Nice workshop!

Just a note when repairing 99/4A computers, those white strips carry power and ground. I always thought they were simply ground strips to help control noise, but I found out the hard way after days of troubleshooting a 99/4A that they carry power as well.

Excellent article! However, I was hoping you could provide some help for my personal problem with the TI99/4A. Just a quick description: the display flashes upon turning on, then displays a repeating static pattern that slowly drifts across the display (mainly dark blue). Keyboard presses all render white to black gradient scan lines across the screen, and the keys 1 and 2 key flash the screen a few times, then change the static pattern. The Quit key still seems to reset things, as it replicated the behavior on startup. I’m totally lost, if you could help me it would be greatly appreciated. Understanding that this isn’t a forum, you can contact me at my email if you would like. Thanks!

@xAD / nIGHTFALL
I have no spare components, unless you count the 74LS04, haha. Where could I get the MK4116N chip to test with? My board’s (what I believe to be) VDP is a TMS9918ANL, so is that a different version than the 9929A? I don’t know much about which chips do what. Thanks for the help, however, could I get a little more information? Maybe which chip numbers I am supposed to look for would help direct me. Just so you know, I have re-seated all slotted chips that I could as my first test. A friend of a friend thinks it could actually be a power supply cap problem, which I really doubt because everything seems to be somewhat operational, just a little screwy somewhere.

@xAD / nIGHTFALL
Well, my power board output to the system with a load and without shows 22, 10, and -10 volts instead of 12, 5 and -5 volts. Should this be alarming?

@xAD / nIGHTFALL
I take that last part back, my voltmeter is what’s screwed up, sorry about that confusion. DC voltage looks just about perfect.

I hate to load this place up with comments, but I think you need to know my recent breakthroughs. Power input is confirmed okay, and I got the sound to operate simply by switching cables. I can confirm that the main system is okay, as I can play TI Invaders! No video, however, but this narrows it down a lot. All I get is a light static black screen that changes ever-so-slightly depending on what screen is being displayed. So do you recommend I replace the graphics chip, which I still -think- is the TMS9918ANL?

Well, I finally got the new VDP. It didn’t work. What next?

I was hoping I could get some further guidance… I want my TI99 to work so badly, and honestly nobody even knows what they are, so if you have any ideas, please do share them with me.
Also, a side note, why does this website always take between 10 and 30 seconds to load? I’m just curious.

@xAD / nIGHTFALL
Sorry for being pesky… I’ll get those chips as soon as possible then.

I just got a TI 99/4a but it doesn’t do anything after the press any key to begin screen. Do you have any pointers before I crack the case? I would love to write some basic on the original iron. My computer is in near mint condition and looks just beautiful. It came with all of the original packaging and paperwork. I hate to see it die.

you’re in point of fact a good webmaster. The web site loading
velocity is incredible. It sort of feels that you are doing any distinctive trick.
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do you have a source for system rom chips. Can you burn them. I need a set. thanks

Patrick

Just found this site and it brought back a lot of memories.

I worked for TI at the Lubbock factory in 1982-83 during the height of the home computer wars as a Lead Electronic Tech on the assembly line. I also created an intro course to train new techs. (There was zero training and worse, the majority of the techs didn’t have schematics when I arrived.)

I fixed thousands of these machines. After I left TI, I repaired 99/4′s in the Seattle area for five years as a paying hobby. If you have any specific questions or troubleshooting problems, post them and I’ll do my best to answer them.

CC (ccclarke1958@netzero.net)

@Matthew H

The white strips are called bus bars, they distribute +5, -5, 12 vdc and ground

CC

@CC Clarke

Have a silver TI-99/4A. I failed to read the instructions and had the Star Trek game cartridge plugged into the console when I powered on. I got the initial screen, hit enter and came to second screen when I could choose 1. Ti Basic or 2. Star Trek. Hit #2 and got the Star Trek screen. Second time I booted up the versame way – all worked. Third time time and I could no longer get the star Trek screen – even though the cartridge was already inserted just like the first few times. Then it came to me that I was inserting the cartridge AT THE WRONG TIME. Since computer does sense that the Star Trek cartridge is actually inserted, any idea why I can no longer get the game to actually run – even when I do properly power up and follow the correct cartridge insertion instructions? Any idea what might have blown and what is needed to make repairs?

Peter

Have a silver TI-99/4A. I failed to read the instructions and had the Star Trek game cartridge plugged into the console when I powered on. I got the initial screen, hit enter and came to second screen when I could choose 1. Ti Basic or 2. Star Trek. Hit #2 and got the Star Trek screen. Second time I booted up the very same way – all worked. Third time time and I could no longer get the star Trek screen – even though the cartridge was already inserted just like the first few times. (Then it came to me that I was inserting the cartridge AT THE WRONG TIME!) Since computer does sense that the Star Trek cartridge is actually inserted, any idea why I can no longer get the game to actually run – even when I do properly power up and follow the correct cartridge insertion instructions? Any idea what might have blown and what is needed to make repairs?

Peter

Peter

I have the same problem with my TI99/4a PAL like 4&5. Blue screen of death with loud sound.
Can you please tell me what to check first 9900 or SRAM’s?

Hi,

I have a friend’s TI99/4A picked up at a garage sale for a few bucks. I have asked him to test the machine but the only thing I can seen is a black screen with a continuous beep sound. The sound pitch may vary on power recycle.
I have repaired some C64 and ViC20 but never a TI99/4A. Is there a good tutorial first on how to correctly disassemble the machine and maybe point out areas where to look at?

Hi I have the same problem with a ti 99/4a pal. Blue screen with vertical stripes and loud sound.
What do you think I should replace TMS9900 or the static ram chips MCM6810P? Or both?

Hi I have the same problem with a TI99/4a PAL.
Blue screen with loud sound.
What to check first? Static ram replacement or TMS9900?

Many thanks!

Thank you very much for your answer my friend!
Ok the 6810P are easy but check all 74LS….

Hi,

I have two TI99/4a, one is working perfectly and the other one gives a black screen + continuous pitch sound.
As far I have checked for power and looks good with 11.7V, 5.1V and -5.01V delivered so no problem. I have also checked all socketed IC’s by swapping them on the working board. VPC, GROM, Sound generator and the TIM clock generator appear to work fine. I have also checked 3 of the 8 4116 ram and they do work. As per your T/S with one of your black screen problem you had a faulty ROM. I will both rom before moving to the scratch pads. Had anyway a question : can the rom be replaced with an eprom 27c256 or 27c512 ? If yes where I can find the bin file to load?

Hi,

My original TI 99/4a joy stocks have quit working. How can I trouble shoot them. I have a volt/ohm meter.

Thanks,

Charles

Hi,

I have two TI 99/4a consoles with PAL modulator (metal and plastic).
Both machines show a red LED when turning them on, but show nothing on the TV-screen.

I tried several measurements on the PSU and monitor exit (6-pin din) but voltages seem OK.

Somebody any suggestions?

@CC Clarke
i have a ti 99/4 {yep, no A} & i’m currently repairing it. i have a question. i’m using a composite cable, when powering up, no video or sound. power is perfect, although i’m using a bench supply. so far, only thing i’m seeing besides a badly soldered board, is the dual clock ((tim9904) 74ls362n)). it gets warm after about 30 seconds….all components seem to pass my tests. i haven’t used my oscilloscope yet, due to that hot chip. i don’t want to let it heat up for any length of time. any help would be appreciated, any info you need, email me. thank you.

@CC Clarke, you sound like you’re the man to talk to, & i have to tell you, i cannot believe how rare these things {the “4″},are. i’m trying to be careful. the only parts i could find on ebay, a empty ti 99/4 box, for $100.00!!! i about fell out of my chair!! Anyway, i also was wondering about the “4″, & the “4/A” parts, & being compatible with one another. For instance, swapping the power boards, etc. my intention is to purchase a few basket case 4A’s for parts. just diving into this 1979 16bit, serial no. c0082, amazing machine, makes me want to see this thing spring to life, more & more. things like, “press 1 for ti basic, 2 for ti calculator”, or how it was never sold with a modulator, & so many other things, just set it apart from anything else. i know it is going to be a struggle. for the price of an untested & used tim9904, i can purchase a complete ti 99 4/a. So, if i can swap “4″, & “4/a” parts, this would be invaluable. that milspec white & gold cpu is just amazing. the tim9904 is socketed, along with the g-roms, and the tms 9918.
for the 4116′s, looking at the board, u notches face up, 9 chips, left to right: 74ls156n(ti), mk4116n-2(mostek), tms4116-15nl(ti), mk4116n-2(mostek), mk4116n-2(mostek), tms4116-15nl(ti), mk4116n-2(mostek), mk4116n-2(mostek), mk4116n-2(mostek).

left to right, u notch face up:
1) ti 74156n
2) mk 4116n-2 {2 = speed. 20, in nanoseconds?}
3) tms 4116-15nl {15= speed. faster than mosteks? is that ok?}
4) mk 4116n-2
5) mk 4116n-2 {i heard that it’s ok to go slower, but not faster.}
6) tms 4116n-2
7) mk 4116n-2 {doesn’t make sense. which are original???}
8) mk 4116n-2 {this seems fk’d up to me….what gives???}
9) mk 4116n-2 {helpp! is this factory????}

Any idea where to find the part in photo 15? (busbar)

First of all, Xad you’re a legend, thank you for all of this.
Next, I have a TI-99/4A that looks completely dead. When turned on, I get a black screen with no sound. I only had a logic probe and checked the TMS9900 CPU: the ready pin is high, the reset is too but i have no signals in the address bus line, and neither on the memory enable pin, but i checked only the basics, can somebody help me troubleshoot this old TI99?