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.



TRS80 Model 4 128K Upgrade - Part 2

The Samsung KM4164B-12 parts arrived from Bulgaria promptly and well packaged.  

After installing in the Model 4, the machine was left running the Model 4 diagnostics memory test for 4 hours and no problems reported. 

The parts were ordered on eBay from electron-bg.  I would definitely order from this vendor again.




TRS80 Model 4 128K Upgrade

The TRS-80 Model 4 is regarded by many as one of the pinacles of Z80 development and as a lucky owner of several of these machines I do what I can to keep them operating, to supplement their capabilities with modern accessories like the FreHD but still retain as much of the original operating experience as possible.

Both my machines are in the "standard" New Zealand 64k configuration and I don't know of any machines imported here that were upgraded with official 128K upgrade kits.  

The machines run well with 64K.  To my knowlege the extra 64K (of a 128K configuration) is used by few applications and in most cases as a RAM disk.... so upgrading to 128K wasn't high on my list of "must do" items.  

Contributing to this was my workhorse machine being a "Non Gate Array" variant requiring the "rare as hen's teeth" U72 PAL chip.

I was surprised when Ian Mavric sent me the definition file to program a Lattice 16V8 GAL for use as U72.

The upgrade just had to be done.  The GAL was easy but it took some research and digging in the workshop to find the required dynamic RAM chips.  The Model 4 uses 4164 parts but they must have 128ms refresh.  If buying parts make sure you download the datasheet for the exact part number and confirm this.  Parts can be either 128ms or 256ms.  256ms won't work.

In the workshop waiting restoration is a New Zealand single board Z80 computer from a company called Computer Specialists so I borrowed the 8 4864 memory chips to test the upgrade.  If it worked I would hunt eBay for some chips.

4864 Dynamic RAM chips installed on the motherboard

The U72 shunt installed in a 64K Non Gate Array Machine

The shunt removed and replacement with a Lattice 16V8 GAL

After all this.... it did work!  I was really surprised at how easy the process was. 

The memory tests on the Model 4 diagnostic disks recognized the extra memory and reported no errors.  Montezuma Micro CP/M recognized the extra memory promptly created a 64K ram disk as drive M:.  PIP had no difficulties copying files to and from M:

What a great way to spend 30 minutes.  

Not wanting to turn the Computer Specialists board into a parts donor I ordered some Samsung KM4164B-12 parts from eBay (from Bulgaria no less).  Hope they work.... I should know in 3 weeks. 


TRS80 Model 4 Rebuild - Part 4

The TRS80 Model 4 works well with the two internal 3.5" drives.  The challenge is sharing files with the other two stock machines which have 5.25" single sided floppy drives.  

In the future I plan to add a compact flash or sd card based "hard drive" to the top drive bay so don't want to install an internal 5.25" drive.  

The Model 4 has an external drive connector so I went searching for a suitable external drive case.... and Trade Me (the New Zealand equivilent of eBay) turned up 70GB DLT drives for less than $10.


The DLT drive occupies a full height 5.25" bay so the case supports 1 full height or 2 half height drives.  It also includes a small switching power supply providing +5V and +12V DC.  

I replaced the DLT drive with a Tandon TM-50-1 drive that was originally in the Model 4 when I first received it.  In the remaining space I installed a hard drive frame from an parted out HP-9000 minicomputer.  The frame holds Frederic Vecoven's Hard Drive emulator (


A very simple and successful project.  Now I need to find suitable cleaning product to remove the marker on the front of the case.


Page 10 of 29

Powered by Easytagcloud v2.1

Contact Andrew Quinn