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

I have received to repair five TI-99/4A with the following defects:

  1. Black Screen
  2. Working but with some dirt on the screen.
  3. Vertical Stripes moving with a deafening sound.
  4. Blue Screen with thin vertical stripes and deafening sound.
  5. Blue Screen with thin vertical stripes and deafening sound.

The first two are repaired with the following faults:

Black Screen Fault: replaced TMS9929A followed by ROM CD2156NL
Dirt on the screen: replaced a 4116 RAM.

The third motherboard was used for spare parts.

The fourth and fifth motherboard with the blue screen have been the most annoying.

I spent 6 + hours to make measurements with an oscilloscope comparing one of the two motherboards with a working one to find the fault but the signals/voltages were absolutely identical.

So i have decided to go for exclusion and with a TI-99/4A Tech Data manual on my hand and 39.5 degrees in my Laboratory i have started to desolder and put a socket of the following components:

  • 1 x TMS9901NL (20 +20 PIN)
  • 1 x TMS9900NL (32 +32 PIN)
  • 1 x TIM9904NL (10 +10 PIN)
  • 1 x TMS 4732/2532 ROM U610 (12 +12 PIN)
  • 1 x TMS 4732/2532 ROM U611 (12 +12 PIN)
  • 1 x 74LS244 (10 +10 PIN)
  • 1 x 74LS245 (10 +10 PIN)
  • 1 x 74LS373 (10 +10 PIN)
  • 1 x 74LS03 (7 +7 PIN)
  • 1 x 74LS138 (8 +8 PIN)
  • 1 x 74LS74 (7 +6 PIN)
  • 1 x 4116 (8 +8 PIN)

After all this work nothing had changed, nothing worked, always BLUE screen.

I redid the measurements, but nothing, everything looked the same.

At this point there are only two static memories MCM6810P but that i don’t have investigated for what are used, because already on the motherboard are 8 x 4116 RAM Memory shared between Memory program and Video memory.

… anyway … i have tried to unsolder and put a socket.

So we add:

  • 2 x MCM6810P (12 +12 PIN)

Et Voilà the two MCM6810P are both dead! both!

Obviously i don’t have this component in my laboratory, then i try to recover from the motherboard for spare these two RAM hoping that they are not faulty which however these RAM can be purchased online at a very low price.

The fifth and last motherboard that had the same defect of the previous one did not have the faulty ram MCM6810P.

To repair i had to replace the following components:

  • 1 x 74LS04 (7 +7 PIN)
  • 1 x TMS9900NL (32 +32 PIN)

  1. September 15th, 2013 at 16:55 | #1

    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.

  2. January 9th, 2014 at 04:30 | #2

    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!

  3. January 9th, 2014 at 16:00 | #3

    @Erik G

    Hi Erik,

    Thanks.

    Try the piggyback method of the RAM bank (4116) , this is a easy and lame method but is fast.

    After this , if negative, you have a spare 9929A (VDP) ? you need to try this.

  4. January 9th, 2014 at 23:43 | #4

    @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.

  5. January 10th, 2014 at 00:14 | #5

    @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?

  6. January 10th, 2014 at 00:33 | #6

    @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.

  7. January 11th, 2014 at 04:59 | #7

    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?

  8. January 12th, 2014 at 16:52 | #8

    @Erik G

    Hi Erik, this is a good place to write and share the problems with peoples.

    Yes, 99% you have the TMS9918ANL dead , the TMS9918ANL is the NTSC version , 9929A.

    http://www.colecovision.dk/PDF/TMS9918.a.pdf

    Unfortunately I don’t have one (9918).

    You can buy it at very good price , here:

    http://www.ebay.it/itm/TMS9918ANL-Manu-TI-Encapsulation-DIP-40-GPIB-Interface-Controller-/110886809939?pt=LH_DefaultDomain_0&hash=item19d15e7153

  9. January 28th, 2014 at 01:51 | #9

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

  10. February 8th, 2014 at 01:53 | #10

    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.

  11. February 8th, 2014 at 14:51 | #11

    @Erik G
    Sorry Erik for the delay but this is just a hobby, I do not have much time to spend.

    About the load time is because i have a lot of visitators and no money to upgrade my server ;-D

    You need to replace the 2 x MCM6810P , these static ram chip might be causing the problem.

  12. February 8th, 2014 at 20:01 | #12

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

  13. Sean Fuller
    May 22nd, 2015 at 05:13 | #13

    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.

  14. June 4th, 2015 at 23:43 | #14

    @Sean Fuller

    Hi Sean,

    it might just be a problem of the keyboard, you have try to open it gently and reseat all connectors and chip (if they are socketed)

  15. December 8th, 2015 at 07:01 | #15

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  16. patrick
    January 19th, 2016 at 01:05 | #16

    do you have a source for system rom chips. Can you burn them. I need a set. thanks

    Patrick

  17. CC Clarke
    October 4th, 2016 at 16:39 | #17

    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)

  18. CC Clarke
    October 4th, 2016 at 16:42 | #18

    @Matthew H

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

    CC

  19. October 5th, 2016 at 00:50 | #19

    @CC Clarke

    Many thanks for the response to @Matthew. I’m very busy with my work in this period.

  20. Peter
    July 28th, 2017 at 06:11 | #20

    @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

  21. Peter
    July 28th, 2017 at 06:21 | #21

    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

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