Then you can take a picture of the board and identify precisely what is or isn't running hot with much better precision than using a "finger-mometer".
I've got a Flir one. I tend to not use it too much as the battery is always dead and normally I find issues with my hands. It's useful in things like monitors though, where touching things isn't always a good idea LOL
@@adriansdigitalbasement if you ever want to splurge you can get the same flir multimeter i have that has 1 built in
Mine is the dm285 but their is the much cheaper dm166
@@adriansdigitalbasement You already discovered the hot transistor around 14:36, so you absolutely do not need a thermal camera.
@@MitzaMaxwell This time - Adrian could smell burning at the start, and a thermal camera would have sorted out what was running hot and saved a good 11 minutes.
True, but IR cameras are still quite expensive. I can't speak for Adrian, but many channels deliberately avoid breaking out the expensive stuff because they want to encourage people to do it on their own without feeling like they have to pay a fortune to get started. For people starting out, the money for an IR would be better spent elsewhere. But they absolutely do work and they're great.
As a teen with my beginner soldering skills (“skill” is probably pushing it) I installed a numeric keypad in my almost new model 1. The keypad came from a weird RCA calculator/alarm clock that had the form factor of a cassette player/recorder. Anyway my Frankenstein hack worked and didn’t even look too terrible.
The more TRS-80 vids, the better! Thanks so much for all those, and more!
A handy technique to find failing chips is by touching the top or measuring the temperature in the very center of the chip (where the die is). Any chip that is very much hotter (uncomfortable to hold your finger on) is probably on its way to failing.
We love this Adrian. Thanks!
I for one never get burnt out on anything Tandy
Back in the 80s I was in a computer club in Tampa. Known issues with the model 1 was cold solder joints, just reflow with new solder. The expansion port always needed to be cleaned with an eraser. This how they got their nickname Trash 80, I hated that name.
My first computer was a TRS-80 Color Computer 32k with extended basic. Paid $499 plus shipping in 1982 from a wholesale warehouse in Texas far less than buying it from Radio Shack.
LOVE the TRS80 content.. I was an old COCO guy (first one was 4k!) but I always wanted a model 1, or model 3.. Someday I'll get one!! :-) Keep it up with this series for sure!!
Thanks for sharing and the reminder of the SAM manual. Completely forgot about it. Loving the diagnostic ROM!!! Super cool!
I enjoy these types of video. The troubleshooting process is good to watch.
3:22 "basically broke right in front of my eyes" - that's what happens every time I try to repair something ;)
30:47 the partially-garbled characters reminds me of Teletext when reception was bad!
Adrian: "Gee this problem always sometimes happens! But I open the case and everything working..."
"I open the case and everything's working"
"Oh it has to be a heat issue! not this aftermarket mess of wires when I put the case back on."
I love Tandy stuff. My first machine i had as a kid was a Tandy 1000SX and some days I miss the simiplicity of Dos 3.2 on that machine. hahaha
1. *Carefully* scope power lines when problem occurs. PSU or Q4 or Q6 or caps partially failed/too noisy?
2. It may be bad connection or even a break inside IC package. Try carefully knocking/bending the board.
3. If power is fine, replace both Z5 and Z6. You don't want to repair it *again* ...
4. IIRC the mod should NOT mess up characters in BASIC. But BASIC may not allow to use l-case.
5. Try to leave it running when the fault occurs and THEN feel if Z5 and Z6 are hot. It the output one is the hot one, it may be that the input one is shorted.
6. This blinking and jitter is suspect. Does model 3 do that? You may need to look into that more.
7. If jitter is due to POTs, replace with better quality ones like ones with screw on top. It'll save your nerves.
8. Check if the sync is actually the right frequency and phase when the fault occurs.
9. Check for flaky wiring in the two other mods.
10. The fault may only occur if keyboard is on top and video connected. This could be:
---- regulators overheating faster due to less ventilation with keyboard on
---- bad connection/bad joint/crack due to slightly bending motherboard
---- some fault in video circuitry that exaggerates the problem with power
Enjoying the series very much Adrian and really looking forward to the next but can you please consider keeping future follow up projects on the same channel as the first. It makes it much easier (for me) to find them all in chronological order and in one place rather than sieving through two channels to find them. For example if I want to re-watch this series of fixing these models together with the progress of the Diagnostic ROM (which both tie together) next year, it is going to be a nightmare.
Love the TRS-80 content, especially since this is SepTandy! This kind of troubleshooting should be very instructive to both those who are fixing old equipment and budding engineers who are building (and troubleshooting) their own new hardware. Having done both, I totally understand the frustrating nature of an intermittent fault. The aerospace industry used to call those kind of faults "gremlins" - maybe that should be the nickname of this Model 1? Cheers!
FWIW, I watch these to see you fix stuff. I am agnostic on what that stuff needs to be, so long as it's old and tangentially related to computers.
Cool video and great diagnostic! Also great editing - I know how long those windows and multi-camera angles take to make! :)
Adrian, try hooking it back up to that Apple // monitor - that’s when the problems began with the video output on the last repair video. Maybe that monitor was putting too much of a load on the video circuitry of the TRS-80. Maybe? 🤔
if that big transistor is cold, its not passing current correctly, likely open circuit collector or shorted emitter to base, so all the 5v current is passing through its emitter-base junction and then the smaller transistor dissipating the full power instead of the big one taking most of it
I really enjoy the variety of machines, and the troubleshooting. Keep it up!
Nice job on that diagnostic rom for the TRS. Will get the code and peek at what you folks have done.
For me it's not so much the specific hardware as just the troubleshooting in general. It's very useful knowledge, thanks!
My very first computer was a model 1 level 2 with the expansion interface. Mom upgraded to an XT clone and I got the hand-me-downs. So no, you're not going to burn me out anytime soon. Actually I might have to adopt a few of these!
Intermittent failures can be infuriating to diagnose. One of the worst feelings I get when I start a repair is when everything is working normally. Can't wait to see what was the issue here!
Hah, what even is a quick ADB video? Look forward to the next episode!
I am curious to see how many old original chips will have to be replaced before this machine begins to function correctly.
TRS of Thesius
This was my 1st one without numeric keypad. That came later available.
When the memory prices dropped by 50% I get my Extension Interface with full memory..
Later up to 3 floppy drives.
Making floppy's double side.
Epson matrix printer.
Newdos 80 is coming in my mind also.
The first computer I was exposed to was the TRS-80 Model 1 with level II BASIC. And a tape deck.
That was the computer my small-town high school had bought to teach us about computers.
I would like to have a Model III...
So close and yet so far. I'm loving the Trash 80 vids.
I've been working on a Spectrum 128K for 2 years now. I've fixed three of them in the past along with a number of 48K Speccys. But this one has me beat. I can get it working with 4 memory chips, but any more than that and the screen corrupts and the system crashes after about 30 seconds.Very odd. LOL.
I seem to remember a chronic problem with The TRS80 model 1 was a Transistor failing in a weird way, but we're going back to 1979, when I started using one in the 6th grade... I was not paying attention, but the example I had did have the upgrade ROM and BASIC upgrade. I still distinctly remember that the Model I Basic did not support DIM array functions that the Later Model II and Model III did.
Love the Tandy content, but especially anything on the Tandy 1000. We never swap them in Ireland, as far as I know, so they are a fascinating part of history for me.
Video sync issues are generally either Z5 or Z6 in these machines. Ian Mavric (ace Australian TRS-80 guru) has a video out on his channel about this very issue. In my own personal experience, you are better of replacing them both. As far as the transistor in the 5v rail geting hot, I am inclined to agree with others that believe there is a short there somewhere among the two transistors. The Model 1 is a very cantankerous machine, can be rather cranky in its old age. Reliability of them has been spotty at best. I akin them to being like an old British car. Great when it works, doesn't work properly more often than not. With that, I still like my machine (but I prefer my Atari 130XE over my Model 1, as it just works.)
You will likely be very surprised to see what the actual problem is wit hthe machine next week :-) And yes, this machine is very cranky indeed!! Cantankerous is a great word.
Z57 is the other chip in that sync chain. They are 74Cxx chips and apparently quite finicky. When I get back to working on mine I'm probably going to socket them, but half the reason is to see if 74HC chips can work instead. It should be educational even if it doesn't work. But I'd be worried about that power supply problem first. And also tap the board all over with something non-conductive to see if there might be a cold solder joint.
I'm definitely interested in the next episode since mine has sync problems too.
My TRS-80 has the same issue. You see the pulses on screen and nothing else. The only difference with mine is it isn't intermittent. It just totally stopped working. I can't wait to see how you end up fixing yours. I'm hoping it will point me in the right direction for fixing mine. I'm a newbie with these machines so I'm in wayy over my head. Lol
Unfortunately it could be anything.
But like he did, start with the sync signals and enable lines.
You will need a scope - or at least a cheap logic analyzer.
Given the signal changing when you touched the chips, a simple test would be to reflow those chips and eliminate the possibility of a cracked solder joint.
Keep them coming, I’ve watched and enjoyed every minute.
Donald Stoner was in part responsible for me getting my ham radio license in 1991. He literally wrote the book on the no code license. He passed away in 1999. I'm an extra now with code.
Hi, thanks a lot for videos. Very good. I suppose it is a Q6 (inverter) sometimes ICs have broken connections between crystal and legs. So, when you cut a leg you applied force to chip and may be reconnect it. But, all in all i'd like to remove Q6 and Q5 both) have a good time.
+1, Good thought, didn't even think of that. Could also be Q3.
Super interested to see more diagnostic ROM development videos if they show up in the queue! Never owned a TRS-80 (any model) but I learned BASIC by rifling through the tons of TRS-80 programming books and converting them over to work with my Commodores. That's not TRaShy 80 my book!
I never liked the nickname "Trash 80" for it, although I realise for most people its an affectionate term, but the TRS-80 earned its place among the top 3 as it was a very competent machine by the standards of the day. And Tandy was very open about the design right from the start, which made making add-ons and writing software for it a lot easier that for other machines.
45 minutes of good video :) I'm waiting for the next part !
My grandfather had a trs-80. I have an original 16k upgrade kit in its original packaging with instructions.
Well, to be honest if I came across one of these I would definitely have to pick it up. Used to have one.
Question/Request, I am new to this skill, and I wear glasses such as yourself. Can you share you journey of choosing your optics/Headband Magnifier. There are so many options, and I have tried a few different kinds. For example, the big lighted magnifier attaches to desk you look down on.. they wiggle and move if you hit anything.. smaller clamp on types that easy to clip near workspace but no light and arm tends to always fall.. Anyways.. wisdom you can share would be awesome. Thanks... oh ya.. learning lots, thank you.
3:50: It looks like a big one slipped past the Sams proofreaders here, because I don't think it was really called the "Expansion Enterface[sic]".
Yeah, long and detailed in-depth troubleshooting. :)
The little board is called "The XRX Modification" which makes cassette loading and saving more reliable. I would replace the interconnect cable between the keyboard and the main board with some ribbon cable. If that connector is flaky, the system won't work right.
When the error happens, check the power rails at each chip directly on the pins. If one has a slightly lower than average voltage reading, I would suspect that chip.
My theory about the tiny circuit board on the keyboard might be a key debouncer. The Model I was notorious for not having a good one from the factory.
The small board attached to the keyboard is the XRX cassette mod
(Radio Shack fix for bad rom timing on the first batch of LEVEL II ROMS)
Great a cliffhanger! 😁👀🍺
Either Z5 is faulty and needs to be replaced, or there is a problem with the voltage regulation, which is bothering Z5. Please measure +5 volts constantly during operation.
Does the voltage change?
Are the solder joints in the area OK?
Does the large capacitor in the power supply still have enough µF ?
Yes, I'd be wondering about dry joints on any of those ICs. Great video, as usual.
I think I'm right in recalling that the Models I and III had their own discrete TTL video timing circuitry, but the Model II used a 6845 CRT controller? I don't know why, having reduced the chip count with the 6845 they'd have reverted to pure TTL for the Model III, but someone will probably know the reason!