Machines with a history / Restoring Arne’s TRS-80 Model 1

I am a collector of 70’s and 80’s vintage personal computers.  Of many of the machines acquired over the years, I have no knowledge of their history or previous owners. I know nothing of how they were used, as work horses or toys, for a long time or quickly replaced.  They are anonymous representatives of their type in my collection. Operational yes, but with little character to distinguish them from others of their type.

A small number of machines stand out as different....

Two machines (Kaypro IV and TRS-80 Model 100) are notable in that they have name tags still attached. Presumable a personal or assigned machine for the person named on the tag.  Given the tags are still attached it seems plausible that the machines have gone from the user to the closet, surfacing 20 or more years later on an online auction site.

Two machines (TRS-80 Model 4 and Amstrad PCW 8256) have a closer personal association

In the early 1980’s I had holiday employment with Porterfield Computers, the Radio Shack agent and later Amstrad dealer in Dominion Road, Auckland. During this time, Bill Porter the owner used a heavily modified TRS-80 Model 4 for software development.  10 years after the business closed I was able to buy the machine when the current owner answered my “TRS-80 wanted” advertisement in a buy/sell newspaper.  A much loved machine, I make a special effort to keep it operational.

By the mid 1980’s, personal computers had become capable and affordable enough for small business owners to consider. Lomas Matheson, a local quantity surveyor and family friend was looking for a machine for Word Processing, Spreadsheets and my school friend Mark to learn something about computers. The Amstrad PCW8256 was the perfect machine for the job and did sterling service for the family. 25 years later Lomas insisted I have the machine, something I couldn't refuse knowing the history. It also still runs well today.

Where this is leading to… is sometimes a machine turns up with history and an owner to tell you about it.

In my case, this was an TRS-80 Model 1 gifted to me by Arne Rhode. This is a machine with an interesting history.  Arne purchased it “new” in Denmark when he worked there as an embedded software developer for B&O and brought it out with him when he relocated to New Zealand.  

What originally started as a standard 16K Level II Model 1 (26-1xxx) with Monitor and CTR-80 cassette went through a series of enhancements, both standard and non-standard to become the machine I have today.


The configuration I received was as follows:

  • Model 1 Level II 16k Keyboard with
    • Numeric keypad with extra function keys
    • Lower case modification with hand made PCB
    • Soldered Expansion Interface connecting cable
  • Expansion Interface with 32K
    • Radio Shack Double Density Kit
    • Clock board with hand made PCB
    • External Drive Bay with 2 half height drives
    • Power adaptor (240V to TRS-80) with sockets for European mains plugs

Arne told me that he upgraded to a Model 4 and later IBM PC’s when they came available and put the Model 1 into storage.  It hadn't been used for the best part of 30 years but did follow him through a couple of house moves.

After a quick email external clean (amazing how dust gets into sealed boxes), I first powered up the keyboard using a known good Model 1 power supply and monitor. It is not good to blow things up on first power up.


The keyboard LED lit but nothing appeared on the screen.

Debugging time....

  • 5v and 12v power supply regulator outputs were working and the values within tolerances.
  • Tested the Z80 processor with a logic probe which showed no activity on the /MREQ or /IORQ pins so it was replaced. This was a MOSTEK variant common in TRS-80 machines and is not the first that I have replaced one.
  • The replacement Z80 showed activity on /MREQ and /IORQ but still nothing on the screen.
  • Installed the Z80 ICE board and tested the clock, RAM and ROM, all of which checked out OK.

Time to look at the video circuit....

I worked backwards through the video circuit with a scope and found that no synchronization signals were being generated.  This was caused by faults with Z5 (74C00) and Z6 (74C04).  


Unfortunately 74Cxx parts are not so easy to get these days.  I tried replacing with 74LS00 and 74LS04 parts.  The video seemed fine when viewed on a monochorome composite monitor but was not so good when connected to an LCD monitor.  Instead of white text on a black screen there was a pink color to all the text.  Substituting the 74C00 with 4011 and 74C04 with 4096 worked well and the video was back to "Model 1 normal".




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