Tandy LT1400 – Power Supply Repairs
On a quest to connect to packet radio, I pull out my 1991 Tigertronics BayPac BP-1, plug it into the 1987 Tandy LT1400 laptop and Yaesu radio. The boot disk is already in the computer… Took a bit to locate the modem software. Flip the power switch…. and…. blank screen…
What!!! This was working a few years ago when putting it into storage.
Troubleshooting:
- Power adapter -> Checked with Volt Meter -> Good
- Power being provided -> No power lights on computer -> Could be a switch?
- Listen carefully -> No sound. The LCD display makes a distinctive sound when powered on
- Not a screen issue
- No drives attempting turn on
- No Beep from the bios
- Remove cover and screen to expose control board
- Check power supply cable plugged in on right side of board
- Note: +12, -12, +16, ground … It is missing the +5 Volt
After removing the internal power supply, the problem became obvious…
There was an electrolytic capacitor that had leaked in the power-supply. It is the gold/yellow capacitor at the top middle section. This 25V 470 uF capacitor was for the 5 volt supply.
In this case, you can actually see the liquid on the board. It has seeped along the PCB and has started to corrode the solder joints and power switch pins at the bottom. Easy to desolder and replace with a new one.
To remove the electrolyte, I washed the board in 70% IPA and scrubbed with a toothbrush and a few cotton swabs. It removed most of the contaminate. Dried it lightly, and finally hit it with 250F Hot Air soldering gun to ensure the entire board was dry before using.
Success!
We have a boot sequence. Simple fix. The board lying on the desk is the internal power-supply PCB. It fits into the tan metal holder above it, on the right side of the computer.
The replacement capacitor is just under the mini-board on the top of power-supply board – it is now blue in color.
Notes for Later:
I have two of these computers. Both computers had the same issue. The first one broke a decade+ earlier and had been sitting waiting for me to figure out what broke.
Two computers, Two different power-supply modules, Two different mainboards. One system had the board style as described in the service manual – with a small breakout board to provide 12volts. The other was a newer style, fully integrated. Both had the same 5 volt system, both experienced the same overall failures with the same capacitor.
It is also apparently easy to accidentally flip the floppy disk drive selector switch when removing the drive/power-supply assembly. When restarting, the drives will appear to fail. In reality, there are two small switches by the connector cables that select the drive from 0, 1, or 2. If you touch the switch it can move from 0 to 1, cause a conflict and prevent booting. Took me about an hour or two figure out that this tiny switch had been moved and was causing additional problems.
Now… if there were just a few more packet radio stations in my area.
