TRS 80 Model 100 Cassette Interface Cable for PC

At the time when I was about to replace TRS 80 Model 100 NiCad Battery. I have a few programs on it that I need to make a backup of before replacing the battery.

I don’t have a null modem cable or any other means of saving the data.

So, I went a head and created a custom cable to record these backup using my laptop through the Mic input and Audacity program. With this cable, it’s also possible to record program on these cheap MP3 recorder using the Mic input as well.

Below is a drawing of the cassette interface which use a simple voltage divider to reduce the line audio to a level which is suitable for laptop Mic input.


To capture a recording, press record on Audacity and type in the following command on the laptop to begin saving the program.

save “”

To reload the program, type in the following command on the laptop and press play on Audacity.

load “”

Common Loading Problems

  • A weak, noisy recording in the recorded audio might cause loading problem. This is because most vintage cassette interface use zero crossing detection circuit to detect 0 or 1 by counting number of crossing in given period. Any noise introduced in the audio might cause false reading. In case of weak signal, increase the volume and most of the time it would fix the problem. If the recorded audio is noisy, try disconnecting the power adapter on TRS 80 Model 100 and run it on battery to reduce hum.
  • If you’re recording in stereo, make sure the channel that feed the ear input has audio. Otherwise switch the channel or record in mono instead. Alternatively connect the ear input to both left and right channel.
  • If you are still having problem with loading, use Audacity to zoom in and look at the recorded audio. Ensure that the recording is not clipping and audio signal is utilizing the entire range. Otherwise increase Mic sensitivity or adjust the resistor ratio to increase the Mic input level. Below is a decent sample of good recording.



Replacing TRS 80 Model 100 rechargable NiCad CMOS battery.

I have a TRS 80 Model 100 and while doing a routine maintenance. I noticed the internal NiCad is showing sign of crystal forming. Before allowing it to leak and destroying the PCB, I ordered a 3.6V 60mA NiCad replacement from eBay to replace it.

I was planning to remove PCB to do it properly, however after the cover was removed. It seem like it can be done without removing the PCB.

Using the 40W solder iron, I heated each leg of the NiCad battery from the top side and gently removing each leg. Then I used the solder wick to clean up the solder points and install the new battery in it place. Afterward, re-solder each leg with sufficient heat to ensureĀ  proper bonding.

Once everything is put back together, hook it up to external power adapter and let it charge. Now unless, one of these electrolytic capacitors start failing. It should extend the life of this laptop another 5+ years.