After completing a fourth batch of SP-4 SCSI cards for some nice patient folks who still needed one, I wanted to find a new electronics project to keep me busy.  I’ve had an old TRS-80 Model III rotting at the bottom of a closet for about the last twenty years or so which I originally purchased secondhand from a guy at work.  Even though it was stored in dry conditions, I’ve read a fair number of horror stories about ancient electrolytic capacitors that go up in smoke when you actually power these things up after sitting idle for so long.  So my new project was to “re-cap”, or replace all the failure-prone capacitors, in the old grey box.

  • Find the Repair Manual. Several years ago I found a large online trove of TRS-80 book scans and virtually all of the interesting software images for the system, which I promptly robo-downloaded and now keep safe for posterity. You never know when sites like this are going to fold up shop unexpectedly, so my feeling is that it’s usually best to grab stuff while you can in such cases.  The archive includes the Model III Technical manual among many other goodies.
  • Take The Thing Apart.  The manual includes good disassembly instructions, which is nice to have because these things are a mass of hand-soldered wires, sheet metal shielding, and odd-sized screws.  They weren’t exactly designed for manufacturing efficiency and near-snap-together construction like most modern PCs.  There’s also a 12″ CRT tube for the screen in there, which is highly fragile and could easily be broken by whacking it against some other internal component.
  • Get All The PCBs out. (Printed circuit boards, not polychlorinated biphenyls.) To remove the old components and replace them requires being able to get at the undersides to desolder the leads.  Below are six of the seven boards in the Model III.  Clockwise from the top, these are:
    • The RS-232 port (a whole board!)
    • The CPU board, which includes three ROMs at lower left, 48K of DRAM at lower right, the Z80 CPU (bottom center), and the video generator (much of the upper section of the board, including 1K of SRAM and the character generator ROM (the largish chip top center.)
    • The floppy disk controller, with edge connectors at the top for internal drives and at the bottom for external ones. The large chip is the WD1793 controller.
    • Two identical power supplies putting out +12, -12, and +5V.  One powers the CPU board, the CRT control board (not shown), and the serial port.  The second powers the floppy controller and the internal floppy drives.  Each supply is rated for about 40W of output.
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  • Figure Out Replacement Parts:  The caps in question are all those little blue or black round cans sprinkled across the boards, and a few others. The caps most prone to “sudden spontaneous disassembly” are the two rectangular green ones on the power supplies – the left most board and the identical one in the middle.  These filter RF noise from getting out of the computer and back into the power lines.Figuring out the right replacements was a fairly time consuming task. The Technical Manual actually includes complete schematics and part lists for the entire computer, something that’s completely unheard of today.  (It’s an enormously interesting and educational read to go through, as they explain the theory of operation behind all of the major sections of the design.)  However, the part lists are not 100% accurate and occasional board revisions resulted in changes to what’s actually populated.  So it was necessary to cross reference the listed parts with what was actually there, and note the changes.   Then it was a long session with the Digi-Key online part finder to choose appropriate replacements – the most important thing to ensure is that the approximate size of the part you’re choosing matches what was there previously so that it fits in the available space – very frequently, the same nominal capacitor (in uF capacity and rated voltage) will be available in a wide range of physical sizes, some of which might be far too small or too large to be a suitable replacement.  Furthermore, since many of these are in power supplies and being exposed to mains voltage and potential fluctuations, choosing a suitable replacement which can handle significant ripple current and is also rated for multiple-thousand hours of useful life is an important consideration. Ideally, I’d like the system to be able to go another twenty or thirty years (of light usage) before needing to consider repeating the exercise, sometime around my eightieth birthday. 🙂
  • Order Parts, and Install Them.  Big box o’ caps arrives in the mail, and then it’s just a long session with the solder wick and the iron, removing the old and reinstalling the new.

Interestingly, this turned out to be the easy part of the rehab, because several additional age-related issues (if not really wear per se) made themselves evident during the reassembly.

First, the floppy controller and the serial port board are connected to the CPU board via some very early examples of flat flex cables.  These are ubiquitous today and are ideal for interconnection of parts in mobile phones when space considerations require components to be split across multiple boards (or not part of a board at all, like iPhone camera modules, for example.)  However, these flex cables were really primitive – they amounted to flat strips of aluminum or tin (something shiny but not copper) adhered to a base plastic layer with some kind of adhesive and then sandwiched under another layer of plastic to protect the conductors, except at the ends where they connect to the circuit boards.  While it was necessary to remove these cables in order to take the boards out, putting them back in proved to be problematic.  The adhesive used to hold the conducting strips in place had completely disintegrated with age, and any attempt to reinsert them in their connecting terminals just resulted in the whole thing delaminating and leaving me with something like shredded tin foil. 😦IMG_0776Luckily, you can buy modern off-the-shelf replacements made from Kapton (polyimide) from Digi-Key with nice robust solder tabs on the ends, so this wasn’t too big a deal.

The other minor repair that turned out to be an unexpected pain was in the latch of one of the floppy drives.  As soon as I had the whole thing back together and working, the latch of floppy #2 IMMEDIATELY broke as soon as I opened the drive, and came off in my hand!  Ugh.  This turns out to be a known weak point of these Texas Peripherals drives. (TP was evidently a joint manufacturing venture between Tandy and Tandon.)  There is a little bracket that connects the floppy door to the internal mechanism that clamps down on the disk, and the whole thing relies on some tiny plastic pins that have to withstand what is likely several pounds of spring force from the latch trying to re-open itself.  The plastic of the latch obviously got brittle with age and gave way instantly.

Some folks on the VCFED.ORG (Vintage Computer Forum) site recommended epoxy to hold the weak piece back in place, but several tries at this proved that it was never going to withstand the stress.  This is where having a 3D printer and a micrometer comes in handy. I’m kinda proud of myself, because after giving up on the epoxy route, I whipped up a replacement piece in Fusion 360 by measuring the original, and printed it out.  Worked perfectly the first time.  The STL files are now posted on Thingiverse, but if you want a few printed and shipped just drop me a note in the comments.Pivot.png

It looks big in the screenshot, but it’s only about 3cm across, and the holes for the pins are only about 2.5mm.  Incidentally, the yellow plane cutting the thing down the middle is something I put there to allow me to export it in halves and print it in two pieces. This way, the outside edges would lay flat on the printer bed without having to print with “overhangs.” 3D printers don’t do so well printing parts hanging in empty space, which is what would’ve happened if I printed it all in one piece.  The two halves glue together with acrylic cement in just a few seconds. It’ll hold up just fine, since there are two big screws holding the flat part down through the oval holes, and there’s no stress at all placed on the glued seam.

Here are the pins, the original piece after the ill-fated epoxying, and the final 3D-printed replacement after installation, just visible toward the top of the interior behind the open door. It looks a little warped because of the funny angle and the close-up shot, it’s actually pretty straight in reality.  And yes, the glued gap between the two halves looks a little ugly, but it’s quite strong and invisible inside the drive, whereas the epoxied original didn’t withstand more than a few seconds in place before going Sproiinnggg.9BD32541-2BC8-40D8-A3E2-67A716F8EBBE

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So, that’s it!  Model III refurbishment complete!

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