I'd strongly advise against replacing the AT PSU for a good reason: the new ATX PSU might not give the -5V voltage... which is required by most motherboards from the AT era (iirc) and some cards... Unless your AT-to-ATX PSU has a regulator to create a dummy -5V voltage...

The switch is a bog-standard double-contact switch, one of the contacts being where the current goes (in that case the green wire) and the other where the current goes out of the switch.

Connect the green wire where the brown wire used to be and the white wire (the one from the ATX adapter!!) goes above the green wire of the adapter. you can leave the two other contacts unused or use them to toggle something else.

Vynix wrote:

I'd strongly advise against replacing the AT PSU for a good reason: the new ATX PSU might not give the -5V voltage... which is required by most motherboards from the AT era (iirc) and some cards... Unless your AT-to-ATX PSU has a regulator to create a dummy -5V voltage...

The -5V line is only required by a few older ISA sound cards, the motherboard itself will work perfectly without it. Maybe the OP can tell us what sound card he is going to use so we can determine if the lack of -5V will be an issue.

Qbcd wrote:

The problem is that the power switch has 4 wires coming off of it (plus a ground wire coming off the AT PSU end) and the adapter only has one brown and one green wire. So how do I connect these? I assume brown wire goes to where the brown wire from the AT PSU used to be. But what about the blue, black, and white ones? And what about the green wire from the adapter?

Just connect the two adapter wires to either the brown-black or blue-white terminal pairs of the connector.

With the AT unit connected, flipping the switch to the ON position connects the brown wire to the black wire and the blue wire to the white. The brown and blue are basically the live and neutral from your mains, while the black and white wires are connected to the power supply PCB itself. So, when connecting brown to black and blue to white you're feeding the mains voltage into the PSU, enabling it to turn on.

When using the adapter, you need to connect the green and black cables for the PSU to turn on since it's ATX, so you can use either pair of terminals as long as they get connected when flipping the switch.

TheMobRules wrote:

The -5V line is only required by a few older ISA sound cards, the motherboard itself will work perfectly without it. Maybe the O […]
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Vynix wrote:

I'd strongly advise against replacing the AT PSU for a good reason: the new ATX PSU might not give the -5V voltage... which is required by most motherboards from the AT era (iirc) and some cards... Unless your AT-to-ATX PSU has a regulator to create a dummy -5V voltage...

The -5V line is only required by a few older ISA sound cards, the motherboard itself will work perfectly without it. Maybe the OP can tell us what sound card he is going to use so we can determine if the lack of -5V will be an issue.

Qbcd wrote:

The problem is that the power switch has 4 wires coming off of it (plus a ground wire coming off the AT PSU end) and the adapter only has one brown and one green wire. So how do I connect these? I assume brown wire goes to where the brown wire from the AT PSU used to be. But what about the blue, black, and white ones? And what about the green wire from the adapter?

Just connect the two adapter wires to either the brown-black or blue-white terminal pairs of the connector.

With the AT unit connected, flipping the switch to the ON position connects the brown wire to the black wire and the blue wire to the white. The brown and blue are basically the live and neutral from your mains, while the black and white wires are connected to the power supply PCB itself. So, when connecting brown to black and blue to white you're feeding the mains voltage into the PSU, enabling it to turn on.

When using the adapter, you need to connect the green and black cables for the PSU to turn on since it's ATX, so you can use either pair of terminals as long as they get connected when flipping the switch.

Thank you very much for the input. I will be using a newer PCI sound card, most likely a Sound Blaster Live!, so the -5V will not be an issue.

Okay, this is something I am not very knowledgeable about, so I just want to confirm this in the simplest terms possible -- I should connect the black and green wires coming from the adapter to either the brown and black or blue and white terminal pairs, correct? And does it matter which I connect to which? That is, for instance, green -> white and black -> blue is the same as green -> blue and black -> white, correct? I mean, I assume it doesn't matter since it's just about shorting the two, but I still wanted to confirm. Thanks very much!

Qbcd wrote:

Okay, this is something I am not very knowledgeable about, so I just want to confirm this in the simplest terms possible -- I should connect the black and green wires coming from the adapter to either the brown and black or blue and white terminal pairs, correct? And does it matter which I connect to which? That is, for instance, green -> white and black -> blue is the same as green -> blue and black -> white, correct? I mean, I assume it doesn't matter since it's just about shorting the two, but I still wanted to confirm. Thanks very much!

Correct, as long as there is continuity it doesn't matter. Also, there is no danger at all if you mess up when connecting the ATX-AT adapter to the switch, unlike the case of an actual AT PSU if you cross the mains wires.

TheMobRules wrote:

Qbcd wrote:

Okay, this is something I am not very knowledgeable about, so I just want to confirm this in the simplest terms possible -- I should connect the black and green wires coming from the adapter to either the brown and black or blue and white terminal pairs, correct? And does it matter which I connect to which? That is, for instance, green -> white and black -> blue is the same as green -> blue and black -> white, correct? I mean, I assume it doesn't matter since it's just about shorting the two, but I still wanted to confirm. Thanks very much!

Correct, as long as there is continuity it doesn't matter. Also, there is no danger at all if you mess up when connecting the ATX-AT adapter to the switch, unlike the case of an actual AT PSU if you cross the mains wires.

Great, thank you so much! I love this forum already. I'll post pictures of the completed build. 😀

I want to see more of this case. It is ridiculous in all the right ways.