TRS80 Model 1 Fixup - Part 2

Included in Ian's box of "things from the junk box" for FreHD testing was a Model 1 Expansion Interface PCB.

Ian tells me that a common problem is people shipping Expansion Interfaces with the power transformer still inside.  Add an enthusiastic courier driver and you are almost guaranteed to have a broken case. This Expansion Interface had such treatment so had ended up a junk box parts donor.

Apart from the missing case, WD-1771 floppy disk controller and connection cable to the keyboard, the Expansion Interface board looked complete and in good condition.

For anyone who finds a parts donor Expansion Interface board and wants to get it going, as of 2013 the WD-1771 floppy controllers are still available on eBay as are the 4116 dynamic RAM chips.  Best to search around because the prices vary a lot.  Unless you are hung up on period authenticity then the vintage style packaging with the really high prices are not required!

With the chips replaced and a cable made up the initial indications where that the Expansion Interface worked fine.

At this point I realized why they put these in cases... where to put the monitor when you just have a PCB? Things get very spread out.

Taking inspiration from the current trend in acrylic project cases for things like the Raspberry Pi I went looking for a source of acrylic sheet and discovered CTS Plastics in Tauranga.

Here is my solution to the no-case problem.

    2 sheets of 6mm grey perspex plastic

    5 x 50mm bolts, assorted nuts and washers.

Total cost about NZ$25.


Much safer than having the EI board floating around on the table, looks smart and with 6mm perspex it is very strong so the monitor can sit on top.


The dimensions and drilling information is in the attached pdf for anyone wanting to build their own.

The drill details are:
    4 holes (1 at each corner) 20mm from the edges.
    2 holes 143mm from the left edge.  One is 82mm from the bottom and the other 202mm.
    1 hole 38mm from the right edge.  This is 142mm from the bottom.
The two pieces will be mounted together with spacers so the holes need to be accurately drilled on both sheets.







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. 


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