IBM PCjr Revived By An ATX Power Supply And Many False Starts

The IBM PCjr was a computer only the marketing geniuses of a multi-billion dollar corporation could love. On the face of it, it seemed like a great idea – a machine for the home market, meant to complement the “big boy” IBM PC in the office and compete against the likes of Apple and Commodore. What it ended up as was a universally hated, only partially PC-compatible machine which sold a mere half-million units before being mercifully killed off.

That doesn’t mean retrocomputing fans don’t still snap up the remaining machines, of course. [AkBKukU] scored a PCjr from a thrift store, but without the original external brick power supply. An eBay replacement for the 18-VAC supply would have cost more than the computer, so [AkBKukU] adapted a standard ATX power supply to run the PCjr. It looked as if it would be an easy job, since the external brick plugs into a power supply card inside the case which slots into the motherboard with a card-edge connector. Just etch up a PCB, solder on an ATX Molex connector, and plug it in, right? Well, not quite. The comedy of errors that ensued, from the backward PCB to the mysteriously conductive flux, nearly landed this one in the “Fail of the Week” bin. But [AkBKukU] soldiered on, and his hand-scratched adapter eventually prevailed; the video below tells the whole sordid tale, which thankfully ended with the sound of the machine booting from the 5-1/4″-floppy drive.

In the end, we’ve got to applaud [AkBKukU] for taking on the care and feeding of a machine so unloved as to be mentioned only a handful of times even on these pages. One of those articles marks the 25th anniversary of the PCjr, and lays out some of the reasons for its rapid disappearance from the market.

26 thoughts on “IBM PCjr Revived By An ATX Power Supply And Many False Starts

  1. A regular notebook psu attached to the input connector should do also as the psu board is composed of a full wave rectifier followed by a dc_dc converter. Nevertheless great job!

  2. Four things to do to a PCjr.

    1. The Tandy 1000 video modification. Many sites and articles claim Radio Shack / Tandy made their PC clones with PCjr compatible video. Not quite! The graphics modes such as 320×200 16 color look the same on a PCjr and Tandy, but there’s a small difference in how the mode is accessed. The hack involves piggybacking one 7400 series TTL chip, cutting a trace on the PCjr and soldering a few wires. Do that and *most* Tandy software will run on a PCjr with Tandy graphics selected.
    But you’ll still be stuck with the PC speaker sound because despite using the same TI sound chip, Tandy and PCjr do things a bit differently. IBM Canada used to have the video mod instructions on their website.

    2. The Tandy 1000 sound modification. This one is a mystery. PC Enterprises offered a kit for it. I couldn’t find anything in the web archive of their site that mentioned it, but I remember it from their catalogs. Some people have told me there’s no difference and Tandy software doesn’t have any way to detect whether or not a PC has the Tandy sound. But that’s BS because I had a couple of games I could not use Tandy video mode on my PCjr because they required both Tandy video AND Tandy audio. The games failed to detect Tandy audio and thus would only work in 4 color CGA with PC speaker. Attempting to select Tandy audio with any game where independent video and sound selection was possible would result in A. some response about there not being Tandy audio, B. the selection would be allowed but result in no audio, C. a crash when the game attempted to access Tandy audio functions that the PCjr couldn’t provide.

    So can someone please figure out how to mod the audio on a PCjr to be Tandy 1000 compatible for all Tandy games? IIRC PC Enterprises wanted something like $14 or $15 (or more) for the kit. I did buy one of their kits for the video mod, then modded a few others by harvesting chips from old floppy and hard drive controllers.

    3. Replace the 8088 CPU with an NEC V20 CPU. Not only does that give a small but noticeable speed boost (though not fast enough to play 688 attack sub, that would require one of the aftermarket DMA mods) it also gives the computer a Z-80 clone CPU. Run 22-NICE and a PCjr will run CP/M faster than most 1980’s computers built to run CP/M.

    4. A second floppy drive. There’s a quick and dirty method that makes both drive’s spindle motors run when either drive is accessed. Then there’s a slightly more complex method that provides for individual spindle motor control. Do that one. Info should be findable on the web somewhere.

    There was a thriving aftermarket for the PCjr. There were things like a 14.4 internal modem, mods to add DMA, sidecars with VGA, Soundblaster, realtime clocks, various hard drive interfaces, RAM upgrades on the side or in a ‘second story’ with a bay for a second floppy, and many others. IIRC the best aftermarket upgrade kit was one of the second story type which upgraded to 640K and added DMA along with 2nd floppy support. That made a PCjr just about equal to a PC/XT, plus if you did the Tandy video and sound mods – you could sit back and wonder why you hadn’t just bought one of the higher end Tandy PCs in the first place.

    Back in the day I had a PCjr with 512K, dual floppies, Tandy video mod, and NEC V20. On the same desk I had a 12 Mhz PC/AT clone with a total of 12 megabytes RAM (most of that on Micron ISA cards), hard drive, 1.2M and 1.44M floppies, Soundblaster and EGA video, and a Xerox 820-II Information Processor – with wide carriage Diablo 630 daisy wheel printer. I also had a Star NX-Multi 9 pin dot matrix printer. That was mainly used in Epson LX-800 emulation mode.

    With the V20 and 22-NICE, the PCjr was much faster than the Xerox so I’d use it for writing with WordStar, then sneakernet the disks to the Xerox for printing on the Diablo. I also tried 22-NICE in Z80 emulation mode on the 286. Yeah… sloooow.

    1. The actual sound chip in the PCjr / Tandy 1000 — the sn76496 — is a write-only device. There’s no way to detect that it’s there or not.

      But maybe Tandy corp. added something to make it detectable. Don’t know. Ralf Brown’s ports list doesn’t mention anything.

      What’s one of the games that detects this? Maybe sitting down with a debugger would let us figure out what it’s looking for.

      1. From Vogons/vcfed/nerdlypleasures blog: tandy F000:FFFE = FF, PCjr FD, tandy 1000 F000:C000 = 21, then there is TANDY string at F000:C078. You probably also need to extend bios int 10 routine for Tandy video modes.

    2. As someone who had the misfortune of actually selling Tandy computers while working at Radio Shack, I never thought I’d ever see someone talk about upgrading to Tandy spec. The PCjr must have been much more of an abomination than I remembered it. (to be fair, I’ve only ever used one to play King’s Quest)

  3. For 4.. What became “standard” for the way PC floppy drives were hooked up wasn’t actually standard.. Older floppies were supplied with switch or jumper blocks to allow you to set the drive ID and motor signal used on the chain, however, this was ignored on many clones, and done instead with a twist in the floppy cable with both drives set to id 1. This of course confuses the hell out of anyone trying to use “PC” floppy drives with non PCs or with PCs that happened to do it the actually standard way (Amstrad XT clones for instance) Indeed it may not be possible/easy to use more recent 3.5″ drives as they may just be hardwired for ID 1, or require digging inside of to find the empty jumper pads and installing a block or rewiring them with solder etc.

  4. “In the end, we’ve got to applaud [AkBKukU] for taking on the care and feeding of a machine so unloved as to be mentioned only a handful of times even on these pages. ”

    I’m sure the Colecovision will get honorable mention.

  5. I’ve definitely done reading the pinout upside down before. Super frustrating when you realize where you went wrong.

    Soaking the board in rubbing alcohol would probably help with the flux problem. Or switch to a different type of flux that isn’t electrically conductive.

    Did you ever figure out why you were getting negative resistance readings? Usually you can only do that if there’s some power going to/stored in the board…maybe a dying battery on your meter?

    1. Hear hear! I have encountered many of the issues you describe as well with vintage electronics. I usually end up cleaning the board with alcohol too as I have little confidence in old flux, for the health of the machine and myself. Have done the pinout upside down as well though these days I would ring it out before going whole hog. I got lucky a few times doing that but feel like I spent my 9 lives of upside down pinout karma points lol.

    1. I didn’t know about the PicoPSU before I made this video. A lot of people let me know about it after so I grabbed one and will be doing some design tweaks to better use it.

  6. I like the project and started doing some research into doing the same. In fact I had researched this previously but was put off by the fact that according to the IBM PCJr Technical Reference Page 2-136 It states a +5,+12,-6. An ATX power supply does not give -6, just -12. Not sure how safe that mod is.

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