TI 99/4a Recap

I figured I would make a post on the TI 99/4a I picked up awhile back. It was one of my first computers. I didn’t think of it at the time as a “real” computer though. I don’t even remember when I got it, but it was older and not current at the time, probably the early 90s. I wanted to get one and tinker with it like I have been with the other old systems from the 80s.

Really I am not sure I would recommend it yet. The system is so limited, I hate to invest in it. With a Commodore 64, short of a drive solution to get to and save data the Commodore 64 is complete. With the Ti if I want to do anything interesting, it seems I have to get a 32k ram expansion for it. That or stick to Basic and original cartridges. Many of the cartridges made now look like they require the 32k Expansion to operate as well.

Anyways I did get it, and I picked up some cartridges and even a tape player that I will look at in the future.

I did check the system out when I got it and took it apart. Inside it looked pretty good. The keyboard doesn’t work properly, it is one of the membrane types.. I am afraid also if I want to do much I will have to find a donor keyboard for it.

I did buy capacitors for it, there weren’t many.

Power Supply Capacitors
Capacitors for the Mainboard. A few spares as well.

Initially I wasn’t going to get out my desoldering iron, but I quickly changed my mind before even getting the first capacitor out. I use an ECG J-045-DS desoldering iron. It is a rather cheap device, but it has done very well for me. I do keep thinking about and looking at those fancier units like you see so much of on Youtube. The ECG is a quarter of the cost of even the cheapest of those though and has done well for me so far. I don’t get it out much, mostly it takes awhile to get heated up and does take two hands to operate.

First was the power supply, with 6 capacitors. I cleaned the power switch with some Tuner Contact cleaner while I had it open, initially I wasn’t sure if it was making nice clean contact. I also replaced the LED as it was so dim that it was hard to see. The original LED had longer legs than my new ones have so I extended them with a piece of solid copper wire and put heat shrink to insulate the legs. The original LED had some heat shrink on the negative leg. I tested the power supply and it worked properly afterward, and the LED was much brighter, but not too bright. I also tested with the 99/4a board attached and it all worked fine. The voltages tested good as well.

I did add the zip tie to the largest capacitor after the picture was taken, although it was quite secure already.
Have to crack open the board. Two holding clip things and three bolts to remove this shield.

Then was the mainboard. That has 9 total. There is a single 10uF 35V, there are five 22uF 25V, and finally three 100uF 16V. To replace them I did get the five axial 22uF caps, but the 100uF I could only get in radial from the supplier I choose. The 10uF I couldn’t get an axial cap either so I went with a Tantalum instead, I could have picked up a radial 10uF though instead. When I did the Timex Sinclair 1000 I ended up getting some Tantalums for it as well.

Here is the board before. The Heatsink compound on the video chip there is replaced as well.
The board after installing the new Caps

The worst thing of switching out from the radial capacitors was making sure I had the new ones installed correctly. Even the Tantalum is a polarized capacitor. The Tantalum marks the Positive, where Axial Electrolytic Capacitors mark the Negative. Then Radial Electrolytic Capacitors have a stripe with an arrow on it that points toward the Negative. So I did a lot of triple checking before powering up the board. I also checked my pictures of before and after to make sure I didn’t flip anything. That is why it is good to take pictures of these things. I then cleaned up the flux from the board and hooked the bare board up to test.

My Trusty little Desoldering iron.

It didn’t power up properly after putting the capacitors in. That was an awful feeling, this is why doing such things are questionable. I checked and rechecked the board, I looked at each capacitor, checked if any were warm and possibly in backwards. I checked all solder points. I pushed in the socketed chips. Then I checked over the board again. I found a few flecks of loose solder, and one of them looked like it may have been shorting two points on the board. I checked again to see if there was any more on it then reconnected the board to the power supply and monitor. This time it came up normally.

I put it back together and checked it, well the keyboard is still awful, but it is working. I can play some Parsec just fine, well the joysticks are still awful. So it seems I have some more work to do with this one.

I pretest the new capacitors before installing them to make sure they are in spec. I also test the old ones I check how close to spec they still were. In this case I think all of the original ones are likely just fine. I just use one of the cheap atmega based component testers.

Still in the end, I had to open it for the new LED and well I say that was worth getting in place. It looks so much better with the new LED. Sure doing just that would not have been nearly as complex a task though. I also replaced the thermal compound on the video chip as well and do recommend that. The RF Shield was reinstalled the same as original. There is the later QI board that apparently doesn’t include it, or part of it at least.

My next project was a Joystick Adapter to use the Atari 2600 joysticks with the 99/4a. It is a simple enough project just taking some connectors and diodes. It did give me a bit of trouble though initially. Possibly a solder bridge issue again, or simply an issue with a poor connection at the TI joystick port, or an issue with not cleaning the flux from the board properly. I have it working now, I am just waiting on some more DB9 connectors to finish it up. I did also make a cartridge with a new pcb and eprom that I may mix in with some post at some point. I am really not quite understanding the way to use the new cartridge boards properly yet. I did get the homebrew remake of Pitfall on it, but I am not quite understanding how to set them up for different size roms.