I can't tell you how to test the PSU, but you have connected it the right way. Do you have a multimeter? That's the only way I can think to test it.

got a cheap AT PSU for my old mobos

DON'T.

Yep @debs3759, I do have a multimeter!

@The Serpent Rider, maybe a little more useful answer would be appreciated.

Minebea made good quality power supplies, but I think they abandoned that line of business in the early 00's. Yours probably comes from an OEM system.

Did the seller know if it was working or did you buy it untested? You can pop the cover and take some pics in case there are any components with obvious signs of damage (make sure it's unplugged first!).

@TheMobRules the seller tested the PSU and says it works. I will open the unit and take a look (and send a pic) to see if everything looks fine.

cprieto wrote on 2020-11-30, 20:20:

@TheMobRules the seller tested the PSU and says it works. I will open the unit and take a look (and send a pic) to see if everything looks fine.

Don't open unless you know what you are doing.
Test voltages under load first.

Properly testing a PSU, whether it be AT or ATX, is moderately involved and preferably involves some sort of load cell (for all supply voltages), a decently accurate multimeter for measuring steady state voltage and/or current, and an oscilloscope for measuring voltage stability, transient response, and ripple. Your initial test should be with the multimeter to ensure the voltage outputs are in spec. Partial full load testing could potentially be performed using a junk motherboard and CPU as a sacrificial goat.

The Serpent Rider wrote on 2020-11-30, 19:03:

got a cheap AT PSU for my old mobos

DON'T.

That's irrational. I have a half dozen or more AT supplies still in service with no issues. As long as you look them over and check the rails there's nothing more to worry about than any other SMPS.

Agree with Max. For a quick test: find some big old hard drives you do not care about (I keep a few with tons bad sectors but still spin up just for this) and hook them up to the AT PSU. Each old drive will draw about 1amp and most old PSU need at least a 2amp load on the 5v and 12v lines, the -12v and -5v do not need a load. Use your Voltmeter and measure all the voltages. If they are within 10% then you know the Vregs are working proper. First I would open it and look at the Caps, if none are leaking/bulging then test it as above.

Disagree with both. AT power supplies are a bomb waiting to go off untill you service them. Might not be today, might not be tommorow, but it will be sometime, and I don't care how good they where made back in the day.

Grab a good multimeter, spare hard drive or something to sip a bit of power and give it a good check, if you wanna make sure it'll survive long term, give it a recap too.

Any old hardware is a potential bomb. It's part of the risk of having and using old hardware.

You can also lift the skirt of your PSU and share some pictures. The general construction of the supply will give some indication if it includes over voltage and over current protection. With detailed photos, we can probably find the supervisory IC.

SodaSuccubus wrote on 2020-12-01, 01:13:

Disagree with both. AT power supplies are a bomb waiting to go off untill you service them. Might not be today, might not be tommorow, but it will be sometime, and I don't care how good they where made back in the day.

Grab a good multimeter, spare hard drive or something to sip a bit of power and give it a good check, if you wanna make sure it'll survive long term, give it a recap too.

🤣 not sure which two you disagreed with but did say this:

Horun wrote:

For a quick test: find some big old hard drives you do not care about (I keep a few with tons bad sectors but still spin up just for this) and hook them up to the AT PSU. Each old drive will draw about 1amp and most old PSU need at least a 2amp load on the 5v and 12v lines, the -12v and -5v do not need a load. Use your Voltmeter and measure all the voltages. If they are within 10% then you know the Vregs are working proper. First I would open it and look at the Caps, if none are leaking/bulging then test it as above.

And in reality you want less than 5% diff off the +5 and +12v mains under load. I just blurted out 10% cause it was on my mind 😀

I think I will return the PSU, I noticed that when I connect the PSU to the power and press the power switch, the fan doesn't spin... And if my memory doesn't fail (it has been ages since I had to deal with old hardware) that is not a good sign 🙁

I share a picture of the inner of the PSU just in case, meanwhile I will fill a return ticket for the unit.

maxtherabbit wrote:

I have a half dozen or more AT supplies still in service with no issues

Good for you. But I don't see good reason to specifically buy old junk AT PSU, when new modern PSUs can be easily used via adapter, which is safer.

cprieto wrote on 2020-12-01, 16:56:

I think I will return the PSU, I noticed that when I connect the PSU to the power and press the power switch, the fan doesn't spin... And if my memory doesn't fail (it has been ages since I had to deal with old hardware) that is not a good sign 🙁

I share a picture of the inner of the PSU just in case, meanwhile I will fill a return ticket for the unit.

That's actually a really well built unit. I see full filtering on the input and output, a decent sized switching transformer, and the one cap I can positively identify is a Chemi Con in the output filter. The only thing I see which could use improvement is the heat sink which is a little on the small side. However, the supply is only rated for 150W, so it won't need to handle too much.

Can you get a better shot of the chip in the top right corner between the primary caps and the rectifier?

We're dealing with decades old hardware now, and it's an increasingly reasonable expectation that hardware will need board level servicing, lubrication, alignment .etc. Stuff that people into IBM XT hardware, CPM-era stuff .etc have long been doing. People into 8-bit home computers often have repair services, especially in Europe, but I haven't seen that around for PCs, Apple IIs or other more serious computers. I'd say everyone needs to learn how to solder, but that's elitist; what we really need are more repair options, and maybe some people in a low wage country rebuilding parts and selling them at a premium to better heeled people in the West and Japan.

For what it's worth, my 486s power supply did die shortly after putting it into use. It was an OEM system with a top quality Astec supply too. I hade to switch all the capacitors out, which fortunately revived it -- I don't have the skills to do more than switch them out and test rail voltages while the system is running. Which I figure should function as a basic real-world load test.

If modern suitable power supplies were made, I'd feel it'd be a reasonable option, like the new power supplies for Apple IIs, C64s .etc are; but Startech made the last and they discontinued it. Anything you're likely to find that's classic matte grey steel and ATX 1.3 or earlier was made fifteen to twenty years ago and therefore is nearly as old as a late-era AT supply.

The Serpent Rider wrote on 2020-12-01, 17:55:

maxtherabbit wrote:

I have a half dozen or more AT supplies still in service with no issues

Good for you. But I don't see good reason to specifically buy old junk AT PSU, when new modern PSUs can be easily used via adapter, which is safer.

This whole "safety" thing is what I object to. The chances of ANY AT/ATX power supply failing in such a way that it destroys whatever hardware it is powering is EXCEEDINGLY rare.

maxtherabbit wrote on 2020-12-01, 19:53:

The Serpent Rider wrote on 2020-12-01, 17:55:

maxtherabbit wrote:

I have a half dozen or more AT supplies still in service with no issues

Good for you. But I don't see good reason to specifically buy old junk AT PSU, when new modern PSUs can be easily used via adapter, which is safer.

This whole "safety" thing is what I object to. The chances of ANY AT/ATX power supply failing in such a way that it destroys whatever hardware it is powering is EXCEEDINGLY rare.

Depends. I don't understand the electrical specifics, but certain brands of late 90s and early 2000s power supplies built up a reputation for taking computer components with them when they failed. In particular, I know Deer/Allied units developed a reputation for this, as did the otherwise decent Bestec power supply units found in many OEM systems. I believe the Bestec units had something wrong with a two transistor 5VSB rail that would take out motherboards as the capacitors aged, and other units of the era used a similar design. The other big issue is that some cheap units had grossly exaggerated power ratings, and little or no overcurrent protection to keep someone from drawing beyond what was electrically sane for the unit -- which may have been as little as 100-150 watts for a very poor unit. Power hungry new Pentium 4/Athlon CPUs and 3D accelerators actually drew that much or more under load, unlike previous era hardware, leading to Intel and AMD power supply approval programs and people with stories of burning smells, dead cards, firework flashes, in rare cases even an actual power supply fire.

Most of this would not concern an AT power supply, where the draw is generally low and the 5VSB nonexistent. It also hopefully has little to do with junk made in the last decade -- even the $20 Ebay specials with blue lights have a working protection chip, right?

radiounix wrote on 2020-12-01, 20:43:

I believe the Bestec units had something wrong with a two transistor 5VSB rail that would take out motherboards as the capacitors aged, and other units of the era used a similar design.

Two transistor standby circuits have a critical capacitor on the output. If it steps too far away from tolerance, standby voltage can go wildly out of regulation thereby toasting the motherboard. Those Bestec supplies were generally complete, but they used a failure prone capacitor in that application.