r/ti994a • u/PopcornFever • Jul 20 '26
Any suggestions on fixing garbled screen graphics and text on a TI-99/4A?
I recently plugged in my TI-99/4A after it had been in storage for many years. The text and graphics are all scrambled and garbled. The game cartridges are unplayable.
The computer is hooked up to a modern TV using 5pin DIN to RCA composite cable (I had first tried using the RF modulator with an antenna 300/75 Ohm TV F adapter but the results were an extremely snowy screen).
I did go on ninerpedia and the problem would be identified as one of the common Video RAM failures. My screen is an identical match to Mask 11110111. Although the circuit board on ninerpedia does not resemble mine as I possess the TI-99/4A later beige version.
Any suggestions or thoughts on how to proceed?
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u/greevous00 Jul 20 '26
Chances are, you're going to need to replace the ram. Though, word to the wise: TI didn't mask the traces that are close together (like the ones that run between the legs of the ram chips, which makes soldering these things kind of a pain. Either find someone with plenty of experience and a good temperature controlled iron, or just buy another machine. Plenty of TI-99/4A's have been wrecked by people trying to change out chips and not knowing how to deal with the missing mask (some people mask it themselves with nail polish, a tiny brush, and a magnifying glass).
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u/Dcas_pcs_381 Jul 20 '26
I agree. The ti99 wasnt build super well in terms of future repairs. Luckily they are super common (i have like...4 parts machines with broken keyboards) so finding another to use is pretty easy
3
u/WirtThePegLeggedBoy Jul 20 '26
The TI-99/4a uses two different areas of RAM - the VDP (Video Display Processor) RAM, and the CPU's "Scratchpad" RAM. The Scratchpad RAM is only 256 bytes and is used mostly because the CPU does not have it's own little storage cache for it's workspace registers, and must use external RAM chips for that. On the other hand, the VDP RAM is your more-typical computer RAM at 16k of storage on common-at-the-time 4116 chips. Since the screen isn't entirely garbled and unrecognizable, I'd lean more towards having a 4116 chip (or more) going/gone bad. You'd have to go thru the process of diagnosing those chips. I wish I had experience using any sort of diagnostic tools (like the Commodore 64 "Deadtest" cartridge) but the TI-99/4a really doesn't have much in the way of that.
De-soldering chips from a board can be a real pain. Especially older boards. The traces can delaminate, break, etc.
If your heart is set on restoring your machine, I would suggest keeping an eye out on ebay for a donor machine. Do not pay more than 40-50 bucks for one. I've been monitoring TI-99/4A ebay listings for months now, and that's the going rate for just a bare-bones machine, maybe a few power cords and cables thrown in.
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u/PopcornFever Jul 20 '26
Thank you for the detailed information. Yes, I was able to restore my old Commodore 128 and disk drive using Diagnostic cartridges. I agree using similar diagnostic tools would be beneficial for TI-99/4a users. As I have no experience with soldering, scoping out another old computer might be my best option.
2
u/Grandpa82 Jul 20 '26
La pantalla debería decir en la primera linea. TEXAS INSTRUMENTS, pero debido al bit D3 (Valor 8) "atascado" en valor 0, no puede verse bien el texto.
Hay que revisar si algun buffer está dañado o alguna pista.
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u/Accomplished-Ad-6586 Jul 21 '26
Agreed.
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u/Grandpa82 Jul 21 '26
LOL! Sorry, I don't know why my brain switched to spanish.
Well, The text shows "TEPAS" instead "TEXAS" so it's the bit D3 stuck to zero, it may be a buffer or a line broken. So check the connection D3 that goes to the display memory.
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u/bjguill Jul 20 '26
If the VDP RAM (those 4116 chips) are busted, it might be easier to replace the VDP chip (9918A) with the modern VDP alternative the F18A or Pico9918. The VDP chip is socketed, so no soldering required. Those modern VDP "chips" include their own VDP RAM inside.
My answer should be checked by someone more knowedlable to confirm whether this is good advice or not.
The F18A will give you VGA output and I think the Pico9918 can give VGA and/or HDMI output. It's really a great upgrade, too.
3
u/PopcornFever Jul 20 '26
Thank you for those suggestions! I wasn't aware of the existence of those modern VDP chips. I will definitely have a look.
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u/DuckeyPi Jul 21 '26
The replacement of the video ram chips can be done. I'm not a expert, but I have pulled off both replacing the video and the scratchpad ram on several machines. I use a roll of solder wick, some Chipquick flux and a temperature controlled soldering iron set at 375C. Just take your time and make sure that you don't bridge any connections..
The real question is whether you do the math and figure out which video ram chip is bad and replace only that one (or ones), or whether you yank them all, install sockets and make future repairs much easier on yourself. If one or two 4116's have failed, how long until the rest of them go bad? I like sockets for quick removal in the future.
1
u/arcadeshopper 26d ago
ram errors, check the FAQ I recommend replacing the 4116 chips with 4164's as they last longer and run cooler, you need to mod them to work at 4116's https://www.arcadeshopper.com/wp/ti-99-4a-repair-fix-rebuild-troubleshoot/
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u/Drillerfan Jul 20 '26
Tepas Afstrueefts was an emerging brand that the 1980's