Replacing an IBM XT power supply unit with an ATX one

[Eudimorphodon]

FWIW, if you're happy gutting your original PSU you could probably get away with using one of these incredibly cheap "ATX to bench power supply converters" they sell on Amazon and eBay. It'll give you some fuses and a set of terminals you can connect the existing wires to and will *probably* fit in the 5155's power supply case.

The one thing these are missing is the -5v power regulator; take this with a grain of salt, but I'm *reasonably* sure that the lack of -5v isn't going to matter in the vast majority of PC/XT-class applications? I think the only thing that used it on the 5150 is the 4116 DRAM on the original 16-64K motherboard. It's *possible* there might be some expansion card that uses it, but I'd be kind of surprised. (Some clones omit -5v entirely from the PSU and ISA connection; the Tandy 1000, for instance.) If there's something else on the IBM motherboard that wants -5v by all means correct me.

That said, I second (third, whatever) the comment that maybe the better idea to do first is finding a quieter fan, perhaps a thermostatically controlled variable speed one. I guess I've never owned a 5155, but if it's like a 5150/60 the fan in the power supply is the only thing in the whole case providing any airflow, so *eliminating* it might be a bad idea. I mean, even if you've gone solid state for storage the 5155 does have a CRT monitor in it that ouputs a few BTUs.

P.S. since I gave a multimeter recommendation, what I wrote is OK for picoPSU, for true AT power supply, provide some load on 12V rail first or else it might damage itself.

Some ATX supplies also aren't completely happy unless they have some load on them; the small flexATX power supply I use for my bench/breadboard stuff falls into that category; it won't run its 3.3v regulator reliably unless it has some minimum load on the other legs. Solution: Xmas lights: (The old-fashioned incandescent ones, wired into multiple series substrings and soldered to the back of the breakout board.)

 

[cjs]

Solution: Xmas lights: (The old-fashioned incandescent ones, wired into multiple series substrings and soldered to the back of the breakout board.)

Exactly. Because using modern RGB LEDs to make your machine look cool wouldn't be period correct.

 

[Eudimorphodon]

Exactly. Because using modern RGB LEDs to make your machine look cool wouldn't be period correct.

Well, that, and you'd need *much* longer strings to pull the necessary amps. To pull as much as this array of xmas bulbs I think you'd be pretty close to something like an LED motorcycle headlight.

 

[Zare]

The one thing these are missing is the -5v power regulator; take this with a grain of salt, but I'm *reasonably* sure that the lack of -5v isn't going to matter in the vast majority of PC/XT-class applications? I think the only thing that used it on the 5150 is the 4116 DRAM on the original 16-64K motherboard. It's *possible* there might be some expansion card that uses it, but I'd be kind of surprised. (Some clones omit -5v entirely from the PSU and ISA connection; the Tandy 1000, for instance.) If there's something else on the IBM motherboard that wants -5v by all means correct me.

Yeah Olivetti M19 doesn't have -5V either, so I managed to make a trivial replacement PSU with pico.

It pays off to check whether OP's IBM machine needs -5, if it doesn't, then he needs only a pico and this

https://www.amazon.com/20-Pin-Power-Supply-Adapter-Cable/dp/B01EDGS0KI

 

[Eudimorphodon]

It pays off to check whether OP's IBM machine needs -5, if it doesn't, then he needs only a pico and this
https://www.amazon.com/20-Pin-Power-Supply-Adapter-Cable/dp/B01EDGS0KI

Need to graft some drive power supply plugs onto that but, yeah, that'd do it.

 

[boggit]

in the worst case you could just solder the wires in place of the on-board switch. (That extra soldering should be no issue since you'll have to do a little bit of soldering/desoldering to use the big red switch itself.)

Actually that would be an issue. I didn't know that I had to do any soldering/desoldering to use the red switch itself, which is why I asked about that in the first post of the thread (3b). I do not have access to any soldering equipment, no experience with soldering other than what I learned in school 25-30 years ago and then quickly forgot, and I have no space in my home (a two room apartment shared with my significant other) in which to solder stuff.

This is a clean switch

https://m.media-amazon.com/images/I/41F2Ac5Pk5L._AC_UF1000,1000_QL80_.jpg

You just slide in power connectors to those rails and that's it.

I see. I'll open up my 5160 PSU then and check to see if the power switch there seems to be one similar to the one you linked to, in which I can slide in the power connectors to the rails without having to do any soldering.

Yeah it does take up an ISA slot, but I wanted to keep the installation completely non-destructive for people who don't want to modify their machines.

Sounds reasonable! It might be the most convenient solution for me to pursue, all things considered.

You can indeed hook up the Big Red Switch to it! I wrote about how to do that in the second half of the guide.

Excellent! I'll take a closer look at that guide.

FWIW, if you're happy gutting your original PSU you could probably get away with using one of these incredibly cheap "ATX to bench power supply converters" they sell on Amazon and eBay. It'll give you some fuses and a set of terminals you can connect the existing wires to and will *probably* fit in the 5155's power supply case.

Interesting! Will doing so be possible without having to do any soldering/desoldering?

Also, it's a 5160 that I have, not a 5155.

The one thing these are missing is the -5v power regulator; take this with a grain of salt, but I'm *reasonably* sure that the lack of -5v isn't going to matter in the vast majority of PC/XT-class applications? I think the only thing that used it on the 5150 is the 4116 DRAM on the original 16-64K motherboard. It's *possible* there might be some expansion card that uses it, but I'd be kind of surprised. (Some clones omit -5v entirely from the PSU and ISA connection; the Tandy 1000, for instance.) If there's something else on the IBM motherboard that wants -5v by all means correct me.

I actually have no idea whether -5v capability is something that I will need or not. I am, again, very new to retro hardware. Others are more than welcome to provide their thoughts here, regarding things that I might want to add to the computer that uses -5v.

That said, I second (third, whatever) the comment that maybe the better idea to do first is finding a quieter fan, perhaps a thermostatically controlled variable speed one. I guess I've never owned a 5155, but if it's like a 5150/60 the fan in the power supply is the only thing in the whole case providing any airflow, so *eliminating* it might be a bad idea. I mean, even if you've gone solid state for storage the 5155 does have a CRT monitor in it that ouputs a few BTUs.

Again, I'm using a 5160, not a 5155, if that has any implications for your advice here.

Yeah Olivetti M19 doesn't have -5V either, so I managed to make a trivial replacement PSU with pico.

It pays off to check whether OP's IBM machine needs -5, if it doesn't, then he needs only a pico and this

https://www.amazon.com/20-Pin-Power-Supply-Adapter-Cable/dp/B01EDGS0KI

Good to know! Again, I'm not sure how important -5v is for me, as I'm not experienced enough with these machines to know what I would be missing out on if I gave up on -5v capabilities. Again, more knowledgable users are more than welcome to voice their thoughts here regarding that matter.

For what it's worth, I'm leaning towards swapping out the original 5160 motherboard and replacing it with a baby AT 286 board, if that has any implications to the sort of voltage variations that will be necessary for the PSU to output.

 

[boggit]

On the topic of voltages, I have a question regarding this information in the IBM 5155/5160 Technical Reference (6280089/March '86):



What does "key" mean? Is that the output for providing power from P8 to the keyboard? If so, what's the voltage of that? +5 Vdc?

 

[FishFinger]

"Key" means the plastic tabs on the P8 and P9 connectors to make sure you don't plug them in 180 degrees out.

 

[boggit]

"Key" means the plastic tabs on the P8 and P9 connectors to make sure you don't plug them in 180 degrees out.

Are you sure?

Both P8 and P9 have those plastic tabs, and only P8 is listed as having such a pin in the image I included above.

 

[cjs]

"Key" when applied to a particular pin position isn't typically tabs on the side of the connector, but a filled-in hole matched with a missing pin. If you try to plug it in backwards, the filled-in hole will be blocked by a pin and you will not be able to push the connector down on the pins.

In this particular case, the connectors do not need to be keyed for direction (the tabs on the side already do that), but they do need to be keyed for connection to the correct headers, since they are both six-pin connectors. The key on P8 prevents it from being plugged into P9. This prevents you from accidentally applying ±12 V to the ±5 V power rails, which would damage parts on those ±5 V power rails.

 

[Hugo Holden]

The ATX4VC and a PicoPSU are pretty much exactly that:

There is no way in a million years you will get the performance, longevity and reliability of the original supply on the left photo, from the tiny modern supply on the right.

Step back and have a look what is there on the original supply. Substantial sized inductors/transformers. Physically large and resilient and long lasting components. Also on the vertical boards is an array of components dedicated to monitoring the supply output voltages, not just one of them, but all of them with tight under & over-voltage windows, and the current too. Also it is a dual line voltage supply. There isn't a chance in Hell that this original SMPS will develop a failure mode that will damage the IC's on your computer mobo, the protection systems are too sophisticated. People see these older SMPS's and somehow think there is something wrong with them, just because they have stopped working and look dusty, and are blind to the design excellence. And if you don't have the schematic it is dead easy to copy it out to aid diagnosis and repairs.

There is absolutely nothing wrong with the original supply and in fact, it is a masterpiece of vintage electronic switch-mode power supply engineering. If I was offered the old one not working and the new one working, I'd take the old one any day. Sure, it might need to be cleaned up and repaired, but I rate these early supplies as very easy to repair, much more so than new ones. I see the one on the right as the pile of junk, and the old one on the left the good one, but the little boy couldn't see the Emperor's new clothes either. If I bought a vintage computer and found the PSU had been gutted like that, I'd be horrified and beyond dissappointed.

 

[Zare]

There is no way in a million years you will get the performance, longevity and reliability of the original supply on the left photo, from the tiny modern supply on the right.

Just by looking at the pictures it should be clear enough how much passive electronics original has and what format they come in, such as caps and inductors.
The original PSU has way higher mean time between fails than pico but pico is new.

I will also test soon how pico behaves when totally in-line, close to a motherboard, and not integrated in a PSU box. It could have a substantial electric field sum, since it's so small and powerful, and it could pose interference in audio hardware. We'll see.

Good to know! Again, I'm not sure how important -5v is for me, as I'm not experienced enough with these machines to know what I would be missing out on if I gave up on -5v capabilities. Again, more knowledgable users are more than welcome to voice their thoughts here regarding that matter.

I believe boards are documented, e.g. certain memory chips use -5V if you don't have them board will work.
You can also check whether there's resistance between -5V input on the board and ground. Or see continuity between -5V input and -5V contact on the ISA slot.

https://upload.wikimedia.org/wikipedia/commons/thumb/d/da/286_AT_motherboard.jpg/2560px-286_AT_motherboard.jpg

This is the 'normal' POV, ISA connections are oriented left-right. First pin on lower side is ground, fifth pin on lower side is -5V.
But easier is to just check online whether the board needs -5V and check all your ISA cards whether they route that fifth lower pin in.

https://ae01.alicdn.com/kf/Ub7bbd43a1460433987b0b5a9e02dd300H.jpg

It's quite uncommon to have gfx cards use -5V, here you can see 1st,2nd,3rd lower pins routed in, (ground, reset pin, +5V) but not fifth.

If you don't have -5V cards and you don't have -5V memory 99.9% chance computer doesn't need it.

 

[ayandon]

In my country, there is a Brand that sells ATX SMPS with -5v channel included.

 

[boggit]

"Key" when applied to a particular pin position isn't typically tabs on the side of the connector, but a filled-in hole matched with a missing pin. If you try to plug it in backwards, the filled-in hole will be blocked by a pin and you will not be able to push the connector down on the pins.

In this particular case, the connectors do not need to be keyed for direction (the tabs on the side already do that), but they do need to be keyed for connection to the correct headers, since they are both six-pin connectors. The key on P8 prevents it from being plugged into P9. This prevents you from accidentally applying ±12 V to the ±5 V power rails, which would damage parts on those ±5 V power rails.

I see. So if that is the case, does that mean that my PSU wouldn't work with, for example, this 286 motherboard that I'm considering purchasing?

In the manual of that motherboard, the following is stated. It needs +5 Vdc on Pin 2, whereas Pin 2 on the P8 on my PSU is "key"?

 

[Eudimorphodon]

Again, I'm using a 5160, not a 5155, if that has any implications for your advice here.

The same warning about heat buildup if you omit a fan applies, although I suppose it's probably going to be a little less of a problem without the CRT in there. I mean, I guess I would probably wager that if you had a *very* efficient pico power supply in there and went entirely to solid-state storage you'd probably get away with it, but it will get warm in there. Although it looks like now you're talking about putting a 286 motherboard in there, and that's definitely moving the goalposts; between the more powerful CPU clocked higher and the additional RAM it's going to run *even warmer*.

(* I would also warn, based on experience, that unless you're willing to take a hacksaw to the guts of your machine you're going to have to be a little careful shopping around for a baby AT motherboard that fits. I mean, there are plenty that do, upgrading these machines was a thing people did back in the day and is essentially what IBM did from the factory with the 5162, but a lot of just slightly more modern motherboards that include niceties like SIMM socked RAM have interference issues with the left drive bay.)

It's kind of obvious that if you look at the PC/XT case that convection cooling isn't really a thing; the back panel is solid, as is the U-shaped cover. All you have is the slots on the front cover, which are positioned fairly low, and the hole that goes into the power supply enclosure. It's my recollection that the fan inside the PSU blows upward; this sucks air through the back of the machine (which is why the insides of these old PSUs are always so incredibly dirty) and exhausts air warmed by the PSU into that space behind the drive bays, and from there the pressure would drive a meager amount of flow through those ventilation slots on the front. (It almost seems like it might work better if you sucked with the fan and blew out the back? Maybe I did get it backwards? I'm too lazy to go open the Kaypro XT clone I have in the garage to check my bad memory on this point.) Aaaanyway, the point is, if you don't have a fan at all you're turning your machine into an Eazy Bake oven; lousy airflow is better than none at all.

 

[Eudimorphodon]

In my country, there is a Brand that sells ATX SMPS with -5v channel included.

The Flex-ATX supply I used for my bench PSU also has a -5v channel. They're not uncommon on old ATX PSUs from the 90's and early aughts, although I think it may have been an *optional* component of the ATX 1.0 standard. It was deprecated with ATX 2.0, it's certainly interesting if anyone makes supplies with the 24 pin connector that include it.

 

[cjs]

I see. So if that is the case, does that mean that my PSU wouldn't work with, for example, this 286 motherboard that I'm considering purchasing?

Having done some further research, that "key" pin does not appear actually to key the connector. So probably you can physically plug it in to the AT motherboard. But there are two issues with that. The first is that it's not clear to me what the key pin is connected to inside the PSU: where a PC or XT motherboard leaves it unconnected, an AT motherboard will put +5V on that pin. But probably the bigger issue is that your original PC PSU is only a 65 W model, far short of even the XT's 135 W much less the AT's 190 W. You'll want to do some careful power draw calculations before trying that.

But you're starting to propose such drastic changes to this machine that it seems hardly an original IBM PC any more. Might it not make more sense, if you want a '286 machine, to buy or build a separate '286 machine and keep the original PC intact?

 

[boggit]

Having done some further research, that "key" pin does not appear actually to key the connector. So probably you can physically plug it in to the AT motherboard. But there are two issues with that. The first is that it's not clear to me what the key pin is connected to inside the PSU: where a PC or XT motherboard leaves it unconnected, an AT motherboard will put +5V on that pin.

Interesting! and relevant answer from Justme in that Stackexchange thread: "XT supply could power an AT motherboard, assuming motherboards do not exceed ratings of the supplies"

But probably the bigger issue is that your original PC PSU is only a 65 W model, far short of even the XT's 135 W much less the AT's 190 W. You'll want to do some careful power draw calculations before trying that.

I have an XT, so 135 W, not 65 W.

But you're starting to propose such drastic changes to this machine that it seems hardly an original IBM PC any more. Might it not make more sense, if you want a '286 machine, to buy or build a separate '286 machine and keep the original PC intact?

I hear you, but I just really like the XT case and really dislike most/all AT case(s).