TRS80 Model 1 Fixup - Part 1

With the success of the FreHD on the TRS-80 Model III and 4 it was time to start testing on the Model 1.

The first step was finding a Model 1 and Expansion Interface.  While they may be plentiful in the US they are a fairly uncommon machine in New Zealand.

Luckily just across the Tasman Sea is Ian Mavric's TRS-80 OldTimer Center.  Ian is a big supporter of the TRS-80 community and the FreHD project in particular, offering both partial kits and completed boards for the FreHD so when we started talking about Model 1 testing he went hunting through his well stocked junkbox to help out.


"I have found you some bits" said the email.... "An early Model 1 keyboard and an EI board. No power supplies so you will have to sort that out. I think they work but you know something about electronics so I am sure you can sort them out"

"How hard can that be?"


While waiting for the parts to arrive I started sorting out the power supply.  Most of my microcomputers use 5VDC and 12VDC so I wasn't expecting much difficulty there.  Then I discovered that while the Model 1 may use 5VDC internally (and -5VDC for the 4116 dynamic RAM), the power supply bricks need to provide 20VDC and 16VAC. No opportunity to recycle an old PC supply here and keep the Model 1 fairly close to a stock configuration.

The solution was an excellent power supply design by Dean Bear that is documented on  There is a good writeup on this design in the September 2013 issue of TRS8BIT available at


For my test setup I built the power supply on prototype board and mounted it in a Jaycar case. As the TRS8BIT article warns, there are high voltages in this project and if you are not comfortable work with this I suggest checking with Ian Mavric to see if a kit or built up unit is available.




With a suitable power supply now ready and a box of parts having arrived it was time to see what Ian had sent over.  It had been 35 years since I had used a Model 1 and my initial reaction was that it was a lot thinker than I remembered.


The machine powered up with the usual Mem Size? prompt but the power switch was difficult to latch on.

Holding it in was fine for the initial test but made using the machine difficult so this was the first thing to address.

With the latching pushbutton not being easily available here I used a miniature three pole toggle switch from Jaycar instead. This is easy to wire into the PCB holes previously used by the pushbutton and mounts nicely in the existing case hole. One day when I find the real pushbutton I can return to a more stock configuration.





This closeup thanks to Dean Bear shows the PCB holes clearly.



Powered by Easytagcloud v2.1

Contact Andrew Quinn