The left-for-dead TRS-80 Model II is finally repaired and working!

[@BigDaddy_MRI]

Hi Adrian. Just FYI. I’m sure you may know this, but just for you in the future- 74 S = Schottky; L = Low power; LS = Low Power Schottky; C = CMOS; HC = High Speed CMOS; F = Fast TTL; HCT = High Speed CMOS with TTL inputs/outputs. There are more, but that’s the lions share. All these IC’s have advantages and disadvantages. For a designer, you’d have to decide how fast the IC needs to respond, and how much power you need to allow the IC to use. A bad output can be loaded by a failed input on another gate. That’s a trap that I often fall into. Corrosion on the pins means the hermetic seal on the IC could have failed and allowed moisture to enter the IC. Always suspect crusty pins. Inverter symbols on the inputs and outputs will convert the logic. This goes back to the Apollo computer. Example: in your drawing the AND gate has inverters on the inputs and an inverter on the output and it becomes an OR gate. LS 32’s can be replaced by the HCT 32 without any problems. Great video!! Excellent logical troubleshooting skills. And the RCT helped confirm your suspected IC failure. Nicely done!! And it’s good to see the machine boot up. Very gratifying!! An old Z80 lives again!!

[@mstandish]

Adrian is the exorcist of cursed machines.

[@rick420buzz]

Just think of the exorcism team that would happen if he joined forces with the guy from the Action Retro channel.

[@jasonhowe1697]

na he just resurects 30-40 year old machines from the grave yard kind of a necromancer of sorts lol

[@minty_Joe]

@@rick420buzz Heh, wouldn't Sean and Adrian cancel each other out? And then Sean would turn around, add a CPU upgrade and overclock the darn thing. Let's hope he signed a waiver before entering Adrian's Digital Dungeon, I mean, Basement. 😆

[@ephemerallyfe]

I can't pretend to understand how you analyze those schematics, but I couldn't be more entertained. Congratulations on fixing such an oddball hardware fault!

[@herrbonk3635]

11:00 _"No way to replace this chip"_ Perhaps, but the keyboard isn't really "toast" just because the controller is bad. With that simple and well documented protocol you could replace it with a small PIC-processor or similar, perhaps augmented by a couple TTLs.

[@g0bzy]

Amazing to think, some school kid from the early 80s skipping classes to visit his local Tandy shop to play with their TRS80, would someday be watching a repair video of the same type of computer on an unknown thing called KZbin!. I was that kid and this was amazing to watch. Thank you.

[@timbald]

Me too! I used to go to local Tandy and play with their model 3 and 4 in my teens. These Tandy videos have been brilliant to watch

[@lindafader9618]

i used to open the suprise kits, got caught and got kicked out

[@maxxdahl6062]

Nah, TRS 80 Model 2's were business only machines, so not a kid, probably an officer worker though.

[@evaDrepuS]

"Let me do a recap again real quick..." I half expected him to get out the solder gun...

[@cjh0751]

It's such a pleasure to watch you diagnose and fix the problems in these old machines. You definitely deserve more subscribers Adrian.

[@donaldcongdon9095]

Marvelous job Adrian! So happy to see the Model II up and running. Makes me want to dig out my Model II and see if I can fire it up. At least it's been boxed up and inside all these years, so I shouldn't have the corrosion issues you had to deal with. The difference between 74S and 74LS is power consumption. The plain 74xx is original TTL--reasonably fast but power hungry. The later 74Sxx series used Schottky diodes internally (that's what the S stands for), which made it faster. But it also needed more power. Then came 74LSxx (Low-power Schottky), which used less power. This is the reason 74LS was so popular. It was fast but not the power hog the original 74xx was. Later families like the 74F and then the CMOS versions (74C, 74HC, 74HCT, 74ACT, etc.) had different qualities. Thanks for this wonderful series!

[@andrasszabo7386]

I have a circuit I wanted to make since highschool. But it woud require 74C chips that are not sold anymore here in Hungary.. Can I replace them with 74LS chips instead?

[@donaldcongdon9095]

@@andrasszabo7386 Maybe. It depends on what the circuit does. The 74C family is actually not true 7400 (TTL logic). It's like the 4000 series CMOS logic but with 7400 series pinouts and part numbers. As long as the circuit isn't trying to exploit some feature of CMOS, it should work with 74LS. But a little-known quality of 4000 and 74C CMOS parts is that some of them can do analog tricks like amplification. True TTL (74, 74S, 74LS, 74F, etc.) won't do those. As long as it's a purely digital circuit, I'd guess it would work. If you want to be sure, however, look around for 74HC parts. These are similar to 74C but have improved speed. They're still true CMOS logic, however. Don't confuse them with 74HCT logic, which is TTL-compatible CMOS parts. Yes, it's confusing!

[@danmenes3143]

@@andrasszabo7386 Mixing logic families can be a problem, as the input and output thresholds for different families are different. Assuming you are using a 5V supply, you can mix different TTL families (74, 74F, 74S, 74LS, 74AS and 74ALS). TTL can also be mixed with 74HCT, which is "high-speed CMOS with TTL-compatible levels": Alternatively, you can try building the circuit entirely with 74HC, which is "high-speed CMOS", and has largely replaced the old 74C. I am not sure about mixing 74HC with 74C--you would have to inspect the data sheets carefully to determine if the input and output voltage thresholds are compatible. If you are not using a 5V supply, your selection is much more limited. I believe both 74C and 74HC were designed to work over a broader voltage range, but again, check the datasheets to make sure that the input voltages are compatible with your design. As long as you are careful about building the circuit out of compatible logic families, the circuit will likely work as designed, as long as the design is relatively conservative and you aren't pushing any chips to their limits or depending on the analog behavior of the chips. As Donald Congdon points out, if the designer is doing something "non-standard," it may not translate right.

[@danmenes3143]

@@andrasszabo7386 Another thing to keep in mind is that all of the TTL families will draw significantly more current than any CMOS family. That may be a problem especially if its a battery-operated design.

[@McTroyd]

Never apologize for a long video Adrian, _especially_ if it was a long troubleshooting process like this was. Too many KZbinrs practically jump straight from "It's not working" to "So here was the problem" in the interest of expediency. Detail at your level is rare. 👍

[@TheDiveO]

So it finally ain't a Trash-80 anymore, congratulations! Good to see the heroes of my youth coming back to (after)life.

[@parjf]

I've been working in some type of IT job for over 3 decades and my main skill is troubleshooting (more on the software end though) - because of my background, I really appreciate and respect what you're able to do with ancient hardware. Kudos.

[@mapesdhs597]

46:30 - Perhaps you should construct a mini trophy shelf of the Most Annoying Chips Ever. :D Little labels to remind you of the pain each one caused. Awesome to see your reaction when it finally worked, well done!!

[@horusfalcon]

What a tool that diagnostic ROM is! Great work, man. The folks who helped you put that together and then port it to the Model II have much to be proud of.

[@BoycottChinaa]

The space knight?

[@hernancoronel]

Love this videos Adam! Also love your dedication and absolute stubbornness on not letting it get away of your electronic claws! Keep up the great work!

[@blockbertus]

Adam? His channel even has his name in it.... Adrian.

[@GORF_EMPIRE]

🤦Adrian.

[@krnlg]

Yeahhh! Well done Adrian, what a marathon series and such a great end with this component level diagnostics. Your channel is just super awesome :) :)

[@jackeldogo3952]

I had a TRS-80 Model III when I was in high school. It had been shuffled from parents' to siblings' houses since the late 1908s but no one ever used it, just got stashed in a basement or attic or garage (horrors). At some point, a rat ate through the plug end of power cord. Finally, I was able to pick it up from my brother a few years ago. My son and I replaced the 3 prong plug and snapped the thing on (on-off rocker switch, so cool) and it booted right up. Damn, I was impressed. I started just typing old TRaSh BASIC off the top of my head and it was running like it was 1984. Then it started smoking, I freaked out, but we took it apart and there was about 1/2" of dust covering everything on the inside. A little vacuuming and liberal use of canned air, and it's worked fine ever since, no KB or CRT issues....rock solid build.

[@der.Schtefan]

What I love about this channel is that a 50 minute video is not considered "too long", when other channels would chop it up for KZbin algorithm reason.

[@HardverGURU]

Thank you for your work, you are doing a lot for the future with these videos. I also save computers and some other old technologies, and in several cases, with what I have learned from you, I managed to fix something that seemed like a dead end before.

[@atkelar]

The letter codes are "seriers" markers; they usually differ in current draw and switching speeds. Which adds up to one important factor: "fan out" - this is the number of inputs an output of a chip can drive, i.e. how many chips you can hook to a line withouth a driver in between. I don't think it's a problem in this chip, as it's driving only one or two gates, but overall, I'd have a peek at what happens with the signal when I change the type of chip.

[@adverschueren]

Correct. The 'original' 7400 series was relatively power hungry but not very fast. So they introduced the 74S00 series (with the S from Schottky), which was more power hungry, had a higher input load and higher output drive but was quite a lot faster (three times faster, IIRC). Then someone thought "let's get the best of both worlds" and they came up with the 74LS00 series (Low power Schottky) which were far less power hungry than the equivalent 7400 devices, had a lower input load (and lower output drive), and were generally a bit faster than the 7400's. These series are I/O level compatible, so they can be used together. But be careful not to overload an 'LS' output when driving 'S' inputs - I recommend a maximum of 2 'S' inputs driven from an 'LS' output.

[@Castaa]

all's well that ends well. Amazing it largely survived with such extreme weather exposure!

[@SimonEllwood]

Great video, bus clashes are the hardest thing to fix.

[@KGE64]

Great 'diagnosing' 👍 my colleagues are also shaking their heads if I am diving into circuits as deep as this 🙂

[@UK_Cobra]

Some of what you say is way over my head, but still love watching you trouble shoot these problems, and keeping the fingers crossed for the end of the video. Just seeing these old computer's get some TLC and fire up at the end is a joy to watch 😊

[@Dukefazon]

I understand the issue after you explained it but it's unbelievable that you figured out the issue and you could zero on in that 1 particular chip. Great job!

[@jwhite5008]

Yes but also he kind of got lucky. Well when I saw disk light flashing when reading every second memory bank it was a dead giveaway of what was happening. If it was lit all the time or not lit at all it would be much harder to find a fault that way. I would rather poke the board with the scope, but finding possible fault through logical schematic analysis is also a possible method.

[@_zzpza]

That's amazing detective work, well done! :)

[@williamwallace9826]

As an end user of personal computers, I have zero comprehension of 90 percent of what you did and did, but this is a great example of the troubleshooting process. Well done, Sir.

[@lohphat]

So many memories of working at a Mod II dev shop in the early 1980s. Before the PC standard it was all computing protoplasm -- each vendor and CPU family was trying to push the tech forward.

[@Starchface]

And to me that was the Golden Age of computing. Every machine had its own personality, unlike today's boring generic boxes. To be sure, that's also why the PC architecture relegated our beloved microcomputers to the dustbin. They could not economically compete in an era of standardization and abstraction, where ROM was there just to get the machine booting and no more. Nevertheless I will never forget learning the quirks of each machine as a kid way back when. Yes I'm "a little" sentimental about it. The sky was the limit. Exciting times.

[@JB52520]

I like both old and new tech. The hardware is more standard these days, and software is generally more isolated from the hardware, but writing software is still really interesting. There are so many frameworks, languages, and libraries, and you can use those to make your own, too. I just started writing a simple graphics engine, not because I have to, but because 3d math is beautiful and I feel like learning something. I've already had an "It freakin' works!" moment, and it only draws points so far. Software can run so fast that you can create emulations of old systems, or even your own designs. GPGPU is still really close to the hardware and should ideally be tuned for every card it runs on. SBCs and microcontrollers let you talk directly to whatever you wire up to them, so that's fun too. There are still a lot of ways to press the boundaries and do something quirky or close to the metal. Some day (with any luck) we'll feel nostalgia for today's technology.

[@Frostfly]

I couldn't care less if this machine was chucked in the bin, but I very much enjoyed watching you fix it. Thanks for the fun ride!

[@hadtopicausername]

I know nothing about this computer and have zero experience with it. But your fascination with it and the way you delve in to find the faults... I think it's awesome.

[@xianox8]

You must be the main celebrator of septandy! I have no personal relation to it as I'm both too young and it was not a popular computer in Sweden. But it's very impressive what you do with these old machines! BR joakim

[@adriansdigitalbasement]

Thanks! Radio shack was such a big part of nerd's lives in North America, so I just love seeing these machines work again.

[@ranieriphoto]

Congratulations Adrian on fixing the machine! A well deserved victory over such such a pesky fault.

[@tony359]

Only we "tinkerers" can understand the joy when flipping the switch and seeing things working after many hours of work! :) Nice video!

[@infinetic]

Your attention span, stamina and enthusiasm are so far ahead of mine its crazy! You really "glow" so to speak, whilst producing content regarding retro computers from even before I was born in 1980. Thank you for this awesome TRS-80 repair presentation, and yes, I love long videos when it means repairing from start to finish!

[@fixitalex]

Great work! I got through similar journey several years ago restoring soviet PC EC-1840. Everything gone wrong, I was looking for parts all around the world. But finally... Like your video.

[@principals16842]

I think this might be the first time I ever saw Adrian despair over a moribund machine. I am so glad you went back, reviewing your camera footage, and found the cause. It's funny to think (spoilers) that a little chip with four OR gates was the cause. Great job, Adrian! This TRS-80 Model II series has been one of my absolute favorites. I only worked on one machine with an 8" floppy drive, early in my career, an IBM AS/400 (CISC) with Twinax cabling for the terminal. I would love to see you work your magic on something like it one day, or maybe an early IBM RS/6000, such as a 7013-540 machine running AIX 4.3 on a ~33 MHz processor.

[@Starchface]

Well done Adrian. I never doubted you would find the fault, and it is fortunate indeed it was not in one of the "unobtanium" parts. The TRS-80s seem pretty reliable in that respect. You all did some fine work on that diagnostic ROM as well. Once again it saved the day. I hope it is useful to many other owners of the TRS-80 machines. It would have been interesting to see the use of that riser card. I'm assuming the Radio Shack service centers must have used something similar, since the test points would be useless otherwise.

[@timbald]

Brilliant! As ever, Adrian. Such a fascinating series these model 2 videos, plus all the other Tandy models. Something about the way you present, talk it all out, show the schematics, testing kit etc etc. Just so easy to follow. I loved studying digital electronics in my late teens at school, so it's great to follow your thinking as you explain your train of thought.

[@Duddie82]

It is very cool you finally found the culprit in the system. So happpy to see that machine that was left out because it was not working. Happy you got it working!!

[@doozer2726]

I didn't understand a thing about this video, but I do like to see the enthusiam/reward when old tech is up and running again.

[@BarnabusMarsh]

Wow, you brought back all kinds of memories. I worked at a Radio Shack service center during the 80's and fixed those when they were new. It made me wish I still had all of my old schematics.

[@anthonydenn4345]

Great job Adrian. You have the patience of a saint 😄

[@cs_fl5048]

Brings back some memories. I started my programming days on the TRS Model 4 using TRS basic. I still remember that hunk. It was quite stylish for the time.

[@gene2024]

My first PC ... ever! I really liked that machine. We actually ran a spreadsheet program on it. And, it printed better than an IBM Selectric ... Wow. You brought back all kinds of memories. Thanks!

[@desiv1170]

That was an epic troubleshooting video!!!! Awesome!!! And congrats on the diagnostics ROM!

[@adriansdigitalbasement]

Thanks, I rewatched my own video last night and I was very pleased with myself!

[@robertgijsen]

Congratulations on this one, as you said a real triumph! I really enjoy these journeys, I learn a lot from them! And the ecstatic moment it actually starts working again, wow I just felt so much joy for you there 🙂

[@tcpnetworks]

Thanks for saving the machine - it's a pleasure watching you get through the machine's logic and coming up with the right steps to fix it. Thanks muchly...

[@Jonoth]

Always exciting to see a resurrection like this! You did an awesome job, Adrian!

[@johncarter2741]

I actually exulted in joyous applause once you diagnosed the fault

[@PapasDino]

It's freeking working!!!!!!!!!! Congrats Adrian, really enjoyed the journey with you on this one! 73 - Dino KLØS

[@pwissink1]

What a devotion ❤ And a very happy Andrian. Nice to see that computer living again.

[@Quickened1]

That's truly cause for celebration! I personally know of no one who could have done that. You deserve a medal...

[@petesapwell]

Sorry for swearing but BLOODY well done, you absolutely figured it out, dead pleased for you. Well done that dude!!!

[@ax14pz107]

I literally just watched all the other videos in the series in the last three days so this is a perfect release for me.

[@stonent]

74LS draws less power than 74S. LS = Low Power Schottkey, S = Schottkey.

[@lindafader9618]

HCT even lower and faster

[@stamasd8500]

@@lindafader9618 actually not really faster in most cases, but just about the same, or slower in some specific parts.

[@gregmcpherson5671]

Such a friendly way to learn about hardware diagnostics. And thanks for the TRS-80 love. My first machine was a System-80... a Model 1 clone. It turned me into the software nerd that I am today.

[@JohnUsp]

Thank you for restoring this piece of history.

[@johnharmer394]

IT'S FREEKING BOOTING!, Yay Adrian, You are the Best. What an Amazing Diagnosis Process. Back from the Dead!

[@rilian226]

I wanted to say thanks for this series. I am coming in late but better late than never. Also neat that you're in the same metro area I am. I happen to work IT for a local district and came across a trs-80 model 2 keyboard (appears intact from the outside. I have not opened it up) in a donation bin years ago. I saved it for nostalgia despite not having the computer with it. It's on display with a few other retro items for kids to see what it was like 40 years ago...

[@alextrusty2585]

Wow, it's great that model II works again!

[@Dirtyharry70585]

Years ago we used Bernoulli drives. The computer would stop run the stored data base. Every time I check in the damn thing was running. We replaced the drive under contract, I took it home for the why…..Turns out the 5v/12v internal supply dropped to 4.5 and it was critical to be no less than 5vdc. I turned up the internal to 5.2vdc, swapped it out, and never failed again.

[@itzcaseykc]

Glad to see the problem resolved. It's always satisfying to see the results of your efforts.

[@nikodevious]

That was one fantastic bit of troubleshooting, dead-ends and all.

[@markevans2294]

The Z80 can use all 16 bits of the address bus for I/O operations. Though it was common to use only the lower 8 bits with CP/M machines since this enabled the BIOS to be written using 8080 assembly code. Since the DRI documentation uses 8080 assembler code throughout and never mentions anything which only the Z80 can do.

[@Torbjorn.Lindgren]

The reason DRI uses 8080 assembler for CP/M is that that was the processor it was designed around, it worked on the Z80 worked because that was (mostly) backwards compatible (there was a few edge cases). Most CP/M program uses 8080/Z80 compatible instructions to broaden the compatibility despite Z80 being AFAIK much more common than 8080 in later CP/M machines. And IIRC accessing higher ports was slower too? (it's been far too long for me to be sure). There's also a cost to decoding more of the I/O address, all devices with I/O ports would need more decoding logic, which pushed hardware designers towards "if we can get away with 8-bit, lets save the money". But it's worth remembering in case the computer in question does use the additional bits. No idea which (if any) did that.

[@JamesChurchill]

The Amstrad CPC uses 16bit io; it actually uses the high bits directly as chip selects without any extra decoding. The only drawback is that it prevents normal usage of several io instructions that use B as a loop counter (ini/ind/inir/indr and the out versions)

[@johncochran8497]

You might wanna look closer at your assumptions. The Z80 is backwards compatible with the 8080. But the Z80 does have some capabilities that the 8080 doesn't and at least one of those capabilities is totally incompatible with CP/M and MUST NOT BE USED. Look at the non-maskable interrupt. CP/M can not support that since 66h is smack in the middle of the FCB that the CCP initializes and supplies to transient programs.

[@DrRChandra]

IIRC, the qualifiers are "L" to mean low power, "S" to mean "Schottky inputs" So an "S" part would be sort-of compatible with an "LS" part, just that if the power delivery is marginal, it might not work because it is not the low power variant.

[@JeffTiberend]

Thank you for the explanation. It's been a long time since I have taken electronics and logic classes.

[@mikemtbeerglass875]

You lost me at hello, but dang this is by far my favorite video so far, I was so intrigued by your deductions on how, what, why the system was not working. I am just blown away how you troubleshoot these computers, great work Sir. I am a new viewer and been binge watching your videos for 2 weeks now, keep up the great content.

[@audreywinter4553]

Congratulations! What an incredible saga. It's been great following long to such a satisfying conclusion. Your determination to get to the source of the problem after all this time and all these troubles is a true inspiration.

[@ceebee23]

I do hope your shoulder is fully recovered.... :( I do enjoy these really early computers.... fascinating to see how they developed

[@radio-ged4626]

Well done on resolving the wierd boot up problem. I had a gut feeling when you mentioned the FDD access problem that it may be an address or chip select problem on the controller but you pin pointed the exact issue. Brilliant.

[@SobieRobie]

Great work! I'm happy for you!

[@CH_Pechiar]

Your happiness shows the hours spent on the project. 👍 I always learn with your videos.. I sometimes find myself shouting at the screen too 😁

[@MrSnooze]

Excellent to see this working now, great detective work 🙂

[@wizard-pirate]

Thank you for the many hours of entertainment you've provided me over the last couple months of me binge watching your videos. Also, did you know that there are spicy ketchup and spicy dill pickle chips in Canada now?

[@UpLateGeek]

It was a long road, but you got there in the end. I always knew you could figure it out and get this machine working! Anyway, I'm glad your shoulder's feeling better. There's nothing worse than having a health problem that stops you from doing your job or a hobby.

[@insanelydigitalvids]

A Tour de Force in troubleshooting! Could not stop watching :-) Great work, Adrian.

[@saturn5tony]

Another awesome debugging process, so cool Adrian, soooo cool!

[@supermarketsweeps]

Glad you fixed it , i was sitting here and when you put second disc in and it acted different from the first disc i was thinking something wrong with floppy disc ,

[@thedungeondelver]

Thank you for another great video, Mr. Black!

[@jeffreyphipps1507]

I appreciate you finding solutions for these problems. This helps people dealing with these issues to read and interpret poor Tandy schematics and repair guides.

[@kdietz65]

The cool thing about the Model II is you could actually use it for real in modern computing. Since it has the square brackets and squiggly keys (which many other computers of this era don't) you could use it as a terminal into a pi and do real work with it.

[@ricardog2165]

Very educational seeing your troubleshooting methods! If it had been unobtainium like the keyboard microcontroller, I suspect a Raspberry Pi Pico could be made to work. When you mentioned that you can't test the board "in situ" I was wondering why you don't try signal injection.

[@jwhite5008]

I feel like inserting a computer running linux OS that is thousands times as fast and millions of times more complex just to be a keyboard controller is a tiny bit over the top. Although it could help save the otherwise dead machines. But the simplest arduino/AVR/PIC/8051, maybe even PLA+some buffers should be enough to replace that one chip, maybe even just a few 74xx chips. It would be harder than writing a C program but with a simulator and an autorouter I think replicating an old keyboard controller would not be that much harder. The original authors of the machine had to prototype it somehow and they didn't have access to microcontrollers or PLCs or a bunch of computers just lying around. They didn't have any electronics design software or simulators either. They only had normal logic chips and transistors and that's it. And they would avoid doing something too complicated because that would be harder to make and debug. So I bet all chips on those early machines are reproducible with just a handful of the chips that could be obtained at that era.

[@Vermilicious]

It's easy to say now, but the very random problems really did indicate a more severe problem. Congrats on figuring it out at last :)

[@FriedrichBrunzema]

Wow, what great logical reasoning to find the problem. Well done!

[@kencreten7308]

So happy for your efforts coming to this, Sir. Congrats.

[@Sloposse]

Your patience knowledge and second guessing is beyond mind blowing. Nothing short of excellent work here well done Adrian when I was about 10 I remember seeing these machines in tandy showing that exact screen screen with the TC in it

[@ddognine]

Great job on finding the faulty chip. My worry is there are probably many, many chips that are similarly teetering on the edge, and you or anyone who wants to put this TRS-80 through its paces will be chasing down fault for a long time.

[@christopherjackson2157]

Its cool to see the scientific method applied like this.

[@Shamino0]

Wonderful news! I realize that it's a year later, but If you haven't already tested those other AND/OR gate chips, you might want to do so anyway, since they do look a bit squirrely. One thing I learned from another channel (Joe's Classic Video Games) is that simple logic gates can be (somewhat) tested with a basic multimeter, even in-circuit and unpowered. Well, you can't prove that the chip is good, but you can often prove that it is bad. Put the meter into diode test mode and then measure the voltage drop between the input and output pins. You should see the usual TTL voltage drop (usually between 0.4v and 0.7v). If you see a dead short or no connection, then the chip is almost certainly bad. At least suspicious enough to remove it and put it in your tester if you don't want to just swap it at that point. Great video. Looking forward to seeing some software running on this thing.

[@Sir_Uncle_Ned]

I love how you made a diagnostic ROM and it actually helped you diagnose something! Keep up the good work!

[@pupaepedorra]

This was an amazing video, this kind of content is the reason why i am subscribed here and in patreon. Thank you so very much mr. Black.

[@ForaPhil]

Really impressive detective work! Congratulations on making a really interesting video and of course with a happy ending.

[@derekchristenson5711]

Hurray! I've been hoping to see this one finally, definitely, fully working again. 🙂 Great job!

[@Cpt_Adama]

I really enjoy listening to your problem solving and being able to push thru disappointing results. Good job!

[@maxtornogood]

That bad chip took you on quite the wild goose chase Adrian!

[@mattalki]

I love your enthusiasm when you get things working. Makes watching the whole video series worth it!

[@chaggydawg]

Love this, and even more little lessons in how exactly older computers function in a chip logic level. Great work Adrian!

[@rod370]

Hi, Adrian. Yaaaaaa Hooooooo you fixed it. Nice one. Thank you for a great videon. I loved it. Stay safe to you and your loved ones.

[@Renville80]

The 74S series of TTL chips features Schottky transistors instead of standard transistors, which made them faster, while the 74LS series was an improvement in that they did not require as much current.

[@davidw.2467]

10:38 In case those mask programmed keyboard chips (the 8021 in this case) are bad, they could be replaced by 8051 microcontroller via emulation (e.g. using the Atmel AT89S51). Years ago I fixed one of my old Apple II clones which had a bad keyboard controller (the chip was KR3600-PRO) using this method. An adapter will be needed since the pinouts are different, and an 8051 keyboard matrix scan program will also need to be written.

[@VoltageGoat]

Great job! It's nice to see your excitement in finding and fixing. I wouldn't have been as calm. :)

[@MoreFunMakingIt]

Fabulous episode! What a journey! Really loved your reaction at the end, helps to remind yourself when you feel like giving up you might miss out on a huge win like this.