Category Archives: Modifications & Repairs

Testing a Texas Instruments TI-99/4A Without Power or Video Cables

I bought this TI-99/4A computer along with around 70 games, 2 monitors, and a Milton Bradley MBX system at a garage sale a few years ago. If I recall correctly, the seller used to be a Regional Manager for Texas Instruments in the 1980s, and used it as his personal computer during that time. The whole set cost me around $90, however, it didn’t include the power or video cables, so I couldn’t test anything before I bought it.

The set

The set

While cleaning out my basement last week, I rediscovered the whole set and decided to take it for a spin. Since the TI-99’s AC adapter outputted AC power and I don’t currently own a variable AC supply, I decided to just disassemble the computer and bypass the internal power supply completely by connecting DC power directly to the motherboard.

Prior to disassembly

Prior to disassembly

After unscrewing the bottom of the case and removing the power supply, I was able to locate the power connector to the motherboard. According to the silkscreen on the power supply board that it was plugged into, the voltages required were +5v, -5v, and +12v DC. With that in mind, I configured 3 variable DC power supplies so that they shared a common ground and outputted the correct voltages. I then used jumper wires to connect the supplies to the computer.

Power supply board - Not exactly the clearest picture, but you can see that the output pinout shows the voltages.

Power supply board – Not exactly the clearest picture, but you can see that the output pinout shows the voltages.

Power supply configuration

Power supply configuration

How to create two positive voltages with a common ground (these supplies have reverse-voltage protection diodes built in)

How to create two positive voltages with a common ground (these supplies have reverse-voltage protection diodes built in)

How to create a negative voltage with a common ground (these supplies have reverse-voltage protection diodes built in)

How to create a negative voltage with a common ground (these supplies have reverse-voltage protection diodes built in)

The connected computer

The connected computer

With the power supplies connected, I began work on the video. I found an old Commodore 64 monitor, and was able to use that for testing since it supported composite video input and mono sound. I also found a 5-pin DIN cable (like those used with old audio equipment), and matched the pinout of the video jack to that of the DIN cable. I’ve included a picture of the pinouts below. Before plugging anything into the monitor, I also measured each end of the cable with a multimeter to make sure I wasn’t about to plug +12v into the monitor’s video or sound jack (the TI-99 has +12v on one of the video pins to power the RF modulator box that was sold with the system). With everything verified, I connected the correct ends of the video cable to the monitor, and powered up the supplies.

DIN Cable Pinout

DIN Cable Pinout

TI-99/4A Video Jack Pinout

TI-99/4A Video Jack Pinout

Coloured ends used on the DIN cable

Coloured ends used on the DIN cable

The connected computer

The connected computer

It works!

It works!

As I had hoped, everything ended up working fine. I had some fun with a few cartridges before putting everything away. I also got the speech synthesizer working with the TI Extended Basic cartridge (Command: CALL SAY (“HELLO”) in TI Extended Basic).

Main menu

Main menu

Hello, World!

Hello, World!

This cartridge had great intro music

This cartridge had great intro music

I had absolutely no idea how to play this game...

I had absolutely no idea how to play this game…

TI Invaders

TI Invaders

A-MAZE-ING! I spent the most time playing this game by far.

A-MAZE-ING! I spent the most time playing this game by far.

Before putting everything away

Before putting everything away

Quite an interesting “blast from the past”.

Apple Time Capsule Power Supply Replacement Mod

After having repaired my Apple Time Capsule’s power supply twice due to heat-damaged bulging capacitors, I went ahead with a mod that I had been thinking about attempting for quite some time. The mod replaced the built-in power supply with a simple ATX Molex connector, allowing the Time Capsule to run without it’s hottest component, on a standard DC Molex connector, like those found on any common computer power supply.

To complete this mod, I first removed the bottom rubber cover from the Time Capsule, exposing the screws below. This should probably be done with either a hair dryer or a heat gun to avoid damaging or tearing the rubber. After exposing the screws, I removed them; as well as the metal cover protecting the logic board.

Bottom cover with rubber pad removed.

Bottom cover with rubber pad removed

Next, I disconnected and removed the power supply. Pictured below, it was a small, black, plastic-wrapped module that according to the label, was rated for 30W.

Removed power supply.

Removed power supply

To confirm that it was in fact the power supply that was causing problems, I removed the cover and measured the voltages with a PSU testing tool (meh, easier than a multimeter). While connecting the tester, I also noticed that the 6.3v caps were all bulging (a classic sign of damaged capacitors). The tester confirmed this, showing an incorrect output voltage on the 5v line, which would explain why the Time Capsule wouldn’t turn on.

Bulging caps.

Bulging caps

Failed 5V Test.

Failed 5V Test

With this confirmed, I began work on the mod. I found a Molex extension cable and stripped the ends; soldering them to the matching connection lines which I cut from the old power supply. If you look closely, the 5V, 12V, and GND lines are all labelled on the power supply board. GND was connected to the black line, +5V to the red line, and +12V to the yellow line on the Molex connector. After soldering, I covered the exposed parts of the wire with electrical tape.

Labelled voltages on the PSU board

Labelled voltages on the PSU board

Cut connectors from the PSU

Cut connectors from the PSU

Stripped Molex extension cable

Stripped Molex extension cable

Soldered and connected power lines

Soldered and connected power lines

With all the lines connected, I then tied them together with Kapton (heat resistant) tape, and closed the Time Capsule back up. Note that although it doesn’t show in the picture, I did connect the fan again before putting it back together.

Tied power lines

Tied power lines

Finished product. Almost as good as new!

Finished product. Almost as good as new!

Finished product. Almost as good as new!

Finished product. Almost as good as new!