Comments on: Texas Instruments TI-99/4A: diagnosing and fixing motherboard faults

By: edomobius

edomobius — Wed, 24 Jul 2024 21:21:59 +0000

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?

By: Matt

Matt — Mon, 30 Jan 2023 01:26:25 +0000

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

By: Mike Francis

Mike Francis — Thu, 13 Feb 2020 20:42:08 +0000

@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????}

By: Mike Francis

Mike Francis — Thu, 13 Feb 2020 19:44:03 +0000

@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.

By: Anthon

Anthon — Sat, 06 Jul 2019 14:43:13 +0000

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?

By: Charles

Charles — Tue, 27 Feb 2018 21:43:00 +0000

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

By: Riccardo

Riccardo — Sun, 15 Oct 2017 16:12:06 +0000

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?

By: Pavlos

Pavlos — Wed, 11 Oct 2017 13:47:54 +0000

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

By: xAD / nIGHTFALL

xAD / nIGHTFALL — Tue, 10 Oct 2017 12:38:32 +0000

@Pavlos

Hi Pavlos,

Doing a remote diagnostic is not possible.

You have to try first to remove the 2 x Static RAM, next step might be one of the TTLs and not the CPU.

By: Pavlos

Pavlos — Tue, 10 Oct 2017 10:43:33 +0000

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!

By: Pavlos

Pavlos — Tue, 10 Oct 2017 07:33:52 +0000

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?

By: Riccardo

Riccardo — Thu, 28 Sep 2017 17:16:22 +0000

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?

By: Pavlos

Pavlos — Tue, 26 Sep 2017 07:34:30 +0000

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?

By: Peter

Peter — Fri, 28 Jul 2017 04:21:48 +0000

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

By: Peter

Peter — Fri, 28 Jul 2017 04:11:27 +0000

@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