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AT PSU worth it for AT computer?

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First post, by kikendo

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Sorry it's me again with questions related to my 386 restoration.
I'll make it brief. The power supply in my unit is gone and I see an AT power supply for sale on eBay but it's rather expensive.
https://www.ebay.com/itm/152431356038

Would it be OK to just use an ATX PSU? I currently am using one, with an ATX to AT adapter. The size seems about the same, it just doesn't have the extra female power plug for the monitor power.
What is your opinion on this? I tend to go for newer parts in these cases, but maybe there is an advantage to buying a brand new AT power supply that I am not seeing.

Thanks again for all your help!

Reply 1 of 24, by red_avatar

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Honestly? That price is pretty OK for a PSU designed to work with 25+ year old PCs.

From what I recall, there's a few problems with an ATX power supply:

- they expect a "turn on" signal whereas AT PSU is either on or off, the button acting like a light switch. In other words, you can't hook up an ATX to a system that has a power button which creates a continuous connection (i.e. a button you can press in and stays in)

- you need -5V which ATX doesn't supply unless you got a really old ATX PSU and that kind of defeats the point since that might break soon as well. I believe -5V is needed for the ISA bus?

Me personally, I'd recap my old AT PSU with improved components and use that instead because it's not that expensive to recap, you get to keep the original PSU and it's often going to last longer recapped than whatever alternative you might find which will likely have cheap components as well ...

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Reply 2 of 24, by cyclone3d

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red_avatar wrote on 2020-06-11, 20:21:
Honestly? That price is pretty OK for a PSU designed to work with 25+ year old PCs. […]
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Honestly? That price is pretty OK for a PSU designed to work with 25+ year old PCs.

From what I recall, there's a few problems with an ATX power supply:

- they expect a "turn on" signal whereas AT PSU is either on or off, the button acting like a light switch. In other words, you can't hook up an ATX to a system that has a power button which creates a continuous connection (i.e. a button you can press in and stays in)

- you need -5V which ATX doesn't supply unless you got a really old ATX PSU and that kind of defeats the point since that might break soon as well. I believe -5V is needed for the ISA bus?

Me personally, I'd recap my old AT PSU with improved components and use that instead because it's not that expensive to recap, you get to keep the original PSU and it's often going to last longer recapped than whatever alternative you might find which will likely have cheap components as well ...

You can use an ATX PSU. You just need an adapter and then you hook up the push button wires on the adapter to the normal AT power switch.

The only real reason to use an AT power supply is if you need the -5v signal and.
1. Don't have an ATX power supply with -5v
2. Don't have an ATX to AT adapter that adds the -5v
3. Don't want to add the -5v circuit yourself to the adapter.

or you need a proprietary power supply form factor to fit your case.

New ATX power supplies are way way way way way better in terms of efficiency, less ripple(cleaner power output which will help extend the life of components and also help them run cooler), and way better voltage regulation.

I would love to see one of those new AT power supplies put on a real tester such as JonnyGuru or HardOCP used to use for testing power supplies. My guess is that it would come nowhere close to handling the power load it claims to output and it might even have parts catch fire or explode. The temperature rating of 25C is hilariously bad as well.

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Reply 3 of 24, by kikendo

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red_avatar wrote on 2020-06-11, 20:21:

- they expect a "turn on" signal whereas AT PSU is either on or off, the button acting like a light switch. In other words, you can't hook up an ATX to a system that has a power button which creates a continuous connection (i.e. a button you can press in and stays in)

The AT-ATX adapter I have works with a "constant on" switch.

- you need -5V which ATX doesn't supply unless you got a really old ATX PSU and that kind of defeats the point since that might break soon as well. I believe -5V is needed for the ISA bus?

Hmm good point. I didn't know that. Since I am currently using no ISA devices, maybe I didn't notice.

Me personally, I'd recap my old AT PSU with improved components and use that instead because it's not that expensive to recap, you get to keep the original PSU and it's often going to last longer recapped than whatever alternative you might find which will likely have cheap components as well ...

I don't know if all that's wrong with my unit is recapping. The PSU seems to be in really bad shape and wouldn't even power up right now, and honestly, I'm not much for recapping PSUs, it's a bunch of work.

I agree ~45 is not that bad, but since ATX PSUs are cheaper, I thought I'd ask.

cyclone3d wrote on 2020-06-11, 21:09:
The only real reason to use an AT power supply is if you need the -5v signal and. 1. Don't have an ATX power supply with -5v 2. […]
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The only real reason to use an AT power supply is if you need the -5v signal and.
1. Don't have an ATX power supply with -5v
2. Don't have an ATX to AT adapter that adds the -5v
3. Don't want to add the -5v circuit yourself to the adapter.

Any idea how I can know if I need it? All I intend to put in the ISA port is a Sound Blaster or an AdLib card, but it did come with a modem/gameport card I don't really need that might require -5V. not sure.
The adapter I have looks like a simple cable, I don't think it makes up for the -5V circuit. But wouldn't it be simple to make one that turns +5V ito -5V?

New ATX power supplies are way way way way way better in terms of efficiency, less ripple(cleaner power output which will help extend the life of components and also help them run cooler), and way better voltage regulation.

Those were my thoughts, too.

Reply 4 of 24, by Windows9566

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I had one AT PSU bite the dust catastrophically, it made a bang, and smoke came out. It did that as soon as i tested it with a HDD. I opened it up and one of the capacitors blew up, I'm gonna recap it at a later date.

Reply 5 of 24, by red_avatar

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cyclone3d wrote on 2020-06-11, 21:09:
You can use an ATX PSU. You just need an adapter and then you hook up the push button wires on the adapter to the normal AT powe […]
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You can use an ATX PSU. You just need an adapter and then you hook up the push button wires on the adapter to the normal AT power switch.

The only real reason to use an AT power supply is if you need the -5v signal and.
1. Don't have an ATX power supply with -5v
2. Don't have an ATX to AT adapter that adds the -5v
3. Don't want to add the -5v circuit yourself to the adapter.

or you need a proprietary power supply form factor to fit your case.

New ATX power supplies are way way way way way better in terms of efficiency, less ripple(cleaner power output which will help extend the life of components and also help them run cooler), and way better voltage regulation.

I would love to see one of those new AT power supplies put on a real tester such as JonnyGuru or HardOCP used to use for testing power supplies. My guess is that it would come nowhere close to handling the power load it claims to output and it might even have parts catch fire or explode. The temperature rating of 25C is hilariously bad as well.

This is all perfectly true - I even asked this question a few years ago and got the same answer but finding those adapters is not that easy - I've seen them sell for €40-50 when I did find them which is rather expensive. I guess it depends on whether you want the benefit of an ATX (which you need to buy as well ...) with better efficiency and components (and often quieter fan) but requiring hacks to make it work or you want a no hassle solution and get an AT PSU and perhaps get it recapped.

And there should be people in your area that recap them - I recapped a bunch of stuff because I can't solder myself and they used quality components.

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Reply 6 of 24, by cyclone3d

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red_avatar wrote on 2020-06-11, 21:27:

This is all perfectly true - I even asked this question a few years ago and got the same answer but finding those adapters is not that easy - I've seen them sell for €40-50 when I did find them which is rather expensive. I guess it depends on whether you want the benefit of an ATX (which you need to buy as well ...) with better efficiency and components (and often quieter fan) but requiring hacks to make it work or you want a no hassle solution and get an AT PSU and perhaps get it recapped.

And there should be people in your area that recap them - I recapped a bunch of stuff because I can't solder myself and they used quality components.

The adapters are available on eBay for as low as $11.

I used to buy the ones that also had the -5v added, but sadly that seller is nowhere to be found on eBay anymore.

Here is a thread about how to know if you need -5v for an ISA card (not a lot use it) and how to add -5v if you need it:
Minus 5 Volts for ISA Slots on ATX Board

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Reply 7 of 24, by brostenen

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I say, if you can get an ATX supply, that works in combination with an adaptor. Then that is the best way to go. 386's do not swallow that much juice anyway. Just be aware, that some ATX PSU's will give some issues. I have used a couple that eighter made the AT machine unstable or did not want to power on. For some odd reason that I did not care to investigate.

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Reply 8 of 24, by 386SX

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I'm using a Enermax EG375AX-VE(W) 24P psu with a ATX to AT switch but even if old as "Pentium4-ready" has no -5v anyway but until now I've only seen problems and it may explain why, with some I/O ISA cards I don't know if they need the -5V but some doesn't boot the system at least one will.
But even if these psu are still quite old themself, I prefer their circuitry compared to old AT ones that seems really too simple and really too old. The psu I'm using have some more protection pcb's and components, even automatic fan running after shudowns..

Reply 9 of 24, by SuperSirLink

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I recently purchased an AT power supply like that off Ebay, brand new with better internal protections over buying an old used one... Paid about the same. Depends on your use, for me I was wanting to maintain period correctness.

https://www.ebay.com/itm/Replacement-Power-Su … 872.m2749.l2649

Edit: they are also nice if you plan to power the monitor off the PSU...

Reply 10 of 24, by red_avatar

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cyclone3d wrote on 2020-06-12, 06:35:
The adapters are available on eBay for as low as $11. […]
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The adapters are available on eBay for as low as $11.

I used to buy the ones that also had the -5v added, but sadly that seller is nowhere to be found on eBay anymore.

Here is a thread about how to know if you need -5v for an ISA card (not a lot use it) and how to add -5v if you need it:
Minus 5 Volts for ISA Slots on ATX Board

Well I meant actual adapters that add -5V - what you mean is just changing one plug to another. Without the -5V, ISA cards can behave unexpectedly. Sound cards may sound off or wrong for example.

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Reply 11 of 24, by kalohimal

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kikendo wrote on 2020-06-11, 21:12:

The adapter I have looks like a simple cable, I don't think it makes up for the -5V circuit. But wouldn't it be simple to make one that turns +5V ito -5V?

It's easy to drop -12V to -5V with a simple 7905 voltage regulator circuit. It's inefficient and will need a heat sink but will get the job done. Or you can use an inverting buck converter (dc-dc converter) module (search for "negative buck converter").

For me I prefer to repair PSUs myself, it's lots of fun and save them from going to landfill. But it does involve high voltages and you need to know what you're doing to avoid getting nasty shocks.

Cheers.

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Reply 12 of 24, by pan069

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kikendo wrote on 2020-06-11, 20:00:
Sorry it's me again with questions related to my 386 restoration. I'll make it brief. The power supply in my unit is gone and I […]
Show full quote

Sorry it's me again with questions related to my 386 restoration.
I'll make it brief. The power supply in my unit is gone and I see an AT power supply for sale on eBay but it's rather expensive.
https://www.ebay.com/itm/152431356038

Would it be OK to just use an ATX PSU? I currently am using one, with an ATX to AT adapter. The size seems about the same, it just doesn't have the extra female power plug for the monitor power.
What is your opinion on this? I tend to go for newer parts in these cases, but maybe there is an advantage to buying a brand new AT power supply that I am not seeing.

Thanks again for all your help!

Don't buy this PSU. I bought 2 about a year ago. They're literally shit.

Reply 13 of 24, by 386SX

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I think I was hoping too soon that the ATX to AT converter without the 5v would be ok with my system. Since yesterday I noticed strange things happening (failed installs, strange crash..) and this morning can you imagine the vga output of any video cards I tried was becoming slowly to black! Tried three different good isa cards, same things! I cannot even imagine how this would be possible but I tried a AT psu and suddendly the board booted ok as expected, even the I/O cards that were not running before now run at boot...
I hoped to use an ATX psu but still have to better study it. Meanwhile another AT psu I had mostly died as soon as I switched the power button (capacitor probably..).
So I stopped my 386 project to better decide which psu to use..

Reply 14 of 24, by 386SX

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brostenen wrote on 2020-06-12, 12:25:

I say, if you can get an ATX supply, that works in combination with an adaptor. Then that is the best way to go. 386's do not swallow that much juice anyway. Just be aware, that some ATX PSU's will give some issues. I have used a couple that eighter made the AT machine unstable or did not want to power on. For some odd reason that I did not care to investigate.

I've found today the same problem using a simple ATX to AT converter without the -5v. I've seen that there're still some ATX psu around (old ones) that had the -5V cheaper than a good AT one (if there're any good at all). Do you think I'd solve those problems using an ATX having itself the negative voltages?
Thanks

Reply 15 of 24, by cyclone3d

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Exactly what atx psu were you using in the first place (brand / model).

If the problem is with it not having -5v, then one with -5v would probably solve the problem.

I suspect it has more to do with the ATX power supply you were trying being faulty though.

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Reply 16 of 24, by 386SX

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cyclone3d wrote on 2020-06-13, 13:34:

Exactly what atx psu were you using in the first place (brand / model).

If the problem is with it not having -5v, then one with -5v would probably solve the problem.

I suspect it has more to do with the ATX power supply you were trying being faulty though.

I was using a Enermax EG375AX-WE 24P and I checked them internally are good and cleaned. Always ran ok and are quite complex inside (really heavy too!). But I begin to suspect something is going wrong on it.. voltages on another mainboard P4 I tried seems perfect (4,99v..11,90v..etc..).
I'm not gonna use a random AT psu that would do just like one I tried this morning on another mobo (a socket 7) and it did what usually happens.. bad sound of something really bad happening inside just once switched on the power button... still have to check what but certainly something has gone inside that AT one.
What about a PicoPSU 120W with a conventional ATX-AT converter? Those have the -5v line?

Reply 17 of 24, by cyclone3d

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That's an interesting idea. They don't have -5v though. I looked at the manual for 2 different models.

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Yamaha XG resource repository - updated November 27, 2018
Yamaha YMF7x4 Guide
AW744L II - YMF744 - AOpen Cobra Sound Card - Install SB-Link Header
Epstein didn't kill himself

Reply 18 of 24, by 386SX

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cyclone3d wrote on 2020-06-13, 14:06:

That's an interesting idea. They don't have -5v though. I looked at the manual for 2 different models.

I have one of the 20pin model 120W with a 60W 12v psu that I use for a mini-itx board but same problem it would be imho the -5v. And a real proof was that with the original AT psu not only the vga came back working ok, but even a I/O that just yesterday could not boot the system with this it magically works.. I suppose most instabilities I was thinking caused by the cache or whatever are caused by it...I'll try maybe another PSU, not certainly the modern one that may be even worse.. considering I've seen the voltages of some (even good ones) going up and down of +/-0,3 volts... no way..
Anyway the PicoPSU may be something to think of.. I checked the power wattage of the 386DX40 with all the cards and it asked less than 38W for the whole pc, and the PSU itself ask probably 4-5W itself.. I suppose it might do the job..

Reply 19 of 24, by 386SX

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Found what happened to an AT PSU (not the ATX ones I was talking about) this morning... it's a power thermistor 5 ohm / 3 A, SCK-053 really gone badly with sound and special effects.

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