VOGONS


First post, by 33mhz

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Hi everyone!

Ive been lurking on these forums for a few months now and its awesome to see that my enthusiasm for old PCs is shared by such a healthy online community.

Now Ive got a question:

I am the proud owner of a working IBM pc 330 450dx2 and the only thing that bothers me about it is having to use the 21 year old proprietary PSU which came with it not knowing how much longer it will hold up. I would like to replace it with a new old stock AT PSU, or an ATX PSU with an adapter.

Here is the machine:
fyQOGaPm.jpg

Here is the PSU:
3Fmvd2pm.jpg

So As you can see this is no ordinary AT PSU in that it lacks a power switch connected directly to the unit. Instead the power switch on the case is connected directly to the mobo via a 2 pin connector like on an ATX setup and the PSU is connected not just by the P8 and P9 connectors (labeled P1 and P2 on this PSU), but also by a 3 pin connector.

Here is a pic of the 3 pin connector from the PSU and the relevant section of the mobo:
RfQTFUtm.jpg

WJVQWyam.jpg
The 3 pin connector goes just to the left of the 2 pin yellow and black power switch wires.(please excuse my god awful soldering skills 😁)

So basically my question are: What options do I have to bypass using the mobo to control the on/off switch? Can I simply power the board through the P8/P9 connectors and use a separate switch? And importantly: Are P1/P2 really the same thing as P8/P9? or is this PSU wired completely differently?

On thing I noticed was this wiring key on the side of the PSU, but being a relative noob to this stuff I can't seem to make sense of it.
6Md1gcum.jpg

And just for good measure Ill add this image of the showing the mobo layout in case it helps:
wvi1JGtm.jpg

So please try to help if you can. If you have anymore questions of need more images I can provide them.

Cheers

Reply 1 of 10, by Brickpad

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You might be able to cut the three-wire connector and connect two of those wires to the ATX ground and power_on (black and green wires respectively). Not sure what the third wire does, but I am wondering if it's the power sensor.

Reply 3 of 10, by airs

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33mhz wrote:

I was thinking something along the same lines. But what do you think would happen if I just powered the board with P8/P9 and just hot wired the green/black wires or used a separate switch?

Did you ever figure this out? I have a motherboard from a 330 that I’m trying to power with a regular AT PSU 😕

Reply 4 of 10, by airs

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Figured it out!

Some IBM motherboards require an additional three pin connector that needs +5v (left, red) and ground (right, black). The middle pin does not need to be connected to anything. You can then use a regular AT PSU by shorting the two pin connector (white jumper), and then supply 5v and ground from a molex connector. This way your AT switch will still work to turn the PC on and off.

https://imgur.com/gallery/oiPbRSh

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Reply 5 of 10, by thp

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Thanks for this, I've tried this on an IBM Aptiva 486 board with an ATX power supply and an ATX-to-AT adapter and successfully got it working again.

I decided to take the 5V and GND from the AT connector but without modifying the AT connector (or a molex connector) so that the board basically acts like a self-contained AT board -- see the photos. After doing it, I realized I could just do the connections fully on the underside of the motherboard, but so it's at least a reminder that those connections are there (and I did have the jumper wires with the plugs from some other project).

Has anyone figured out if there is a function to the middle pin and/or the two shortened pins? Would it be possible to connect a switch there for e.g. the fast resume, or is this all handled elsewhere anyway?

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Reply 6 of 10, by Kozality

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Hi! I've been following this with great interest, so thanks for revisiting these! I recently came into an old IBM Aptiva myself. It's using a similar board with what looks like a 3-pin molex connector, but same wiring.

I'd like to replace the ancient PSU with a new ATX one. But I'm a little perplexed as to how I should handle the switch. I'm using an ATX to AT converter, but I'm unsure with what to do with the green and black AT power cables. I have no AT switch, and the Aptiva's power button does nothing. I've confirmed that I can get the system to start by shorting the AT power switch cables, but the unit powers down the moment I break the connection. Any suggestions on how I should approach this?

Thanks!

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Reply 7 of 10, by quicknick

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I'm 99% sure that on the 3-wire molex black is GND, white is PS_ON# (green on ATX) and red is +5Vsb (purple on ATX).
For the 1% uncertainty you should use a multimeter: see if +5 volts are present on the red wire with the system connected to mains, check if PSU powers on when shorting white to black (all this pertains to the 3-wire molex).
When you're at 100% you just need to connect three ATX wires to the molex. You already have two of them hanging from the ATX-AT adapter, the effort will be to find a way to connect the purple one.

Reply 8 of 10, by Kozality

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quicknick wrote on 2020-03-17, 23:45:

I'm 99% sure that on the 3-wire molex black is GND, white is PS_ON# (green on ATX) and red is +5Vsb (purple on ATX).
For the 1% uncertainty you should use a multimeter: see if +5 volts are present on the red wire with the system connected to mains, check if PSU powers on when shorting white to black (all this pertains to the 3-wire molex).
When you're at 100% you just need to connect three ATX wires to the molex. You already have two of them hanging from the ATX-AT adapter, the effort will be to find a way to connect the purple one.

Thanks! I'll give that a shot. I've ordered some molex crimpers; I'm going to see about attaching some new ends and then modify the wiring harness. I think I can probably get the purple in since it has no corresponding pin on the ATX to AT adapter, so I'll run a new wire off of it to the three pin molex. Updates to come.

Reply 9 of 10, by Kozality

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That did it! I wired it up (It's quite messy) but after having reversed my 5v and ground lines, it powered up fine. I'm hoping to make it a bit neater in the future by getting the right crimper and connectors (the one I got didn't work) but for now it'll let me test.

Here's my pictures for future reference.

Thanks!

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Reply 10 of 10, by AlessandroB

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if I don't connect the white cable to anything ... would it work? I have a similar problem with a Pentium60. I connected an AT power supply as explained by you but it doesn't always start, sometimes yes and sometimes no. I don't understand if it could be the power supply that is old or the computer that is broken.