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Thread: IBM PS/2 35SX missing PSU - what might help?

  1. #1
    Join Date
    Feb 2020
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    Default IBM PS/2 35SX missing PSU - what might help?

    First of all, I'd like to say hello to everyone, as this is my first thread here

    And now to the point - I've recently been given an IBM PS/2 35SX with original keyboard (is it the "Model M"?) and I'll maybe get an original monitor as well for it. I noticed it's been sitting for a while, but it's in good shape overall, except it's missing a power supply. I think, as these PSUs were somewhat problematic, at one point someone took it out, and then left the whole PC to just lay on a shelf. My question is - can it be somehow replaced with modern PSU? I just want to check if it's functional and that everything is ok (I did check the HDD in another old PC and it's working fine - booted no problem into Win 3.11). I found the service manual and checked what each pins on motherboard connections do. I carefully tried hooking it up to a 200W ATX PSU (I wasn't sure about 2 or 3 connections and I didn't have the -5V rail), but nothing happened, the PSU wouldn't turn on (even if I manually bridge the green Power On cable to ground). Is the -5V power necessary for anything to start? Are there any other solutions? Original PSUs are very hard to come by on the internet and pretty much impossible to find localy.

    Thanks in advance for any help. I'd love to add this old gem as a working unit to my small collection

  2. #2
    Join Date
    Apr 2015
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    Austin, Texas
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    ATX power supplies have all of the voltages AT power supplies have, minus the -5v rail. ATX power supplies did at one time have the -5v rail, but this was removed from the ATX spec sometime in 2003-2004.

    Many systems up well into the Pentium era require the -5v rail to even power on, not because anything uses it, but because the motherboard has circuitry to check the rail and prevent startup if it is missing. This rail is brought out to the ISA bus, but most later ISA cards don't use it at all. I've had several 386, 486 and Pentium AT style motherboards which refuse to power on without the -5v rail.

    If you want to convert an ATX power supply for use in an AT system, you have a couple options.

    1. Find an older ATX power supply with a -5v rail. These are harder to find these days, but are available if you look hard enough. It's fortunately very easy to spot if an ATX unit has the -5v rail because it will have a white wire going to pin 20 on the Minifit Jr. ATX power connector. Just be sure it's a quality unit, and not a Chinese IED. You'll then need to either find an ATX to AT adapter harness, or wire up one yourself.

    Here's a ready made one: https://www.ebay.com/itm/282810637191

    2. Find a newer ATX power supply with a sufficiently beefy +5v rail (as computers back then drew most of their current from this rail.) You'll need to generate your own -5v rail though, and the cheapest method to generate it is to hang an LM7905 (not LM7805) off the -12v rail. The power draw is so low that this should be fine in most cases. You can modify the above ATX to AT harness to include the regulator in-line fairly easily, just make sure you properly insulate it so it doesn't short out.

    Or if you want to go the lazy route, you can just buy a new AT power supply: https://www.amazon.com/dp/B017O243SQ/

    These units are of dubious quality though, beware of dragons.

  3. #3

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    Corei, where are you located in case someone near you has something for you to try.

    BTW, your Profile conveniently holds that info so you don't have to mention it every time you post a message.
    PM me if you're looking for 3" or 5" floppy disks. EMail For everything else, Take Another Step

  4. #4
    Join Date
    Feb 2020
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    Thanks for answers, they are really helpful. I think I'll try Gigabite's idea with a LM7905 as i just bought a couple pf these.
    I'm located in Poland, I will add that info to my profile once I have some spare time.

  5. #5
    Join Date
    Feb 2020
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    Poland, Lubelskie
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    I've added the location to my profile. Thank you all for help, I got around that PC today, and I thought, I do have an old broken mobo with a good ATX socket, so I made a little adapter (with Gigabite's idea about -5V rail, photos attached on the bottom) hooked it up and bam! It started right up, RAM tested OK . The happiness is big, but it's not POSTing, as I get 161 and 163 errors. Looked it up, it does have a Dallas DS1287 so I will do it like in that tutorial thread: http://www.vcfed.org/forum/showthrea...in-an-IBM-PS-2
    However, the site mentioned there, for the reference files has expired or something, it can't be found. Is there any other link to those BIOS reference files? My IBM is a PS/2 Model 35SX (type 8535-043). Thanks in advance, you bring great help guys 84468499_491426881519118_8053951806222368768_n.jpg84779167_2819571064747794_6924406086325764096_n.jpg

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