Repairing and reconstituting a Texas Instruments TI-99/4A


The TI-99/4A was an early 1980s home computer, notable for being the loser in a vicious price-war with Commodore International in the U.S. home computer wars of 1983.

I'd never aspired to own one of these machines. They were dead on arrival by the time they reached our shores here in New Zealand in early 1984 hence had no market impact whatsoever. However, I was given a badly beaten up model a few weeks ago along with some other gear. Getting to touch and see one wetted my appetite. I decided an example should be in the collection. After all, they were popular in the U.S. and are nostalgically remembered in that market.

TI-99/4A number 1

The machine I got for free had had a rough life (See below). The case was dirty, dented and scratched. However, it was a PAL, had an external PAL RF modulator and the PSU was 240 volts. That was good start. The idea was to see if I could get the machine going, then order another clean and undamaged machine from the U.S.. If it was untested maybe it would be cheap? I'd then replace the U.S. NTSC mainboard with my (hopefully working) PAL one.

beaten up ti-99/4a

Figure 1. A battered and bruised TI-99/4A. Unacceptable for my collection wasn't to be. Turning on the machine showed a blank screen. In the TI-99/4A this could mean practically anything. I used this guide to do some basic testing but not having any replacement chips to swap out made it a painstaking 'scope and logic probe mountain I just didn't want to climb. I formulated a plan B. The video was working and the external PSU was delivering the right voltage so all I needed was the console. I decided to look for a PAL "console only" machine in Europe somewhere.

TI-99/4A number 2

Poking around on I found a working PAL TI-99/4a from Spain. Gawd..the shipping was eye-wateringly expensive though! Also, it was looking worse for wear...much like the one I'd been given. However, in talking with the seller we struck a deal. He would remove the motherboard, and send just that to me. Shipping then would be MUCH cheaper. I still would need a nice case from somewhere but at least I would have secured a PAL mainboard.

ti-99/4a mainboard

Figure 2. PAL mainboard from Spain via e-Bay

The board duly arrived. I plugged it in and tested. It went! showed a nice clear screen for about 45 seconds or so, then the image started to break up. Looks like the e-Bay description "in a working state" had a time limit.

It crossed my mind to hassle the seller about this but I felt it might be something simple. Experience had taught me that if a fault develops a short time after switching on it was often a passive component, such as a capacitor. I started on some testing...

I noticed that the screen break up was caused by a sudden drop in voltage on one of the video pins. Working back from this, through a couple of transistors then the video chip itself I finally decided the problem was with the special clock circuitry for that chip. The circuit diagram I had didn't quite match what I could see but after replacing the crystal (in case it was that) I finally found the culprit. A large, fat ceramic capacitor that was going out of tolerance after it warmed up.

tracing the fault on the circuit diagram

Figure 3. Tracing the fault in the video circuit

ti-99/4a faulty cap

Figure 4. Faulty capacitor identified and replaced

With that capacitor replaced I now had a stable and working PAL mainboard. Now to get a nice case.

TI-99/4A number 3

This was a straightforward purchase from the U.S. The unit was untested but the case was in good condition. What's more it came with a voice synthesiser, a common peripheral with these machines. You can see it in the top right in the photo above. As soon as it arrived I disassembled the unit, and replaced the NTSC mainboard with my now-working PAL one. I closed it all back up, switched on and the result was an acceptable-looking, working TI-99/4A.

TI-99/4a working

Figure 5. TI-99/4A up and running

I checked all the keys. Perfect. I was given 4 cartridges with that first unit. They all worked except the very one I wanted most to work; TI-Invaders. I gave all the contacts a clean but to no avail. Oh well, maybe one day I'll try to replace the cart ROM with an EPROM containing the game.

It took a while and some mixing and matching but now I've got another fine machine to add to the official collection.

22nd February, 2014

P.S. I discovered after the third unit arrived that to modify an NTSC mainboard to PAL was actually very easy providing you have the PAL 9929A video chip (which I had from unit number 1). So I didn't really need the Spanish mainboard at all. Oh well..I wasn't sure that the video chip was ok on unit number one anyway (turned out it was) and it's always good to have spare parts!

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