Using our new diagnostic ROM to fix a TRS-80 Model III!
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- Опубліковано 30 вер 2024
- #trs-80 #repair
The TRS-80 Model III is fixed! As you saw in part 1, I was really stumped by the problems this board had -- so I reached out to friends Frank and David about making a diagnostic ROM for this machine. I posted a companion video on the second channel taking a deeper dive to the ROM, I recommend you watch that after watching this video. The repair showed off how even someone as "seasoned" as me can make fundamental mistakes in the troubleshooting process.... So watch to find out what mistakes I made, and what it took to get this machine running again!
Thanks for Frank IZ8DWF and David KI3V for making the diagnostic ROM a reality.
Part 1: • TRS-80 Model III garba...
Part 2: This part!
Diagnostic ROM deep dive companion video:
• Let's talk: Our brand ...
--- Video Links
TRS-80 Model 3 (and Model 1) diagnostic ROM:
github.com/mis...
2364 adapter PCB:
www.pcbway.com...
Frank IZ8DWF's channel: (for really amazing repairs)
/ iz8dwf
When good caps go bad: (t-shirt donated by Jesse)
www.teepublic....
Adrian's Digital Basement Merch store:
my-store-c82bd...
Adrian's Digital Basement ][ (Second Channel)
/ @adriansdigitalbasement2
Support the channel on Patreon:
/ adriansdigitalbasement
-- Tools
Deoxit D5:
amzn.to/2VvOKy1
store.caig.com/...
O-Ring Pick Set: (I use these to lift chips off boards)
amzn.to/3a9x54J
Elenco Electronics LP-560 Logic Probe:
amzn.to/2VrT5lW
Hakko FR301 Desoldering Iron:
amzn.to/2ye6xC0
Rigol DS1054Z Four Channel Oscilloscope:
www.rigolna.co...
Head Worn Magnifying Goggles / Dual Lens Flip-In Head Magnifier:
amzn.to/3adRbuy
TL866II Plus Chip Tester and EPROM programmer: (The MiniPro)
amzn.to/2wG4tlP
www.aliexpress...
TS100 Soldering Iron:
amzn.to/2K36dJ5
www.ebay.com/i...
EEVBlog 121GW Multimeter:
www.eevblog.co...
DSLogic Basic Logic Analyzer:
amzn.to/2RDSDQw
www.ebay.com/i...
Magnetic Screw Holder:
amzn.to/3b8LOhG
www.harborfrei...
Universal ZIP sockets: (clones, used on my ZIF-64 test machine)
www.ebay.com/i...
RetroTink 2X Upconverter: (to hook up something like a C64 to HDMI)
www.retrotink.com/
Plato (Clone) Side Cutters: (order five)
www.ebay.com/i...
Heat Sinks:
www.aliexpress...
Little squeezy bottles: (available elsewhere too)
amzn.to/3b8LOOI
--- Links
My GitHub repository:
github.com/mis...
Commodore Computer Club / Vancouver, WA - Portland, OR - PDX Commodore Users Group
www.commodorec...
--- Instructional videos
My video on damage-free chip removal:
• How to remove chips wi...
--- Music
Intro music and other tracks by:
Nathan Divino
@itsnathandivino - Наука та технологія
This was more entertaining than anything I can currently find on Netflix, Amazon Prime, Viaplay or Disney+
Bro, this was better than those south american soap operas that my grand mother used to watch XD
Stranger things was pretty good.
And yet we still don’t pay for UA-cam premium
Maybe it's the vodka!
Well yeah. The streaming services are so scripted and calculated, yet with people like Adrian and LGR it's not too scripted and just casual enough to connect with the audience.
As someone who's worked on Bally Midway machines (like Tron), those cheap ribbon cables go in the trash immediately, even if they appear to work. They're just too much of a liability.
That cable reminds me of something at work , where a connection on a signal tranformer was loose solder wise. Got voltage (continity) but could not do any current. it's one terminal was broken solder inside
Replace them with kapton flex cables or an actual ribbon cable. This 70s paper crap goes in the garbage.
might pass dc current, but a high frequency square wave with no crosstalk: Some of the time
I don't deal with old electronics like this, but I definitely did not like those cables at all. I can't say I'd have caught this sooner, because frankly Adrian knows more about these machines than I do, but it reminds me of how it's often a good idea to preemptively replace capacitors when there's a chance they might be bad.
Still, it's great that this has lead to a diagnostic ROM being created.
Signal Generator @ 2 MHz and oscilloscope, how does the ribbon mangle the communication?
We want a follow up!
Absolutely cracking video. I loved the thought process, the theories (and their revisions), and your grounded (pun intended) logical approach to testing. That you were able to suspect and challenge initial assumptions was great to watch. Then to top it off, you have made a diagnostics ROM too. Simply stunning!
Thanks!!
We're tool making monkeys. The real magic comes in sharing that tool with others and showing how it's used, both of which he's done. Think of the many hundreds of vintage devices that work could save and the dozens of previously frustrated techs that it'll help. Yay!
you are of course joking. eeny meeny miny is not a logical approach. really really BAD. Inteligent, yes , sense.......... nope.
Not a mindless rant, I've been doing this type of work for 40 yrs for a living.
I just bought a not functioning TRS-80 Model III for repairing. These videos are to me one of the most precious things in the whole world right now. Thanks for doing this!!! (I'm a happy Patreon)
The audio was just fine, not a worry at all! What a fascinating episode, wow, really shows the dependency of many chips on one chip or a ribbon cable. Rock on Adrian!
Yup, audio was fine, for what it was at least.
I had the impression of an ever so slight syncronization glich. Maybe I'm overly sensible to this.
@@mvcube I got that too, but my setup could be shifting it just far enough out to be perceived.
It's always the simple things, Having owned a TRS-80 from new. Modifying it, building the LNW-80 interface and writing heaps of machine code back in the day. I was so involved back then that I also acquired a KAYPRO-10 which was a Z-80 machine with a 10MB HDD that had failed. Replaced the drive with a 20MB drive, designed and built an EEPROM board that I could switch addresses and enable and disable writing to it etc. Re wrote the ROM code to support the 20MB HDD into an 8MB / 8MB and 4MB partitions running CPM. Built a high resolution graphics interface for it (640 x 400) extended the code in the ROM to support accessing the new graphics and it was a good fun project. Sadly I don't have the machines any more But I learned alot about digital electronics in my youth.
I modded my long gone MIII to the extreme with all sorts of homebrew including an 8MHz Z80H, overclocked of course.
I think I'd run permanent bodge wires on the back and never have to worry about the ribbon cable again. The homemade replacement cable works, so I can understand not wanting to fix what ain't broke, but if I hit even a lick of trouble, I'd hardwire it.
@@mal2ksc Hardwiring sure fixed it for me but those cables do allow the sections of the MIII to be split for diagnostic purposes.
Aargh that ribbon cable! Yeah it probably got worse from being put in and pulled out so who knows when it actually failed. But that logic you resurrected would have been the main fault for sure.
At least we got a diag ROM out of it, and a couple of great troubleshooting videos and not just a scattergun replacement of parts
No parts cannon here! :-)
Actually the camera mic is not that bad, still clear enough to hear.
There is a car repair channel run by Eric called south main auto channel and he always recommends testing a circuit continuity with a bulb for a load rather than just a meter because, whilst the circuit may test out as fine it requires a real loading to show if it will carry enough current for the actual circuit loading.
Heh yeah I watch all of his videos. And indeed, he does recommend that. It's a bit harder with a computer since it's not really designed to drive load like that ... but perhaps I can do something with a resistor and a LED.
@@adriansdigitalbasement Try bridging across suspect lines to add more current carrying capability.
I wish I had seen videos like these when I was a lot younger. This sort of tinkering would have been fun. But I simply lived under the assumption that what goes on inside a pc, even the first generations, is just an inscrutable black box whose inner workings are for a select few experts only. So I focused on software only.
It always fascinates me the places where you see Amateur Radio callsigns! IZ8DWF and KI3V 👌
A fascinating video, Adrian! Well done all!
George, M1GEO!
Nice job! I mean repairing one computer is great, but building tools to make it relatively easy for the next guy... _that_ is the part that will have a lasting impact.
Don't apologize for the bleeps and beeps either, they're part of the process, and they're probably much more annoying to you than to us, because you've heard them so many times.
I was waiting for this one. I already from the beginning was convinced that the flat cable was involved, or at least instable. I remember that they were very vulnerable because their material was kind of non-elastic plastic and the cable could break half an inch from the connectors or get micro fractures since they had to put in a very steep angle into the PCB-connector, probably pushing too hard... This cable was not meant to take in and out frequently.
Good work though! I learned a lot from you again, especially because you shared your thought process and faulty thinking, from which we learned the most.
And, off course, triggering the good memories I have of my adventures with my Model III and Model 4p. The latter I still possess, since I was never able to put it out with the garbage, and now it is a very rare to find piece of history.
Thanks for that, I really appreciate it!
I am looking forward to your other Z80 projects.
I think my phone might have gone deaf. I was shouting "put the @#$%^& ribbon in upside down to see if the fault moves!" Wouldn't have affected the bit 7 problem, but should have moved the bit 3 fault to bit 4. Shows 2 separate problems, and instantly identifies the ribbon as one fault. Why? Because the only thing changed to move the error is the ribbon
Model 1and 3 were spooky like this.
@@robertharris2262 Brilliant idea! Actually, would'nt it move from bit 3 to 5 when turned around? Or is the first bit, bit zero?
EXCELLENT!!! I've heard you make the disclaimer that you are "just an amateur", but your knowledge and ability transcend the "professional" level. Amazing work, as always :)
Good to see the problems were found. It's not trivial to find errors in a computer since you can't always rely on a program to actually run like it should.
I guess the lesson learned here was that if there are bus transceivers present, you'll have to check the individual buses. It's a bit trickier.
That was amazing. Seems like a case of the system is always isomorphic to the organization that builds it. Somebody must have really not wanted to learn about DRAM and/or a processor!
Very, very cool diagnosis on the ASCII characters! (16:40)
Thanks!
Hi Adrian, with the bus transiver can be that on the pin of the Databus 7 was a so-called cold solder joint. Since you soldered out the IC and then soldered in the socket, the cold solder joint has disappeared or repaired. I suspect that was the error.
Wow, that was seriously hard core! Writing a diagnostic rom to fix a main board! Really cool!
Loved this video! Writing a diagnostic ROM is a great achievement. You guys make it look easy. Tools that can simplify troubleshooting speed up the process. I didn’t like that ribbon cable at all, but hey, it’s an early model home computer, and it simplified the trace design of the motherboard.
The "confused look" made me laugh out loud :D 35:25
Have loved this series, although I daresay you not so much! It really shows the value of having the test ROM - with that you were able to home in on the elusive problem. Did half wonder if the ribbon testing OK might’ve implied what it was plugged into was sick - and your new, sturdier cable made better contact. The bus transceiver coming back to life is just plain weird.
These videos rock. Nobody does better vintage computer repair videos
Great video, as always!
Thats why after testing CPU, other data and adress buss tests needed to be done, especially, if there is buffers and multiplexors.. 😉
The "Eureka" moment at 41:30 is why I love working with IT and love watching your videos. Outstanding effort, well done 🙂
What a roller coaster of emotions watching this repair video! So glad you were able to find the problem Adrian. That diagnostic ROM will help many troubleshoot their machines, so your frustration was not for nothing.
Watching your Commodore 64 videos got me to dig into my old C64 and fix it (it only needed a couple of chips replaced and still needs the keyboard looked at). These videos have made me want to get out my old Model I and see if I can’t figure out what is wrong with it. To know there’s now a diagnostic ROM to use makes me more hopeful.
Awesome. I think all of us who repair these things regularly go through exactly the same thought processes sometimes. Very refreshing to watch, particularly the 'IT WORKS' moment, splendid :D
That was certainly an adventure! You should get a blue ribbon for effort (but not a ribbon cable!)
Great video, Adrian, and congratulations on successfully repairing it! You learned a lot along the way, too, always good.
A few suggestions / comments:
(1) Don't forget to look for poor or cold solder connections on things like the IC pins. I've seen weird results, and that buffer chip (8T26?) might have been something like that.
(2) Broken circuit traces. Don't assume the signals actually go everywhere they're supposed to be. I've seen a trace under an IC cut by one of the pins being bent under, with that pin itself working fine soldered to its pad. (That was in a new board manufacturing test environment, admittedly.)
(3) Last but not least, when tracking down signals stuck high or low, I used to desolder just the signal pins affected on each of the chips and try to isolate them from their pad or plated through hole. That way I could try to determine where the fault occurred, which chip caused the fault without fully removing the chips. Obviously that doesn't always work and you can't isolate the pin, but it's something to keep in mind.
Again, great video. And I had no issues with the audio you were worried about. 🙂
Glad I found your comment. The bus transceiver chip may indeed be flaky, but since it has gone from hot to cold a zillion times, it could have invisibly wedged itself a cold joint in the PCB.
Re-soldering the pins may have been the real fix.
@@GnuReligion Glad to have passed along a tip or two.
One more related to isolating an IC pin. An open TTL logic input will not be high or low (+5V/0V) but typically something around 1.4~1.7 if I recall. Meanwhile an isolated CMOS logic input can look like a zero volt low on a scope yet not be. I would sometimes take a 100k resistor on a "Porter pick" back to the +5v power so I could provide a low-current input, then switch it to ground to tie it low. I caught a few cases of bad CMOS inputs that way. That was on the old 4000-series CMOS logic chips, trying to recall how the high speed CMOS TTL-equivalents ("74HCTxxxx") acted.
@@bobblum5973 What would REALLY be OCD, is to take the output of a trimmer, feed it though a trusted opamp, and roll the voltage to extents, and record exactly where the new Chinese logic chip flips HL, and LH, and where its output is at high and low. Ok, I do this. Been burned too many times. Modern CMOS makes life easy compared to the days of NMOS and open collector.
Great video! I really like deep in-depth troubleshooting and fault analysis. Keep up the good work :)
It just goes to show what the right diagnostic tools can do! I have found myself going down similar rabbit trails many times before. When you don't know the ground truth (pun?), it can be very difficult to separate coincidence from correlation. Fantastic job getting to the root of it, and extra kudos for contributing an invaluable tool to the community!
Nice Job Adrian! I'm glad I could help!
You are the real expert here Frank! I'm only an imposter :-)
@@adriansdigitalbasement Nahh you're one of the few experts I know, believe me :)
Good things (like the diagnostic ROM) come out of trials. Your work helps others.
This is the proper amount of Geeky! Creating your own diag-rom with friends to diagnose this is just beyond :)
For your diagnostic, just auto configure the memory test. If the test reports less ram then you have installed, that in and of itself is important information. It's easy to test at say a 1K boundary address, to see if ram is there.
Awesome video, be quite Interested in exsact ribbon cable fault, off-chance perhaps corrosion led to bleeding wire/conductor may have caused the chip issue?
Adrian you should get yourself a thermal camera. It would help a ton when looking for warm chips and stuff. They are not as expensive as you'd think. I got a UNI-T UTi260B on Aliexpress for $300. And it's not one of those crappy 32x32 resolution things. It's 256x192. I'd post a link but UA-cam would delete my comment. A similar FLIR camera would cost like $1,500.
Really enjoy videos like this Adrian. Great work on the test ROM too. Early days but the kind of thing that really benefits the fixing community. Little wonder you struggled with this one 👍🕹️
Is there any chance that marginal chip was fine the whole time and you were dealing with a cracked solder joint?
Been binge watching your videos.. you should do some kind of Collab with Ben Eater, not sure of the topic but I'm sure you could find something fun 😊
It reminds me of what they always teach us in troubleshooting networking problems. They always say, start at layer 1 and work your way up. Which would be analogous in this context to starting with the obvious physical connectivity stuff like the old janky ribbon cables. Great video, very interesting to follow you through the troubleshooting. Thanks!
I said there will be a third and so glad too! It's great seeing you didn't give up and used your resources to diagnose the problem. This is what the world needs more of and not to just chuck things away. Great work as always. 👍
maybe there was a bas solder joint and when you desoldered it and put in the socket it fixed it.
Just a reminder, initially look for signal integrity with your scope, THEN on socketed chips, pull them and re-seat a few times to remove any oxidation on the pins and sockets, this can look like
a problem that fixes itself magically, but it was the re-seating/possible pin to socket oxidation connection that was the real issue. So always scope your signals when first starting your trouble shooting.
AND of course re-seat all socketed chips using de-oxit to ensure future reliability. Just a refresher comment to all... Good work Adrian, enjoy your videos... Chuck
Video ram on the model 1 was not 8 bit - only 7 bit to save money! - so you can’t do this on the model 1…
You made me laugh. Freakin ribbon cable! 🤣 But like you said the faulty ic probably stopped it booting. Great video, your face said it all. Nice work on the diagnostic ROM.
The nerd level is really high in this video. I fell off the ladder early but I watched it all 🤓
Adrian I have to be honest I thought the sound was fine the whole way through.
Also had the audio problem. Now always tap the microphone and watch for VU meters on camcorder display before I commence recording.
How come the 'Green-Sometimes-Good-Caps-TShirt' isn't in Adrian's Digital Basement Store? Where did you get that T-Shirt? 🖥
That was actually a viewer submission on SMMC 0026. ua-cam.com/video/TuDZK110nbw/v-deo.html
Great video Adrian. I totally enjoyed the process! (And good to see I’m not the only one forgetting to plug in the mike!)
That rom chip will certainly help repairers out this #septandy !
Hi Adrian: been watching your channel for a while. I owned a model III and it developed a video issue, I don't recall what the symptoms were, but I found that those video cables CAN be dodgy and think that your bodge solution may actually be a good permanent fix just solder them on the bottom of the board to the pads :-) But those data bus cables are the first thing i replace before troubleshooting those old TRS-80s. I have fixed the 4 or so I have owned. My last model III was sold to a local collector in perfect working order with normal wear type cosmetic issues. To my knowledge still works now and it was sold over 6 years ago.
My solution was to solder ribbon cable on the back of the PCB. For the strips that went to the FDC and RS232 card, I replaced the PCB headers so I could use normal IDC type connectors and ribbon wire. Tandy really made a poor choice with the use of that flex cable stuff. Every single MI / MIII that I have had my hands on has suffered from it coming apart and even poor connections where the pins were crimped onto the flex itself.
Hi Adrian,
the stock Model I has only 7bit memory for the video RAM (no lower case) and thus using video memory as stack will fail. :(
Don't mind, the explanation ist on your latest 2nd channel video.
Your best videos are the ones where you never give up! Great work!
It's like digital Clue. It was the ribbon cable, with the corroded trace, in the basement.
This channel really is one of the coolest! I really enjoy seeing you repair all these computers.
That’s really cool, Adrian.
Thanks for the tutorial!
Great and interesting video as always!
For the ribbon cable, it looks like it's the normal 1mil pitch. Perhaps it is better to upgrade it to something more standard, like two single row pin headers plus an 8-way Dupont cable?
These repair videos teach me so much, and remind me how little I know. Keep these amazing videos going!
btw, the good way to find a shorted IC is using the current tracer ;) When there's a real shorted IC of course
How about a diagnostic ROM that twiddles the cassette/speaker to play 8-bit dance party? 😂😆🤣
Maybe a Z80 TRS-80 expert could whip up some ASCII art and the music. LOL!
Great video (I knew video #2 would result in a working board, but I'd never have guessed that it would result in diagnostic ROM's, and more exploration of the Model 3's architecture)! Also, is it me, or is the TRS-80's architecture strangely formal, yet the board layout is bizarre. I'm not very familiar with the TRS-80, but it looks like someone designed the system architecture and then thew the general specs at the actual EE / board designers, and maybe not all at the same time.
I'm not sure your diagnostic chip will work on a stock Model 1. The Model 1 only had seven bits installed for the video display. I modified mine by adding the 8th chip so the monitor would display lower case characters.
It does :-) I'm already using it while working on a 7-bit M1 on another video. The latest version of the ROM that is.
@@adriansdigitalbasement Glad to hear it. Just noticed the lower case characters on your display and knew my Model 1 originally did not display lower case.
I'll just say, you're the first person I have heard referring to chunks of Z80 memory as pages. In my experience, it's only processors like the 6502 that referred to things like the zero page. And of course, the x86 line of processors started out with 16 byte segments, and in later (protected mode) models, have full on virtual addressing and the idea of 4K or larger (2M? 4M?) chunks of memory called pages.
It's good to get different perspectives.
Whew! Glad you figured it out!
this is one of those "throw it in the oven" type fixes that actually worked. lol
shouldn't be line 201 a DB 00000001b instead of a 00000000b? there's already a testone 0 at the very beginning of the pattern set.
Yes, those early tests were pretty haphazard. Worse, they didn’t work well: the tests would not find bad chips. Turns out there are a lot of subtleties to detecting bad RAM when you need to. We’ve switched to a much more effective test (called March C, for the way it marches through the memory) invented by people who know lots more about memory than we do.
You beat me to that comment :)
I'm always looking for inconsistencies in sequences just in case I messed up in a way that's hard to detect later. It's become more of a subconscious habit than I thought.
@@JB52520 I even can't say why I noticed it.
I still have the original Tandy ROMs from my MIII after I rewrote parts of them and burnt to eprom to let me boot from my homebrew SRAM based ram disk. Combined with the Z80H CPU and 1:1 interleave on physical floppies that the CPU speed allowed, the machine could be up and running programs practically before you could blink. It certainly raised some eyebrows when I showed it off.
Tremendous! Enjoyed this one so much!
What creating a Memory Module using core memory that can hold 64KB of RAM to replace the RAM chips on a TRS80
So, why have different ROMs for memory sizes? If you know your machine only has 48k it’s obvious if the next bank higher returns a failure result. Is it a concern those with less RAM have to wait longer for another pass?
The current version has just one version for all memory sizes. The original concern was that 4K machines use a different bank size than 16K, and for decent memory tests to work you need to know the bank size. The ROM is now able to make a decent guess about the bank size.
Great video, like the learning points you made about buses, and thanks very much for the TRS80 diagnostic ROM - a new tool for the box 🙂
Hey, at least you got 2 full videos out of a bad cable.. remember Adrian, in computer repair (any repair really) KISS. Keep It Simple, Stupid! Great job though!!
The black snow on the first few characters of each line is down to the Z80 not being halted early enough by the video circuitry, ie once the CPU gets to a certain stage of execution the halt line is not effective until the next instruction. Tandy could have done some additional logic to widen the window in which the CPU is halted that would have prevented it but that would have added more chips to the already hefty count. There is a bit on a port that controls the halt mode. It can be set to model I mode where the CPU is never halted which gives noise across the whole line and maximum CPU throughput when accessing video, or model III mode but at the penalty of video access causing the CPU to halt, breaking time sensitive code but reducing the noise to the first few characters.
Awesome! Hindsight is 20/20... Always check your data bus, esp. After the buffers..
So did you check the ribbon for shorts from track to track?
I think I learned from Ben Heck to check for this.
This video series was very interesting to watch and reminded me of all the times I've chased my tail in the world of both computers and automotive mechanics, only to find out the main problem was a part that would have taken a few minutes to replace (a flaky cat5 cable or old vacuum hose for example). So now I've learned to replace anything that's both suspect and cheap & easy to replace without even bothering with testing. That being said the tail chasing makes for great content and helps teach others how to troubleshoot.
I'm only 31 minutes into the video but was wondering if it could be possible to program an Arduino to act as an "in circuit chip tester" : knowing you're testing an 'LS00 (8 inputs, 4 outputs), it sniffs all 12 pins and makes sure it acts as an 'LS00. It even tells you how many of the 256 combinations it has encountered so far. Now, knowing it is U48 you're testing, it tells you (maybe with the help of a custom ROM) if it behaves as U48 should. That custom ROM is written to cycle through as many combinations as possible... Or use the standard ROM and (automatically) compare with a working machine (record-compare or database)
Well done - persistence pays off in the end!!
Always follow up a continuity check with an Ohms check. :) Good fixing and troubleshooting!!
I know this is an older video. The diagnostic ROM will not work for a TRS-80 Model 1 unless it has had the lower case mod and it is turned on. The problem is the unmodded Model 1 is missing 1 bit of video RAM. A program or a stack won't work in video RAM without the mod. The cure was to piggyback a RAM (there were only 7) and wire a separate data bit line to the extra chip, cut a trace that forced the missing bit, and add a switch to enable or disable the mod. A look at the manual for the Model 1 will verify this. Just maybe someone published the details of the lowercase mod or you can probably find it in an old magazine archive. It was a common mod back in the day so you could use the Model 1 for word processing.
Also, it was the Model 1 with level 1 BASIC that could have as little as 4k RAM. Level 1 BASIC was little more than a modded version of Tiny BASIC and only used about 4k of ROM. Something to keep in mind should you try to do a diagnostic that might work for a Model 1. All Model 1s were limited to 16k in the keyboard unit. You needed the Expansion Interface to get 48k and the ability to use peripherals. You might want to change the diagnostic to be a little smarter about doing the RAM test.
The Z-80 directly accesses memory through the 16 bit address bus which limits it to 64k. There was a little bit of bank switching logic to access the RAM in 16k banks which is invisible to the user or programmer as it is done in hardware. Those 2 bytes shown for the diagnostic program are just the upper and lower bytes of the address. The Model 4 had paging that allowed up to 128k of RAM using a port for page selection. I don't remember any programs for the Model 4 that could leverage the full 128k of RAM and the early PC's were available making the entire Z-80 line obsolete by then. The Model 4 boots up in a Model 3 compatibility mode so the Model 4 features are invisible unless you know how to access them. At a very limited level, the Model 3 is also identical to the Model 1. The keyboard, video memory and cassette port are in the same places. The ROM and the RAM usage is different enough to cause compatibility issues for programs that try to directly access things or load where they probably shouldn't.
i am sure you already figured this out..but with those ribbon cables instead of doing continuity check it is better to o-scope and see if the signal is the same on both ends while in the machine ..also when testing it like this make sure you flex the cable a bit..sometimes we are afraid to flex a cable in case it breaks or something but heat, RF case, pin corrosion, internal cable corrosion etc will bend the cable whether we want it to or not and cause strange problems...so it is better to put it under a bit of stress when testing...
Found your channel a month back and im slowly working my way thru your back videos. @Vidfavne is correct. your content is more entertaining then anything on TV/Cable. Wish i was around for the trs 80. i made my start on a ZX Spectum. always found coding is basic back in the day relaxing. Keep up with the amazing videos. And thank you for the very interesting content. Dave
I would guess this should be adaptable to also work on Dick Smith System 80 / Video Genie. Probably some differences in memory mapping? Wonder how it would get on with memory expansions being installed with DRAM stacked on top of each other?
These 47 minutes went so fast! LOL! Really enjoyed the video! Greetings from Argentina!
The way you pronounce "TRS-80" sometimes sounds like "tear city". I guess that's quite a fitting name for this board😂
You just confirmed a suspicion of mine- I do commercial low voltage and I've had CAT5 lines that tested fine on a certifier or the basic Ethernet tester, but wouldn't pass a signal- and a new cable run fixes the issue. I thought continuity meant continuity, and was confused when swapping the cable fixed the issue.
Every two-video series for a bad ribbon cable is a learning experience! :D
Hi Adrian! For future Z80 repair videos, why don't you give an opportunity to the ROMulator-Z80? It saves you the burden of havin to write a new Eprom for the system testing program.
The only thing I really don't like with Adrian is that he keeps calling ordinary printed circuit boards "motherboard"...
PCBs that doesn't have a single little daughterboard, neither any kinds of slots for it.
This particular board is clearly a daughterboard in itself, so calling it "motherboard" is... what should I say... :)
I have seen the 74LS367 chips fail on my TRS-80 Model 1, not surprised to see the same problem on the Model III.
Moisture ingress into old silicon chips can exhibit the bad-->good symptoms you have experienced. In your part one video, many of the high with 'glitch' partially low on those 138 devices is quite normal as address changes settle at the outputs.
Well done Sir. We all learn by failing. Good luck with final re-assembly.
If enough interest, I reckon you might enhance your diagnostic ROM with larger memory and on-card address manipulation to cover the various options with mini-shunts or whatever.
It's freakin' working!
Ribbon Cable.... you are the weakest link. Goodbye!
By the end of the second video, I knew the ribbon cable was the problem. The squirreliness of the machine everytime it was rebooted is what convinced me. You tested for continuity, but that sketchy, delaminating cable was carrying rapidly changing data signals, not straight DC.
BTW, I'm addicted to your videos. So nice to see old school component level repairs. Did you ever test that second cheap o-scope? I couldn't find the follow-up video if you did.
it seems that you were using the same techniques of troubleshooting C64's to troubleshoot a z80. A cold solder joint can cause the error you were seeing on that transciever chip.I always thought the ribbon cable was a bit flaky, Well done in getting to bottom of the fault.
Just amazed that they used a ribbon cable to connect major components together...
There was no CAD layout system to help, and it is just a two layer board, so the easy/quick way to get the data bus over to the other side was to used the cable.
Long time watcher. I just had a random thought. Adrian Black is a very cool name.
This video is amazing! Keep it up! Been watching you for years now and the videos only keep getting better.
Hey Adrian.
I am trying to figure out what different paste pads or foam used on GPU heatsinks.
It's a tricky question.
I know what thermal pads are as well as thermal paste/grease. But there is a third substance I found on the gpu waterblock coldplate. It looks like gray paste but it seems more like a foam/cream, thicker 🤷🏼
It's in the large voids of the gpu copper plate. I believe it's on top of the chokes.?
The Model I and Model III are different computers hardware wise. You can't plug Model I ROMS in a Model III for instance. One is port mapped for hardware, the other is memory mapped.