Power to the TRS-80 Model III

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A couple weeks ago I picked up a nice condition TRS-80 Model III from a fellow in Antioch via a Craigslist ad. The seller was also the original owner and had used the machine in high school. That’s usually a good thing to hear when picking up retro machines as they tend to be in better than average condition and often have more accessories such as documentation and software.


When I arrived he had the machine set up on a workbench in his garage. A prompt, “Cass?”, was on the display. Everything seemed to be in order, so I gave him his cash and loaded the machine, a couple manuals and a nondescript box of diskettes into my car.

When I got home I ran into a couple minor disappointments. For one, the box of diskettes was actually old MS-DOS software from the late eighties/early nineties. The other was that the “Cass?” prompt was due to the internal disk drives not being recognized - it was in fact prompting for a cassette. Indeed, no power seemed to be getting to the drives at all.

I confirmed this by checking for power on the floppy drive molex connector and traced it back from there to the 35 watt ancillary power supply that runs the floppy logic and drives.

My first thought was to simply recap the supply and go from there, but as luck would have it, I found a vendor on eBay (computernews80) that had new “old stock” 35 watt power supplies suitable for use in both the Model III and Model 4 at a reasonable price. While I was at it, I ordered a few additional items from the same seller - TRSDOS 1.3, LDOS 5.3.1 and LSDOS Systems Utilities/File Utilities.

Shortly after ordering those goodies, I discovered a couple TRS-80 web sites of interest. The first, and perhaps most important to those without the means to create floppies from disk images, is Ira Goldklang’s TRS-80 site. He offers a service where you can send in a donation and receive premade floppies of several TRS-80 operating systems. I was graciously provided diskettes for DOSPLUS 3.5, MULTIDOS 1.6, MULTIDOS 4.01, NEWDOS/80 2.0 and Super Utility 3.2. He also provided a couple additional diskettes with some nifty games software.

The diskettes from both sources and power supply all arrived on the same day. After a quick removal and replacement operation, I fired up the Model III and the drives sprang to life!


I was a little concerned at first as the Model III apparently doesn’t display an on-screen message if it can’t find a bootable diskette - it just sits there, screen completely blank. After inserting a bootable diskette into the drive and pressing the reset button, I was in business.

Below is a photo of the failed power supply - after removal it was pretty obvious that one of the capacitors is a little burnt. There’s no residual odor so I’m guessing the damage didn’t occur recently. I’m planning on recapping the supply and keeping it around as a spare.


One interesting thing about this particular Model III is that both floppy drives are in a single bay, leaving the top floppy bay free for something else. Free for exactly what, I’m uncertain - but hey, it can’t be a bad thing!

The games that Ira sent along are actually quite good; I haven’t played them all yet, but the ones I tried were fun, simple arcade games in the spirit of Space Invaders.

It should be noted that if you do have the means to create physical floppies from disk images, there are several websites that offer all of the operating systems I noted above - and more - in downloadable disk image format. This requires a PC with a compatible floppy controller and disk drive. I myself don’t currently have such a machine, though it’s becoming evident that I’ll need to put something together, particularly if I hope to get my Model II up to snuff. It craves Pickles & Trout. (The Model II itself is another story I’ll relate in a future entry).

Some sites offering disk images for the Model III:

Timm Mann’s TRS-80 Pages - Offers an incredible amount of technical software including LDOS, compilers, assemblers, utilities, etc. All of Misosys’ catalog has been released to the public, and there are a lot of awesome things in there I’d love to try out.

System 80 - More interesting things from games to word processing.

Dave’s Old Computers - Disk images for various vintage machines, including the Model III.

  • Note: If you do need to get back to the “Cass?” prompt (to use the built-in ROM BASIC for instance), just hold down the BREAK key while hitting reset.


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This page contains a single entry by nekonoko published on August 2, 2008 8:39 AM.

Of TransWarp and Loose Oscillators was the previous entry in this blog.

Installing Pascal on the Apple III ProFile is the next entry in this blog.

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