The Super Rare TI-99/4 Lives!
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- Опубліковано 14 тра 2024
- I know last week I said we were going to take a break from the TI-99, but this one hit me right in the obsession, and I was losing sleep at night over why it wasn’t working. So, I gathered up the handful of brain cells I have left, walked back out to the garage and go to work. And, while my meager brain cells couldn’t figure it out, the folks over on the AtariAge forums cracked it! Coming along as figure out just what went wrong.
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Intro Music adapted from:
Artist: The Runaway Five
Title: The Shinra Shuffle
ocremix.org/remix/OCR01847
Thanks for watching!
Chapters
0:00 Why I did what I did last episode
2:32 What’s next?
3:24 Setting up the Logic Analyzer
5:34 Analyzing the logic
9:30 Okay, what does that tells us?
11:19 I think we found it!
13:14 Will it work now?
15:52 Cleaning it up a bit
17:32 All plugged in and working!
20:52 A subtle cry for help in finding unobtanium TI sidecars
21:43 There is tube content coming, I promise
22:21 Wild bunny! - Наука та технологія
Glad we were able to help get this thing working! - Greg
Thank y'all so much for the help!
(for whatever reason my other comments got deleted here) but this video is very helpful if something happens with it (e.g. the keyboard as you see is different than the /4A system), also Parsec will not work (different Video chip - 9918 - not 9918A). FinalGrom does not work in this /4 model.@@UsagiElectric
Seeing a button on a piece of high-end HP equipment that says "DON'T CARE" is hilarious to me.
I want to thank you for awesome content. I came here for the Centurion and discovered the Vacuum Tube Computer and stayed for everything else.
Thank you so much!
And fortunately, there's a lot more of Centurion and Vacuum tube computer to come!
The broken "bus bars" that you replaced with wires are not simply power wires - they are also decoupling capacitors between ground and power in the area where they were connected. In addition, they rather cleverly also act as a grounded anti-interference shield for FCC compliance. I suspect if you connect a 100nF cap across each IC that the bus bar originally connected to power, you might get an improvement. I really hope you kept the broken one!
If I recall, they were made by a company called Mektron...? The company I worked for in the 80s used them on a few PCBs, strangely, also with the TMS9900 cpu.
I was about to write something similar!
You are not a "normal" guy ...you are a computer hero !! Compliments !!
Thank you!
But really, my only real skill here is that I'm just stubborn and don't know when to quit, haha.
Love this series. My parents bought me a TI-99/4A back in 1982. I loved that thing. Tons of support at the time for cart and cassette games. I had the same Acoustic Coupler. It oddly has a very advanced processor, crippled with a horrific memory system.
Loved my TI-99/4A too! Extended BASIC was a fun language to learn with. I have a TI-99/4A emulator on my Windows PC that I occaionaly code on for a trip back to the 80's! 😊👍😊
congratuatlions! I have a tendancy to toil away endlessly with non working systems, giving myself pure pain and annoying my friends with endless updates. But the feeling when you get a complex failure like this working is one of the best feelings in the world. BTW i totally feel for you. I had a non working apple III that I spent months troubleshooting and it was a solder bridge between two address pins on the character gen srams.
Sometimes complex faults like the one you had on the Apple III have the simplest causes.. I got a case like that with a Socket 7 PC that would act erratically and crash with a Pentium 233MMX chip.. Turns out that the motherboard had a jumper that when open would cause the Vcore to get bumped up slightly, just enough for the P233 to freak out, yet the original CPU (P133) worked just fine, if not ran a bit hotter (seems like non-MMX Pentiums are quite tolerant of higher voltages!)
I thought the master solution to Apple III was to pick it up and drop it on the table, to reseat the chips?
@@editingsecrets it didn’t work that one time for me. LOL
You know Tursi! Almost 30 years ago we used to hang out in his apartment on O'Connor Street in Ottawa and listen to people dial into his TI-99/4A powered BBS - three floppies and a RAM disk! He is a master programmer and made an amazing demo. I can only imagine that poor Workspace Pointer when his demo is running.
My skills are more analog video, and I did manage to get a TI-99/4A to genlock to an incoming source using an LM1881. It never really left the breadboard stage, but using the TMS9918A's transparency "color", I was able to superimpose text and sprites over incoming NTSC video and record it to a VCR. In TI Extended Basic glory; CALL COLOR and CALL SPRITE. LOL.
Your video noise: check those 2N3904 transistors you just changed. I feel like they might be the source. Check the biasing resistors, especially the ones which bias the base of the first transistor - resistors can get noisy. DO NOT use high precision resistors, they are laser-cut resistive material in a way when tends to increase inductance which might prevent high frequency noise being bled off to ground! You should be able to hook an oscilloscope to the output pin of the TMS9918 and see clear video; even get clever with the oscilloscope and its gun brightness (often called the Z input, sometimes on the back, if the scope has it) and actually create a full image on your oscilloscope. If the noise is coming right from the TMS9918, just leave it, the machine is more special with an imperfect TMS9918 than perfect with a TMS9918A. And at some point, just stop. The machine boots and works. Every repair risks future damage to a very rare machine - "can I accept this as good?".
Your noise will increase with heat; if you're not running the TMS9918 with a heat sink, that might cause noise long before a catastrophic failure. To that end, a little cooling spray on those discrete output transistors might help to isolate the problem.
ALL shielding has a purpose. It is one thing to remove it for troubleshooting and repair, but it cost money to put it there; there is no such thing as "an extra part" in manufactured goods. Every shield and every screw has to be back in place before you can really judge the output. You think Nissan would have really spent the extra $0.11 for that bolt if they didn't think it was necessary in the car? Same idea.
As for the ghosting and fringing on the TI Color Monitor, send it a good signal from a professional NTSC signal generator. Make sure you're in 75 ohm mode on the monitor and the signal generator. If it's not perfect, double-check your cable from the signal generator to the monitor. Something is ringing or not recovering immediately from a high-contrast transition around the characters; I would start looking for high ESR or open capacitors in the emitter circuits of transistors in the video chain. Those monitors were made by Matsushita (Panasonic/Technics) and are commercial-quality video monitors you could see with a Panasonic label in any TV station of the era.
Peaking coils can also go bad, I don't think this is what is happening here, but keep it in your mind as a possibility. If you have a schematic for the Matsushita/Panasonic monitor, contact me privately through the "About" page on my own UA-cam channel, I will go through it.
The original TI-99/4 monitor - probably even more rare than a TI-99/4 - was a Zenith color TV set without a tuner installed. From what I have heard, the original video modulator did not pass FCC spec, so TI offered the TI-99/4 only with the Zenith monitor. I would imagine the surviving TI-99/4 machines got saved while the bulky and heavy monitors tended to get pitched.
Another great video; thank you. Keep the TI-99 alive!
Lawrence
Tursi is an awesome guy. I used to work with him in Virginia and California.
What the hell was all that negativity about for just replacing a bunch of IC's? Even if it wasn't more time efficient, if somebody decides at a certain stage that it will work better, who cares. It's his machine, not yours.
It's actually a technique very often used on a professional level to just work from a fresh new start. You otherwise run the risk of going in circles when troubleshooting. (I have been doing this kinda work for 15 years).
Keep up the good work and don't let those grumpy people decide what you have to do! 👍👍
Awesome video as always.
A lot of people oppose socketing chips because getting them out risks breaking traces. And with multiplayer boards that can be difficult to find and repair. So diagnose before just shotgunning everything.
Personally, i think there is some value in that argument, but it should be made as a suggestion, not a criticism. We don't all have the same tools and knowledge. And if you have spares, socketing can be useful for quick diagnosis. But as the previous video showed you risk lifting pads.
This is great thanks! I'm going to add "Test resistance between pins on CPU" to my checklist of things to try. Any two pins that are tied together might be a chunk of something shorting something else. Seeing a cut trace or shorted trace like that would be hard when it's under the sockets, but checking that data lines and address lines shouldn't be shorted together would be a good general check to try and catch really frustrating edge cases like this. Nice!
2:50 seeing that "forest" of probe leads warms the cockles of my heart curiousMarc and Jerry Walker would be proud. Leave no stone, no pin un-probed! 👍
The solder likely splashed there as you were fixing the power bus bar. (in previous video) The socket seems melted a bit in the corner right next to the splash. Doesn't really matter how it go there, nice work! ...and good luck fixing the video out.
You're probably right about that. The solder splash is too shiny to be very old and looks exactly like what happens when an excessive amount of solder is on a soldering iron tip while maneuvering to get to something that's at a difficult angle.
I would suggest that the solder blob is the result of someone doing some work with soder on the board, not being particularly careful and accidently flicking a drop of solder under that socket.
Makes you wonder if this machine even ever worked.
that looks like a dropped blob of solder, it's much too shiny to be old solder.
I'm thinking solder splatter - it was a bit that got flung while unsoldering something.
Hey, that's MASSIVE progress! One small blob of solder, a huge pain in the ass before you find one. Bet video recombobulation will be easy peasy lemon squeezy... and I love seeing Hellorld again!
Next one, water chip :)
Solder splatter. Typical situation is unsoldering stranded wire by heating the joint and pulling it out. When it suddenly comes free, it can fling a bit of solder. That's the most common, but that's not to say it won't happen when unsoldering anything else.
love that your reference for typing is "it's like using a teletype" - for most of us, not sure that helps 😂
Love your vids :)
all the best from the UK
Haha, I never thought about that, but now that you mention it, chiclet keyboards are probably way more common than a teletype!
I remember using one on the museum, its something between a typewriter and an old spring keyboard.
@mattsword41 perhaps it's like typing on an oric-1 then ;)
I just loved the way he said it as though we all knew - it tells of a man who spends a lot of time with old gear :)
(Not a criticism at all, it's exactly why I watch the channel). The funny thing is I suppose, a more-than-normal proportion of people knew ;)
As a teen aged ham operator in the late 60s who owned and used a Model 15 TTY on the air, I knew just what you meant
I look forward a lot to the vacuum tube computer. I absolutely LOVE that computer and i wanna see more of it
That's the TU99/4, then.
I've been so delayed on it because I wanted to make sure the redesign was going to work and was mostly sorted before I started filming on it again. But, I think it's really starting to shape up, so we've got some good stuff coming!
Current video plan is:
CPU5 Centurion video
Mystery secrete tube project video
UE14500 tube computer video
Great to see you followed my advice. As I said, that's going to eat some time, but it will also deliver answers. I would have made a second trace to look it the deviation occurs at the same place, but first checking some obvious ideas is as good and it resolved the issue. Great episode, as usual!
I loved all that logic analyser stuff... made me quite jealous... back when i had address and data busses, that sort of equipment was way to expensive for me... and these days i do a lot with microcontrollers and my address and data busses are hidden away inside the silicon. Being able to watch each instruction execute on the analyser screen is fabulous.
I can't imagine trying to troubleshoot modern stuff!
Being able to see each instruction is such a huge benefit when trying to troubleshoot, if it were all hidden away in the silicon, I'd lose my mind, haha.
"... address and data busses are hidden away inside the silicon"
Many modern MCUs have a software debug interface that allows to do that with software instead of a logic analyzer.
@@physnoctCan you please point to some resources for this?
Love to see a healthy machine with an infinite "HELLORLD!"
Very excited for more vacuum content!
Nothing sez 1979 engineering quite like a backup space bar...!
This is the only machine I've seen with a backup spacebar, and hilariously, it's the only machine I've needed a backup spacebar on, haha.
@@UsagiElectric it may not be so much of a coincidence as it first seems
12:33 I ran into exactly this issue with a Stern Meteor pinball machine. I about pulled my hair out trying to figure out why the game ran but some sounds were getting garbled. I eventually traced it back to something like this where two address or data lines were bridged due to a miniscule amount of spilled solder. I have no idea how the machine was even able to run at all but it did. Congrats on finding this!
It's crazy how with such a bad fault, the machine can still mostly run!
I was blown away to see just how far into the boot process we were getting.
Also, kudos for getting another pinball machine up and going, I got a soft spot for a good game of pinball!
I have one with the Original Box , Box is in fair shape . I bought it at a Sale . It's in the basement on a shelf .
Isn't that rush when you finally figure out the problem worth it all, though? Omedetou gozaimasu, and a big ol' "Thank You" to all the folks who contributed so freely of their knowledge. (I'm glad it didn't turn out to be the buss bars that were the actual root problem... that would have been more difficult to resolve.)
To all who contributed to this solution: Otsukare sama deshita! {Thank you for your hard work!}. You made bringing this old and rare hardware back to life possible.
The solder is too shiny to be from the production date.
It is probably from the last repair trials.
"If you don't like this bar, there's another one just around the corner" Cheers for max patience with troubleshooting and I certainly enjoy seeing some logics getting analyzed :D
That was premo-troubleshooting with the logic analyzer. Brings back great memories, both of using high tech instruments and having a resource available to provide tips.
Thank you!
I certainly wouldn't have been able to get it done without the help of Tursi and the guys on the AtariAge forums!
For anyone wondering about the system's funky name, the TI-99 is from being the little brother of the TI-990 minicomputer, and I don't remember if I read it on Wikipedia or elsewhere, but apparently TI had a whole range of machines planned, and the /4 was meant to fit smack bang in the middle of that range. This never happened. There aren't really any TI-99/1, /2, /3 or /4+ computers except maybe some plans or prototypes, but the /4 still stuck.
(The TI-99/4A's letter A is from the TMS9918A video chip, the second version of the TMS9918, which added bitmap mode support.)
One common cause of jail-bars is inadequate shielding of the video circuitry. It picks up the clock signal as RF interference. Of course it also helps to have a monitor that works well, to make sure all the issues are coming from the sender.
The 9918 chip should output something close to the composite video spec: 1Vp-p between white and black, and another 0.4 to the sync pulse. It will have a DC offset, but after going through the first cap in the circuit that's gone. Or rather, replaced with another DC offset. Looks like it should be about 1.34V. The "brightness" trim is actually for calibration to make sure the final signal output is according to the above specs. Assuming my analysis is correct, the output of Q200 should be about 3.65V DC. That would put the DC offset at Q201 closer to 2.95V DC. Assuming these biases all read correctly, you should have a smooth transition of picture data throughout the amplifier circuit. Just remember that the trim should be set for 1Vp-p between black and white at the final output.
As for the TV, I expect that the "Hi-Z" setting removes the 75-ohm pull-down resistor from the circuit. This has the effect of doubling the signal voltage that the TV sees. It also increases the likelihood of signal reflection, which causes a different type of ghosting. It would be best if the TV could be made to work in the 75-ohm position.
I bought the 99/4 as my 1st Computer. About 1977 or so. I was also going to Grad School at night. My Project class in Programming was pretty open. We had learned Basic, Fortran, Cobol, Pascal, PL1, APL and some Assembler. I wanted to do a Linear Programming Model. I found the code listing in Basic so I checked with my Professor. He was fine with it. And I got an A. The Machine was fine for me at that time. I got the expansion box and also controlled a Little Robot. Dennis
Glad it works now! Clever sleuthing, as well, on that bit with the solder joint, that would've driven me absolutely _mad as crackers_ for months on end if it'd been me.
Have to admit, as well, I like how you addressed the criticism leveled at you in the last video. Don't blame you for being salty -- we all were, clearly, after all -- but it amuses me no end how you went through and were very, very defensive about how you troubleshot everything by going through the basics and all, power, clock, address and data buses... and _completely bugger-off ignored the entire fact that you had gone through it all with all the level-headed orderliness of a coked-up meth head running a fairground ride in dire need of maintenance_ -- which, of course, was a far more significant issue at hand than whether or not you actually hit off the base-level stuff. It's beautiful, you didn't even lampshade it, you just completely ignored the issue entirely, even though it was "even Stevie Wonder could see this" kinds of obvious that you were going through it like a half-starved monkey with a hemorrhoids problem going after a banana.
Absolutely brilliant, good sir. You should consider a foray into politics.
That monitor is just like the one my grandfather had on his 4A. Great repair job!
Seeing all the probes hooked up to that poor thing, it reminds me of a line from Star Trek 2, where Scotty says that that the reactors are "bypassed like a Christmas tree."
Good job! 👌
That 'TI' never imagined it would come back to life at the head of such a cool peripheral train. 🙂👋
Great debugging there, love the channel.
Congrats man, really happy to see you beat this. Gotta say, your dedication and enthusiasm are inspiring.
I loved watching you use the logic analyzer to step through the boot and see what each step was doing. I found that fascinating.
That solder looked fresh.
Like I said, could have been from me, but ultimately, it doesn't matter because we found the fault and got it working!
Great channel brother, Ive recently been delving more and more into oldschool computers, and the TI-99 looked interesting. Looking forward to checking out your other content.
One of the interesting things I see with Retro computer folk is that they almost never use a scope on the video outputs.
There are expected voltages on the video path and frequency response can also be a factor, both of which can be diagnosed with a scope triggering on the video sync pulses. (Easier with a scope that has video triggering)
Oh I'll definitely be scoping the video outputs! I just was running short on time and needed to button this project up for the time being to make room for some new stuff coming in. So, once it was booting and mostly clean, I put it all back together and wrapped it up here.
I'll be revisiting to fix the video issue on the 99, the ghosting on the monitor, and the bad spacebar in the future!
Is there some opamps amplifying the video output?
Scoping the video signal is of limited value as a rule. You already know the output is bad. There are so few analpg parts (everything comes out of a chip) that there is little left to check.
All that for a little speck of solder. But ... the knowledge gained through analyzing the boot sequence is priceless.
your enthusiasm and positivity remain contagious :D
It's great to see the start up sequence for the 99, I have 2 TI's and neither are functioning yet. Great work from Tursi and all the TI'ers, all your video's are classic computer investigations on hardware that only enthusiasts can appreciate.
Love this as the TI-99 was my first PC as a kid. Learned basic from the owners manual.
I would have said "99.4 percent was up and running." haha... Great video, and the TI home computers were fascinating and weird! All good wishes.
Congratulations. Way to go! I ❤ your videos.
Thank you so much!
Very interesting, just come across your channel & it rekindled my old love of old computers before they became smart and took over the world. Also feel the need to invest in another hp device.
Love your videos!
Thank you!
Great outcome! Top notch detective work all around.
Thank you!
Congrats on cracking a really tough problem. Those marginal crosstalk bugs are so hard to diagnose
Great problem solving.. That was my number one guess - either a dry solder joint or a short.. 😃
Noise in the video circuit.. Maybe a high frequency capacitor, could also be the cable or a ferrite bead. The same with the monitor.. Maybe a tube saver could help as well - if you can get access to one.
Your posts should be on the Tenacity channel. You are brilliant and have more patience than Job.
LOL! Using a logic analyser just for a solder splash. Total overkill!
Very very well done 👍 😊
If you have things like led lights in that room, it might be that this version of the ti99 is more sensitive to their interference.
I have seen multiple tv or radio restorers loose a lot of interference by just turning of the lights.
Great to see you find that issue. I think many would have given up!
The video signal looks very weak to - I'd not be surprised if it wasn't even fully connected, and your just picking it up via coupling. I'd just buz that video signal back though to the chip, and check its actually connected. I've seen plug sockets on other systems get partially detached from the motherboard for a similar effect.
congrats!
men...you are the beast...the best...
the best beast!
WOW!! I have so been waiting for this! Figured it would be something simple (this is why I have no soldering confidence...if I can make something worse I always will). My most hearty congratulations, you actually made my day.
I don't think there's a TI enthusiast who doesn't know the name Tursi, and ALL of us have benefitted from his expertise either directly or indirectly. And as you stated, the Atariage forums are chock-full of folks who will jump to assist (been there, done that, have the now-working peripherals as a testament)
Just an aside (that I may have mentioned previously - if so apologies, old-fart syndrome), I am SOOOO jealous every time I see your thermal printer...I have slowly re-aquired all of the TI hardware I owned in 1982, but the thermal printer remains my most lusted-after Holy Grail. Then again, I also always wanted a /4, but now I can live vicariously through Usagi Electric.
And I would be so into watching the process of constructing the inards for a P-code sidecar!, so yes please!!!
Thou hath prevaileth! Bravo to you!
That's fantastic fault finding. Congrats. 👍
I remember Target was selling these TI-99 computers for $99 dollars. The catch was that the other peripherals were quite expensive.
My high school had these for computer classes. They were ridiculous machines to program on in BASIC, so I'm glad we didn't have to open them up for surgery! My understanding is that the chips could actually do a lot more than could be reached from BASIC. I learned hexidecimal pretty well using graph paper to design sprites.
I've been there before. I had a solder bridge on an Ohio Scientific 540B replica PCB, and it threw off the timing of the video circuitry pretty badly. Didn't notice the fault for quite some time. Nice work on the 99/4!
Yay! It seemed a bit dark in the last episode. It's always darkest before dawn!
love me some TI-99 videos. the community around this little computer is insanely knowledgeable
I'm curious, other than the available expansion card cage, what in particular about this machine keeps people enthusiastic about it up til today?
@@editingsecrets I'm not exactly in the community myself so I can't speak for them. but for me, I just think its neat.
You did it! Hell yeah!🎉🎉🎉
Thanks!
Lol I love mine. I don't have the rare daisy chain but the power tower. It was amazing they expected people to have 6 foot desks and chain everything in a line. Lol
Don't worry about defending your actions so much! Anyone can see you're not a new-comer and the equipment is in the best care it could possibly be!
Ooh... you've got that monitor perched on "my old scope"... I forget the model number but I could never forget the layout... I loved that scope. :)
It's a great old Tektronix scope!
I think this one is a 465M, but there was also a military version called the AN/USM 425.
@@UsagiElectric yeah, yeah... that's it, 456M... LOVELY!
Congratulations. I have loved the TI since it first came out. I am not sure if the keyboard has the same design as the 4A. I had to use a silver scribe pen to redraw some of the keyboard traces on one of my 4As, but it worked perfectly. I did not need to have a TI for parts only.
Thank you!
The TI-99 is a really interesting machine, and I really want to build a homebrew out of a TMS9900 someday. I think it's a fascinating chip and cool architecture.
Kudos for getting the Mitsumi membrane keyboard back to life, those are notorious for failing!
Good finding and very patient and organzied troubleshooting. The spot of the solder blob/bridge is strange, but the solder itself looks "more recent" then the solder on the pcb. Could it be a drop of solder the "splashed" out while desoldering some of the chips on the board? The relativle high resistence between the address lines seems to be due to some corrotion or coating on the old solder with the little splash just on top of it. It cooled down too fast to make a proper join with the old solder.
My father has one of these in the attic.... I have some really strong memories with this. I think I might have to go digging into his attic.
I thought this series was supposed to be a break and then we were going back to the PDP, now you're talking vacuum toobs and whatnot.
Nothing compares to that first time a failed system comes up. As a matter of course I like to begin with looking for shorted adjacent pins / traces. My friend revived a donated 520ST this way, sometimes its just that simple, especially in the era of thru-hole when leads could be folded over and touch intermittently causing all kinds of grief.
That TI-30 on the end of the train is a nice touch, that was my last basic scientific before having a programmable 57.
Good job! Pesky solder bridges!
A rather unique cold solder joint.
A TI-99/4A was my first computer that I got for Christmas about a year before they were discontinued. I had it for a few years before upgrading to a C64.
If you have both computers there are two interesting books to read: 1. The orphan chronicles (about the TI-99 series) by Ronald Albright and 2. The Home Computer Wars by Michael Tomczyk
That solder blob looks fresh. So my guess is it fell in recently. Even if it was you, that hardly matters. It's just annoying faults that can happen to anyone.
Maybe if you had a better desoldering station available, with a vacuum pump. That might give you less broken pads too. Never broken a pad since i got one. But that is just a suggestion, i accept you might have different priorities. If you do ever get over, get one with the pump in the station and not in the gun. A loser weight gun is easier to handle.
Normally, a short between address or data lines will be seen as some invalid or slightly out of the ordinary voltage levels at times when all the other similar signals are working with normal voltages. I'm not sure what you would see, though, when the short has significant resistance. I suspect this would make the incorrect voltages much less apparent.
I was also surprised - it's a long time since I looked at 9900 assembly language, but I still recognised a few of the op-codes. 😊
I like how the logic analyzer has a button labelled "Don't Care" xD🤪
Nice catch finding that short between vias. Definitely an unusual type of failure and hard to find given its location. To catch this type of issue would require an oscilloscope to watch the address lines. With two of them shorted the scope should show that two lines are not always going fully high or fully low. Best of luck getting the video signal looking better. It should be an easier problem to fix.
Would it have been faster for him to have started by using an oscilloscope to test each line, rather than using the logic analyzer first? What's most expedient or advisable?
@@editingsecrets A scope and an LA are used in different ways. The scope would have been the first thing to use as it would have shown a conflict, or short, between an address line and something else. A multi meter would then be used to try and find the short. It still would have been tricky as it wasn't a dead short. I usually rely on the continuity tester beeping when there is a short but at ~40 ohms between the two address lines it might not have been noticed by the continuity tester.
@@kevincozens6837 Thank you!
Best Sunday nap time channel.
おやすみー :)
Bien joué beau travail
that solder bridge was definitely not done at the factory.. that socket has been melted on the one corner so obviously someone has been in there with an iron before
I’ve had my Hakko desoldering iron spit or drip solder from time to time after letting off the trigger. Luckily I’ve caught it every time but I’m afraid something like this will get me one day.
I remember these. I used to have one when I was little. The first computer I ever used.
Glad we got this working! Woohoo!
I have three drives to my setup. My rig takes up 70" on my desk! Soo big! I gave up on the video controller a long time ago (you also need one of three laser disc players made for it that are almost as rare), but will hunt for the PCode until I am old and gray most likely! lol I have thought about making one too, but the cards are so rare that the Indiana Jones in me won't let me destroy one to make it.
LED lighting is a major source of RFI, pop it back into its case, hopefully it is shielded enough to move forward...
I like it when things work. Cool
When in doubt, blame the PCB not the chips. You could have found that by just checking for shorts between pairs of pins on the CPU. Hindsight is 20/20 though - good work with the logic analyzer.
I was confident you could resolve it. Everyone was expecting a chip malfunction. This was caused by a user trying to do a mod or repair. The rom socket looks like it was burned by a soldering iron. Bridging the 2 address lines makes sense. This mimicked an interrupt problem, but seeing the cpu executing code in consecutive memory addresses without a JMP ruled out a NMI . You could done this repair without help. I'm sure of it. You just needed to know what trouble shooting steps to take and in what order. You should build some home brew computers using a Z80, an mc6809 or a mc68000.
This was my very first Computer.
My parents bought a TI-99 in '81, I think I spent more time on it playing Android from a cartridge than I ever did on any arcade game. Also installed a pinball game via tape recorder and tried to program a haunted house game like Zork but discovered I'd consumed all of the memory just on the house and surrounding yard and had no memory left to play the game!
let see you get TI-990 working. I know a lot about TI-990 and the case with they TI-BUS
Good memories of the 90s, all like the Amstrad CPC or the Commodore 64.
So that ended up paralleling my Tempest board troubleshooting in a different way -- before I had the issue I mentioned in the comments in the last video with the ROMs, I had an issue with the state machine logic that I had to diagnose with a logic analyzer. And that also ended up being a shorted trace! In my case it was an issue with the lack of solder mask on the main PCB, and a couple of traces had gotten scratched, bridging them together with a near microscopic piece of metallic debris.
16:20 check the quality of the clock on video chip, nowadays you can just buy a tiny surface mount 4pin oscillator chip 3.3 and 5v tolerant, which puts out a nice cleaner square wave instead of a sawtooth like one, also check all the video mixing circuitry, nowadays you can just rip it out and use a single mixer ic with a few passives and get clean composite and svideo and rub if the chip spits it out...also check how much noise and ripple dat psu spitting out