It's AT, buy an ATX to AT adapter and a ATX Power supply to test it. many Pentium 1 boards are AT, LPX also uses the AT power supply as well.

That's not ATX, that's standard AT (P1 + P2 connectors) + 5V standby for soft-off. You will need an ATX to AT adapter, but also a way of getting the +5vSB (purple) and PS-ON (green/black) ATX lines connected to the motherboard. Based on your picture, I would assume the yellow and black cables are used for the +5vSB that comes from the PSU (this would be the purple/black wires in ATX units). The 3-pin connector with red, white and black cables probably goes to the power switch?

I'm sure it's possible to rig something up and get it to work with an ATX unit, but it doesn't seem trivial. If you can post more pictures showing where each of those cables go I may be able to provide more help.

I meant AT I wrote wrong. I wanted to know if I can replace this IBM power supply with an AT standard.

If the power supply still works, you can make a pinout diagram using a voltmeter. Just check the voltage coming out of each pin and write them down. If it doesn't work, you'd need to find the IBM documentation. IBM was known for having really good documentation, so the pinout is probably in the manual for the computer.

PSU is totally dead... i can’t find anything about it. I was hoping that the color of the electrical wires would suggest to someone whether or not they used the standard layout of common AT power supplies

Hi AlessandroB, See here:
https://en.wikipedia.org/wiki/Power_supply_un … Wiring_diagrams

Same colors as standard AT, yes a standard AT (not ATX) supply should work fine. I find IBM likes to be pretty much standard, unlike some other brands...😮) Cheers! Bellarmine

thank you my friend, they would seem to match except for the P1 connector which instead of having the pin8 orange (power good) has the black ground wire.

I think it works ... can someone confirm this? because burning this IBM would be a terrible thing ... it's a very rare computer to find.

Aside from the two P1 and P2 connectors, in the photos I put in the first post you see a Yellow-Black wire coming from the switch (there would be no problem because the switch would be operated by hand separately), but there is still the other Black-White-Red cable that comes from the power supply and could be a power supply supplement ... does anyone have hypotheses?

Thank you

Did you open up the AT psu to check if it was a fuse that blew up or if there is more visible damage?
Perhaps only a fuse or some fuses blew and it is a very easy repair..
If you doubt about it post pictures here, there are some very smart guys around here that can easily identify if it is worth repairing.

Nvm1 wrote on 2020-02-26, 10:05:

Did you open up the AT psu to check if it was a fuse that blew up or if there is more visible damage?
Perhaps only a fuse or some fuses blew and it is a very easy repair..
If you doubt about it post pictures here, there are some very smart guys around here that can easily identify if it is worth repairing.

i will post a pics but i went to my frend that repair smartphones and we see large black area on the PCB, for sure two component called "MOV1" and "MOV2" are blow. they are varistor from what can i know... if the standard AT power supply works I could send my burnt PSU to someone who can repair it ...

Hi AlessandroB,

I looked at your photo more closely, a few ideas:
- If you mean that small yellow-black wire from the motherboard, I'm guessing that would go to the power supply fan so the motherboard can control the fan speed or something. If you look inside the PS that would confirm.
- As for the orange/black wire on P1, am I mistaken but it looks like a piece of shrink tubing with the Orange wire going in and Black coming out? Maybe someone had to replace the pin and used a black wired pin from another connector. Just cut open the shrink tubing to check.

Cheers! Bellarmine

AlessandroB wrote on 2020-02-26, 10:09:

i will post a pics but i went to my frend that repair smartphones and we see large black area on the PCB, for sure two component called "MOV1" and "MOV2" are blow. they are varistor from what can i know... if the standard AT power supply works I could send my burnt PSU to someone who can repair it ...

If you or your friend can remove the blown varistors, do that and also replace the fuse, then check if it works (with the 110/220V switch in the proper position this time). The varistors and fuse probably gave their lives to protect the rest of the components. If it works, remember to replace the varistors afterwards as they help protect against voltage spikes. If it doesn't, you may want to start checking for other blown components (the big caps, switching transistors and so on).

Regarding the other cables that go to the motherboard besides the usual AT power connectors, read my previous post. They are used for the soft-off, in a similar way as an ATX power supply. The +5VSB line connects to the motherboard which has circuitry to handle the on/off states, with this they avoid having mains current going through the power switch.

I'm betting the PSU can be repaired. The sacrificial components (MOV, fuse) probably took the worst of it. The primary stage may have suffered too -- that would be things like the rectifier, mains-side bulk caps, possibly one or more switching transistors. Nothing that can't be replaced. Most likely, nothing happened past the isolation transformer and the DC side is 100% fine.

That said, having a backup plan is not a bad idea regardless whether the PSU survive(d/s). Best thing to do is open the PSU and take some good pictures of the PCB. PSU manufacturers tend to be helpful and provide silk-screen labels all over the PCB, particularly right next to where the "special" wires go. They might say things like "POK", "5VSB", "PON", etc. This makes it quite unambiguous what each wire does.

Never trust the colors. They're somewhat commonly standardized, but it's never a guarantee. The PCB often tells you everything you need to know, so there's no good reason to make assumptions.

I would also not simply replace this with an AT supply. It's probably fine, if you work out a way to switch the mains without having a mains-rated switch in your case. But if this is a board that expects to have control over when (or if!) the main DC supplies are enabled, then it's rather rude to bypass that control. ATX supplies offer the control you need and would be a more suitable replacement -- with the requisite ATX-to-AT adapter, of course -- provided you can definitively reverse-engineer the soft-off mechanism and ensure it's compatible with the ATX standard. (It probably is. The ATX way is a very straight-forward approach, and the OEM of your original supply was probably aware of the ATX standard when manufacturing that supply, and why reinvent the wheel when a perfectly good reference wheel exists?)

Tonight I tried to connect the standard AT power supply to the computer. A series of things happen that never repeat themselves, as if there were random mistakes. I'll explain:

Turning on from the power supply switch (not from the one on the case which obviously has no effect on an external AT power supply) the keyboard LEDs light up for a moment then go off immediately and the keyboard is as if it were not connected, it shows no signs of life and no led lights up.

I turned it on and off at least 50 times to understand the combination of errors but each time the keyboard behaves in the way I have described.

The monitor that I connect to the computer instead exits standby but after 3/4 second it returns to standby as if the signal did not reach it anymore. Not all the times, because 2/3 times I managed to display a message indicating the name of the video chip but then the computer did not move from there. Once it even started as if it worked well, did the ram count and then stopped with "178 system error VPD not available". This only happened once out of 50 on and off that I did.

Other things I noticed: the CPU heatsink always becomes lukewarm and removing the whole ram makes the following series of beeps: beep-mute-beep-beep-beep-mute-beep. from the times when the computer seems to be stationary and the monitor is in standby by pressing the original power button which is on the case and which is connected to the motherboard by two wires it turns the led back on the case and lets the monitor go out of standby but without see nothing on video. sometimes the power LED on the case lights up and then goes off after about 3 seconds without turning on again, sometimes it stays on.

For safety I measured the voltages at the output of the power supply connector and I measured:

-12,15v
+ 12,27v
+ 5,05v
-5,39v

What worries me is the reality of all these states that I have described, turning it on and off 10 times different things happen.

Check this thread. As mentioned by previous posters, your PSU is closer to an ATX one despite using the AT connector.

quicknick wrote on 2020-02-26, 23:21:

Check this thread. As mentioned by previous posters, your PSU is closer to an ATX one despite using the AT connector.

WOOWW!!! adding this 5v on the separate connector seems to work!!!! Thanks a LOOOTTT!!!!

after some problems with the bios which is really unfriendly I managed to start the pc with the boot disks that I had.

the windows95 boot floppy works
the memtest 86+ boot diskette crashes at the beginning of the test (even with a working ram) but I think it is some incompatibility because it marks me with Pentium 0Mhz.

In any case it seems to work, I just have to be able to boot from its hard disk that does not boot but is correctly seen (even the files) if I start the PC with the Windows 95 boot floppy.

Congratulations Alessandro, you must be happy your IBM is all right! QuickNic's comment about the 3 and 2 pin connectors must have been a big help! Now it is a comparatively easy task to either replace the power supply with an AT unit or get the original repaired.

As for the HDD booting, but depending on what OS is loaded on the HDD, you can boot off a DOS 6.22 / Win 95 floppy, and type "SYS C:" (no quotes) to fix up the boot parameters, that just might fix it. Check what OS is loaded first, then use the appropriate boot disk. Any questions let me know.

Cheers Alessandro! Bellarmine

If that happens to you again, if you have ISA slots onboard, you can poke around to check the continuity between the psu's pins and the ISA power lines. Since they don't move, finding the right voltages on the PSU connector will be easy 😀

yesterday afternoon it worked and I did a thousand tests between trying different disks, boot floppies etc. Last night instead doing other tests I don't know what happened but I can no longer show him any hard disk, and he doesn't even record the presence of the floppy in the bios, I noticed that the bios is really difficult to use, for example it doesn't record the fact that you have installed a floppy drive until you boot from floppy properly. I think the computer works well but it has a really difficult bios and I have little time. I will try again today and then decide whether to keep it or not, I have too many retrocomputers and I can't dedicate myself to just one.

HI Aleessandro, Sorry to hear that! Is there a coin-cell battery in the motherboard, maybe in needs a new CMOS battery. I'm trying to remember the BIOS on my old IBM PS/1 ValuePoint 486, I recall like you're saying it had autodetect stuff. Perhaps a new battery and a photo of the BIOS screen here would be helpful? Especially if it needs a new battery and keeps losing settings (does the clock keep time?), you might have to (re) enter the HDD's parameters manually.

Let us know how it goes Allessandro - sure hope you don't have to ditch this one in the bin after all this! Cheers! Bellarmine