Repairing A TRS-80 Model 1 And Expansion Interface
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- Опубліковано 20 чер 2024
- This TRS-80 Model 1 has keyboard and video issues. The expansion interface isn't powering up. Let's see what the issue is and get this system working again!
TRS-80 History Video: • TRS-80 - The Most Popu...
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Chapters:
00:00 - Keyboard Repair
10:43 - Video Repair
19:39 - Expansion Interface
33:15 - Testing
51:58 - Memory Repair
54:58 - Playing Blackjack
Music used by permission:
“Fakebit World” by Malmen
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"Love On A Real Train" by State Azure
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Other music from the UA-cam Audio Library
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I love the huge badges Radio Shack put on the model 1, it reminds me of the 10 square meters of stickers that they put on PC compatible's in the late 90's early 00's
There was also a DEBOUNCE routine that was available on cassette, and was incorporated into most DOS, of course the best dos, NEWDOS/80!
Another advantage of that new ribbon cable is that if you ever get sick of having to disconnect it to service the motherboard, you can just swap it out for a long one until you know there's nothing left to work on and you're ready to put it all back together.
Even as I'm from Germany and Tandy was never a thing in our country, it's pure joy to see your work and repairs. And seeing an old system back up and running is always a big smile my face 🙂. Keep going!
50:11 had exactly strange behavior on an repaired CRT TV, where I was sure the input is functional. Ok, helps to plug it in 🙂
Thank you very much!
G'day, nice video. The "high" voltages you saw on the supply are normal for a no load condition on an unregulated transformer output. The diodes wont "wear out" and give you higher readings, unless it had gone short circuit, typically they will fail open. I should go fix my TRS80 model 1 now too, same configuration as this.
Back in the day we got Silver Solder Kits and went over all the pads on the edge connectors. It worked very well.
Great job getting those TRS-80 model 1s working! Great video!
This is one to best videos you have done on along while. Please more like this !!
I might have built that computer! 😱😁🤪👍👍🇺🇲
DIP switch is ON or OFF, but ON doesn't need to mean logic 1. The switch may be shorting to GND, so ON (short) will mean logic 0 (0V via switch), and OFF (open) will mean logic 1 (5V via pull-up resistor).
Exactly this. Zero being on is correct in a logic system with pull up resistors. That makes the most sense to me.
On AVR MCUs a fuse bit set to 0 is called "on", 1 "off".
If I recall correctly, the keyboard bounce issue was a keyboard switch issue. The 1st gen keyboard had switches that looked like 3 tang jaws and they'd contact and literally bounce. This was fixed by hall effect switches later, but the workaround was done by cassette program and then in ROM before that. I also recall that service centers would replace the ROMs on demand, but upgrading to Level II solved the issue anyway.
I was a MOD I owner in 77, and a Shack employee 79-81
Great video as usual. ❤ the Chevy Chase insert. Probably wouldn’t have had a good talk with your neighbor.
I thought for sure I was going to have to redo that whole sections, but the mic didn't pick it up.
Nice, after the fried memory chips, I'd be checking the power supply rails, it seems like it may be drifting high.
I ended up fitting 4164 chips to mine, then removing all memory from the expansion board. Having all the RAM on the main board made mine more stable. So I could say I have a 64K Model 1, except only 48K is accessible. I could let a big more in, there is a gap between the ROM and the video RAM, however no standard software would use it, so I never bothered.
That's a GREAT idea... I never thought of that approach. I'd had loved to engineer that and show it to my fellow techs when I ran the computer repair center in Alexandria!
I learned Backgammon, just to write THAT program! (My first trip down to Tandy Center, BTW)
The ROM that displays "MEMORY SIZE" and the full copyright banner is the old original ROM. The newer ROM with the keybounce fix had to abbreviate things to fit the new routine. i.e. "MEM SIZE" and "R/S L2 BASIC"
Thanks
Good vid! I wouldn't worry about adjusting down 5.2V. Under load it will probably sag to 5V, anyway.
Yeah. I left it that way.
My dad has a Popular Electronics magazine with an ad in it for a computer course which had a TRS-80 that you could keep as part of the course.
Thanks!
Wow! Thank you!
I have that wizard book!!!
'Ooh...What's that smell?'
Never a good thing. And of course, by the time you smell it, it's already too late...
Yep!
39:20 As I understand it, the way the TRS-80 determined how much memory was available to it was to attempt to write something into the RAM and then read back from that location, going higher in RAM until it received an error. It would then "assume" that the top of RAM was one byte before it received the error. With less than the full 16K in the main unit, there would always be a gap in the memory which would cause the computer to stop looking for more RAM at that point.
This was the "cheap & easy" way of handling RAM since any way of "ignoring" the gap would have been far more expensive than simply requiring there to be no gap in memory.
Edit: That's probably why there is a longer pause when the extra 32K is installed. It takes that long for the CPU to find the top of memory.
Kinda rare to find a Level 2 upgraded Mod 1, with only 4K ram.
I always use the phrase The Quick Brown Fox Jumped Over The Lazy Dog to test the letters on a keyboard as the phrase uses all the letters of the alphabet. Just saves going through each key.
It's "jumps" not "jumped" otherwise you miss out on testing the "S" key.
Check the voltage on the memory chips.
I loved my Model I. My favorite games for it were from Big-Five software.
Which ones in particular?
@@RetroHackShack Galaxy Invasion, Super Nova and Robot Attack. They all have sound via the cassette port and are excellent for the graphic limitations.
Great with the Alpha Joystick !
@@johnmonin I did a over clock mod on my Model 1 back in the day as described in the book "The TRS-80 and other Mysteries". That made the games slightly more challenging!
If it is late, your neighbor shouldn't be using a chainsaw.
Well, if you don't have a light saber handy, whatcha gonna use?
The keyboard unit needs 16k of RAM to see the 32k in the Expansion Interface.
Yes. Found that out 😀
Maybe you need spray some contact spray on the keyboard switches ?
I'm curious if that buffered expansion connection ribbon works. Since it never showed back up, maybe not?
You should only used the Buffered Cable on an e/i that didn't have the mod built in as later ones did.
Arg, don’t replace parts unless you’re reasonably sure they are dead! A cheap oscilloscope and/or a logic analyser is a much better starting point to determining where signals start or stop. Removing a chip risks board damage each time so is best kept to a minimum.
Thos 4116 RAM chips are awful, you can replace them with hacked 4164s, but I upgraded mine to 64KB internal. You may want to consider a FreHD disk drive add on.
Yeah. The FreHD seems to be the defacto standard upgrade these days.
Maybe BASIC will only recognize consecutive blocks on memory, and you'd have to expand the 4KB CPU to 16K. EDIT: and then I unpaused the video and you said just that.
Correct, once the 'wall' is hit, basic doesn't continue to see if any memory 'grew'. That is is if you hit "Enter". If you plugged in a value, it would use that as it's limit, and reserve any memory (if it really existing, of course), and the memory above would have been reserved for 'machine code' i.e. Z80 code that BASIC wouldn't touch - reserved memory.
It is hard to believe a hardware engineer made the video and power use identical DIN connections. Maybe it was their first job?
Being Tandy/RadioShack the engineers were probably forced into bad designs by retail focused accountants somewhere along the line.
If I ever have the chance to speak to Don French or Steve Leininger, this is what I want to ask. 😀
@@user-yr1uq1qe6y That would be my guess. However you would figure they would "key" them somehow to prevent mixing them. I would guess that a LOT of failures were caused by this.
They (the connectors) were cheap and available. Guess you don't know the backstory of the TRS80. They weren't expecting success in sales. Originally they only expected to build around 1700 units, 1 for each store, and when the things didn't sell, the stores would get an inventory system for free. Boy, did we miss the mark on that one! I actually learned backgammon, just to write the program that got distributed!
@@garthhowe297 You would think so, but actually no. I ran the RS computer repair center in Alexandria and I can't recall a single innocinstance of this happening!
If you don’t connect the keyboard does the system boot properly? I have a Model 1 that just displays the random characters on the screen and never gets to the memory screen.
When I don't connect the keyboard, I get "memory size?" And a random character will appear on the screen.
I guess it's too late now, but I don't think you waited long enough for the asterisk to appear during tape load.
From years of experience using a C64 and Amstrad CPC, commercial tape releases typically have 8-12 seconds of lead-in, so you should probably wait until at least 15 seconds before calling it not working.
I did a lot more testing than I showed 😀 so I knew exactly where the program was by looking at the tape counter.
NO! that's not rust. It's Torque seal! Youv'e voided your warranty! (we ol' timers are shaking in our boots!) {grin}
Some (probably the earliest ones) were sealed with a clear red,very brittle shellac. This was to protect the warranty from hobbies its who zapped something when "exploring".
The repair center across from my store (Columbia, SC) would reseal with silver paint that matched the case co!or. They didn't have the reseal stickers shown in the video unless they were installing an upgrade or a buffer cable.
@@earlgriffith3592 There was an actual26-inventory code that only the repair centers could order,one was an 'official' orangeish color, and the other was 'silver' like. They were formulated to break and shatter if someone used a screwdriver to open the screws after it dried over them. There was an official method of application, so trained service people could easily and readily see of someone broke the seal to service their own computer. Back in the day, we could actually decline to service a system if the 'warranty seal' was broken, or charge different service rates. I once had a customer call my District Manager. trying to "get one over us" , but it didn't work. He tried doing this on his Model 2, which never had anything other than a thin paper label as it's warranty seal. The guy also claimed that his floppy drives didn't work. Actually bought a service contract to cover his computer afterwards. Drove 2.5 hours to his site, only to find his secretary was "storing" the floppies to the SIDE of the File Cabinet using magnets to hold the floppy disks. (Sigh)
come on, no link to the precious video ?
I put a link in the player. I'll add one to the description if that helps.
The keyboard DOES need to be connected for the system to work.
Well, not to get display output. I think that is what I meant.
If I recall, the keyboard and video are mapped to memory addresses. That's why there can only be 48K of RAM