You say you have a multimeter. Connect the PSU to mains and find the two of the four wires that have main voltage. These are switched to the other two wires by the switch (order doesn't matter, since the mains wall plug is not keyed in your country anyway). How the switch works is already known.

Since you have a multimeter, you can first do a continuity test between the connectors and the plug on the back of the power supply. That way you can identify which of those are the live (brown) and neutral (blue) coming from the mains.

The other two wires are the ones that connect to the PCB of the power supply, with the convention being black = live and white = neutral. So you can open the PSU case and see where each of those goes.

Once you have identified those, you can connect them to the switch so that when you turn it on, "brown" is connected to "black" and "blue" is connected to "white" (i.e. live to live and neutral to neutral).

I am still bit confused. I tested continuity. So the one I thought is blue doesn't connect to any of the 3 pins at the back. The one that I thought is white connects to right pin and one of the other 2 wires connects to left pin.
So do I make the switch short between those blue and white and also short between the 2 other wires?

Also is it brown or black that is connected to the left pin in the back of the psu?

Baoran wrote on 2019-12-29, 18:55:

I am still bit confused. I tested continuity. So the one I thought is blue doesn't connect to any of the 3 pins at the back. The one that I thought is white connects to right pin and one of the other 2 wires connects to left pin.
So do I make the switch short between those blue and white and also short between the 2 other wires?

Also is it brown or black that is connected to the left pin in the back of the psu?

The colors are just a convention, just use this picture as a guide:

So, by doing some continuity tests, you can identify which of the 4 wires connects to the mains hot and neutral. Let's call these "brown" and "blue" respectively, the remaining two will be the ones to carry power back to the PSU when you press the switch (the convention is black/hot and white/neutral). So you must connect them to the switch so that when it is turned on, brown connects to black and blue to white. It should also work if you connect brown to white and blue to black since it's AC, but in some places like the US they care about hot/neutral when wiring the outlets.

In any case, identifying the wires that are connected to mains is easy, so you must take care that you don't attach them to the switch so that there's a short between them when you press the button.

Thank you for the help. The reason I asked about the colors was that I could mark the colors on the wires that I would not have this kind of trouble in the future with this psu.

this is the typical layout to switch in pics, rare that yours are all brown but have also seen all black wires too. if there is also a green wire it attaches to case or screw at the switch

Yeah. it is like there is just small ring of color below the clear plastic that is suppose to indicate which is which and that clear plastic has yellowed which makes it impossible to see which one is suppose to be brown and which one is suppose to be black.

Only multimeter action will help here and you only have to determine which of the two pins are AC input ones, connect up one probe to one of the pins on input and find the wire connecting to it on the switch end, mark it and find the other. Those two connect to the unmarked one, and it doesn't matter which way. If you have AC output on the PSU you can use it for determining the polarity.

OT, but I recently acquired an AT PSU with only two power connectors instead of four. Was this common? Is it more or less safe than the usual 4-connector type?

The only AT supplies I'm aware of like this are semi proprietary. A have one from Compaq. It supports a form of remote power toggle that predates ATX but provides the same safety benefit of line voltage not escaping the PSU enclosure.

It's an Astec SA145. The main advantage I assume is that it allows for a narrower switch allowing installation in smaller cases.

I have a similar problem, so I'd rather post here rather than making a new thread. I'm building my first retro PC and my AT case looks pretty much exactly like this one:
http://reeseriverson.com/rriverson/pics/IMG_2588-1500px.jpg

Problem is, I have an ATX PSU and a GA-5AA, which does support ATX. Is there a way I can connect the power switch to the motherboard without nuking the whole thing?

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Errius wrote on 2019-12-31, 18:13:

OT, but I recently acquired an AT PSU with only two power connectors instead of four. Was this common? Is it more or less safe than the usual 4-connector type?

These are later ones when the AT standard was already old. They were cheaper to made. They are also less safe, since they may not switch the live wire, depending on whether or not the wall plugs in your country are keyed.

Vic Vos wrote on 2020-01-01, 09:58:

I have a similar problem, so I'd rather post here rather than making a new thread. I'm building my first retro PC and my AT case looks pretty much exactly like this one:
http://reeseriverson.com/rriverson/pics/IMG_2588-1500px.jpg

Problem is, I have an ATX PSU and a GA-5AA, which does support ATX. Is there a way I can connect the power switch to the motherboard without nuking the whole thing?

That motherboard will require an I/O plate, so fitting it to an older case requires surgery. I'd be inclined to look for an ATX beige box and mod it with retro bling, like a frequency display.

gdjacobs wrote on 2020-01-01, 10:48:

Vic Vos wrote on 2020-01-01, 09:58:

I have a similar problem, so I'd rather post here rather than making a new thread. I'm building my first retro PC and my AT case looks pretty much exactly like this one:
http://reeseriverson.com/rriverson/pics/IMG_2588-1500px.jpg

Problem is, I have an ATX PSU and a GA-5AA, which does support ATX. Is there a way I can connect the power switch to the motherboard without nuking the whole thing?

That motherboard will require an I/O plate, so fitting it to an older case requires surgery. I'd be inclined to look for an ATX beige box and mod it with retro bling, like a frequency display.

It fits just fine on the case, though. Will it be simpler to just replace the switch entirely and put a fitting button in there?
I forgot to mention that the case is actually Baby AT, not regular AT.

My bad. That motherboard is AT, so go nuts. If you use the ATX header, you'll need to replace the latching DPST switch with a momentary switch (can be SPST). Alternately, you can get an ATX to AT harness and reuse one side of the latching switch. Depending on your requirements and what you've got for a PSU, you may want an adapter that also generates -5V DC.

Yeah, I figured out as much. By -5V DC adapter, do you mean one of these DC-DC Buck 5V 12V

I've been doing some research and found out that modern PSU's with 20pin ATX connectors still support -5V. Any advice on that?

Vic Vos wrote on 2020-01-02, 03:43:

Yeah, I figured out as much. By -5V DC adapter, do you mean one of these? [link removed]

I've been doing some research and found out that modern PSU's with 20pin ATX connectors still support -5V. Any advice on that?

Well, they're not that modern. The -5V minor rail was made optional in the ATX12V 1.2 revision. This was released back in 2002. After this point, manufacturers could eliminate the rail at their discretion. The only new off the shelf PSU I'd consider is the ATXPower series from Startech. There is another brand or two that might have PSUs based around the older standard, but they're bottom tier manufacturers who might (1) be lying or (2) end up destroying your gear.

There used to be a guy who sold AT conversion harnesses with a regulated -5V output. He doesn't have any listed anymore, but maybe you can contact him?