shadmere wrote on 2020-08-04, 02:20:

Hello! […]
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Hello!

I've recently starting trying to build a 486, because it's the first computer I had growing up. One thing I wanted was to find a new PSU, because 30 year old power supplies seem. . . risky.

I ended up getting one of these: Sparkle SPI-300G

Well, today I finally had some time to try and piece things together, and . . . nothing happens.

The power supply fan comes on, the keyboard lights blink once, but nothing else. No beep, no activation of the monitor, nothing.

Curious, I hooked up the other AT power supply I had. The one that came with the case I found. it is . . . not something I would use. It looks pretty bad.

But using that, I get a POST. I get a beep. I can enter BIOS. (Nothing else is hooked up, so this is as far as I'm testing right now.)

Is the Sparkle power supply just DOA? Or am I missing something about this PSU versus the type I actually need, or something? I don't want to return it for a replacement if there's something I'm misunderstanding about the product itself. (I'm aware that I could use an ATX power supply and an adapter, but since this seemed to be what I needed, I thought I was in luck.)

This forum has shown up in probably 80% of every google search I've made trying to research how to snap a 486 system together, so I decided to finally make an account and ask a question directly.

Thanks for your help!

I would guess DOA . To test, you would need, at minimum, a multi-meter to test the various power rails . See https://en.wikipedia.org/wiki/Power_supply_un … Wiring_diagrams for pinout .

The output rating of the +5V and +12V rails of your new PSU looks ok, so there is no reason why it won't work, unless like you said it is DOA, or it's output is overrated. If you have a voltmeter, you could measure the voltages to see what is happening. Sometimes it could also be just bad connections on the P8/P9 connectors. For the old PSU, what's more important is its internals. You can pop open the lid and see if the corrosion has gotten inside, and whether any capacitors are bulging (or you can post a picture here and we'll help you examine). If the inside is good, the outer casing can be cleaned up using white vinegar on the rusts and clean with soap water (doing so with the circuit board removed, of course).

The rusted PSU with the broken seal was already opened and repaired at least once but it doesn't mean that it's reliable.
The other one is simply junk, send it back.

Sparkle units are good quality, it should work without issues. Try attaching at least one 12V device, such as a hard drive, and see if it works.

Maybe without additional load on that rail the voltage is not stable enough and the power good signal is not asserted high. That would prevent the motherboard from starting even though the PSU appears to be "on".

The rusty power supply probably wires PG directly to 5V as many cheap units tended to do back then. It sure works, but makes the actual purpose of the PG signal useless.

TheMobRules wrote on 2020-08-04, 03:53:

Sparkle units are good quality, it should work without issues. Try attaching at least one 12V device, such as a hard drive, and see if it works.

Maybe without additional load on that rail the voltage is not stable enough and the power good signal is not asserted high. That would prevent the motherboard from starting even though the PSU appears to be "on".

The rusty power supply probably wires PG directly to 5V as many cheap units tended to do back then. It sure works, but makes the actual purpose of the PG signal useless.

Attached a hard drive, and it did spin up, but same lack of response from the motherboard.

I did notice that the wires on the Sparkle PSU's are a different color from the ones on the old PSU and the diagram linked above. Instead of "orange-red-yellow-blue-black-black" and "black-black-white-red-red-red," they're "white-red-yellow-brown-black-black" and "black-black-blue-red-red-red." I'm assuming that's just a different color scheme and not actually inherently different.

I'll probably just send this one back if it should otherwise be working.

I'll open up the old PSU and post pics when I get home from work tonight; no time this morning. Thanks for everyone's help so far!

I decided to test it with my multimeter anyway, because I've never done that before, and might as well learn something.

Results, going from top to bottom of the diagram here: https://en.wikipedia.org/wiki/Power_supply_un … Wiring_diagrams

PG: +4.8
+5: +4.8
+12: +12.7
-12: -12.4
Ground: ~0
Ground: ~0

Ground: ~0
Ground: ~0
-5: -4.6
+5: +4.8
+5: +4.8
+5: +4.7

The grounds all vacillated slightly between -0.5 and +0.5, but I assume that's usual? And might be my multimeter's fault (I could get that much of a reading my moving the wands around in the air.)

The other readings were steady and consistent. I checked them three times and got the same readings each time.

In comparison, the rusty PSU has these:

PG: +5.0
+5: +5.0
+12: +11.95
-12: -11.3
Ground: ~0
Ground: ~0

Ground: ~0
Ground: ~0
-5: -4.9
+5: +5.0
+5: +5.0
+5: +5.0

The rusty supply is definitely is more accurate. I guess I'm a little surprised that the readings on the Sparkle were far enough off to prevent POST, but huh.

Here is a picture of the inside of the rusty power supply: https://i.imgur.com/9Iz4BBv.jpg

I admittedly don't really know what I'm talking about, but that sorta scares me.

SPI used to make good supplies but maybe you got a bad one or their quality has changed. I would send it back. added: I have some very old outside corroded AT that still work just fine, as long as the caps are good and corrosion is not inside they are good !

Help you attach the picture here.

Looks like at some point in time some unknown fluid had gotten inside the PSU and left the white residue. It's amazing that it is still working though. The caps look ok, no bulging. The AC input stage is typical of the cheap PSU during that era - line filter transformer was replaced by 2 wires, safety caps were substituted with normal ceramic caps (lower right corner). In all, it is in quite bad shape, there are even rust on some of the power transistors.

Horun wrote on 2020-08-05, 02:36:

added: I have some very old outside corroded AT that still work just fine, as long as the caps are good and corrosion is not inside they are good !

Yeah agree, I have repaired many old AT & ATX PSUs, many are the cheap ones which skimmed on the AC input stage (no line filters and safety caps) and usually their outputs amperes are overrated. But they do work fine if fixed up properly, and the PWM controller ICs they used do have proper over voltage and short circuit protections. They are not as bad as everyone perceives.

Going to raise this dead topic.

Starting the retro-hobby I supposed it would be something quick and not so expensive. But now, when I have a pile of precious old trash, I started thinking about having the proper power supplies for several of my builds. That's not a problem for the ATX as I can get either new or pretty good old ones (even with Penitum 4 connector when needed) but I am still using several old AT PSUs without estimate of the consequences.

So my question is: why old AT supplies are on such a bad account? What's so dangerous in them that can burn my equipment? And what has changed since then?

Like, I can buy several Athena PSUs as AP-AT30 ($50 + delivery) with 300W that is surely enough for 386-486-early P1, but they say 'circa 1988' on their site - does it mean the design never changed since then?

What is the usual failure point of such PSUs? What can be replaced / added just to put the old ones to a good shape? Recently I disassembled one of my PSUs to clean it properly, so I washed and dried it, replaced the fan, removed the starting corrosion and painted the case black (just to look better in the black chassis and prevent further rust). It was dusty inside but pretty normal with all caps looking good. Very similar to the design of the PSU on the photo of shadmere. With several wires instead of missing elements.

My opinion only: many old AT PSU were built with quality but age deteriates them. Typically caps fail, sometimes the mosfets, rectifiers, etc have been over heated multi times and also need replacing.
Athena is not a true quality builder IMHO and is why their price is about same as what a mid 1990 Good AT supply would cost (with inflation that Good AT would now be $100 like Good ATX psu today)
In the last picture above: transformers and heatsinks are small even for an old 250-300watt, looks more like a 150-180watt...just my opinion but that is also branded TransWorld who I have never heard of...

Horun, ok, so what is good modern AT replacement for now? Not a fan of ATX->AT adapters. Is there anything built on modern technologies?

rkurbatov wrote on 2023-04-05, 22:38:

So my question is: why old AT supplies are on such a bad account? What's so dangerous in them that can burn my equipment? And what has changed since then?

There's nothing special about AT power supplies. The main issue, as I see it, was people were buying the cheapest stuff, using it for a looong time without any sort of maintenance, then possibly upgrading their PCs with more power-hungry CPUs and cards only to see the thing fail. I had way more no-name ATX PSUs fail during Athlon/Duron era than any AT ones, and that's including those rescued from e-waste dumps in pretty sorry state.

In general nothing last forever and PSUs are no exception. Electrolytic capacitors in SMPS work hard and die eventually, and the '90 caps were not as good as what we have now. Lack of input line filtering and undersized heatsinks is nothing compared to things I've seen in the currently manufactured "power bricks". I've thrown out some of those while they still worked after seeing the insides, I'd like my house not to be on fire. Frankly I'm more worried about connecting a modern ATX PSU to AT mobo. Reason being there aren't many good lower power models, and with the higher power ones (and for AT mobo 400W is already high power) you can easily fry tons of stuff due to a short that is not going to register with the PSU capable of driving 20A+ into load during normal operation. Not to mention poor overall regulation due to 3V3 line not being used at all.

Yes, not all AT PSUs are worth repairing/restoring but you can get a few maybe and keep the best ones. It helps to be able to fix them, or at least do the basic things like fan and capacitor replacement.

Deunan wrote on 2023-04-06, 18:19:

There's nothing special about AT power supplies. The main issue, as I see it, was people were buying the cheapest stuff, using it for a looong time without any sort of maintenance, then possibly upgrading their PCs with more power-hungry CPUs and cards only to see the thing fail. I had way more no-name ATX PSUs fail during Athlon/Duron era than any AT ones, and that's including those rescued from e-waste dumps in pretty sorry state.

In general nothing last forever and PSUs are no exception. Electrolytic capacitors in SMPS work hard and die eventually, and the '90 caps were not as good as what we have now. Lack of input line filtering and undersized heatsinks is nothing compered to things I've seen in the currently manufactured "power bricks". I've thrown out some of those while they still worked after seeing the insides, I'd like my house not to be on fire. Frankly I'm more worried about connecting a modern ATX PSU to AT mobo. Reason being there aren't many good lower power models, and with the higher power ones (and for AT mobo 400W is already high power) you can easily fry tons of stuff due to a short that is not going to register with the PSU capable of driving 20A+ into load during normal operation. Not to mention poor overall regulation due to 3V3 line not being used at all.

Yes, not all AT PSUs are worth repairing/restoring but you can get a few maybe and keep the best ones. It helps to be able to fix them, or at least do the basic things like fan and capacitor replacement.

Big +1 to this, the fact that modern power supplies can deliver huge amounts of current continuously without short circuit protection kicking in is something that I don't see being mentioned very frequently. Especially if you add dodgy ATX->AT or SATA->Molex/Floppy adapters to the equation, loose connections can result in molten components.

Also, regarding the "power supply dying and frying your hardware" stuff, in most cases was due to the +5VSB line on ATX units going horribly out of spec and killing the parts that are related to the standby state (such as motherboard chipset, RAM, and so on). This was due to critical caps failing or just a poor design of the +5VSB circuit and it is exclusive to ATX, so an AT power supply would be "safer" in that sense. Of course, a terribly designed AT unit could still cause damage, but a decent one will probably die when its time comes without any collateral damage... even in that case, AT units are simpler and quite easy to repair if you have some basic knowledge (most of them use the same topology).

Thank you for your opinions.

So if I decide to review this PSUs I can try full recapping and overall examination and that would be enough?

rkurbatov wrote on 2023-04-06, 22:48:

So if I decide to review this PSUs I can try full recapping and overall examination and that would be enough?

That rusted one? Well, I'd look for a better candidate for restore. This one can be a backup unit. Rust on case is one thing but I don't like how that PCB looks. That is usually leftover flux after poor factory cleaning chemically reacting with something - should not be conductive but you never know. And it will attract moisture. If I were to guess this PSU was used near some salt water body and the humid air had salt in it. As a bare minimum you have to inspect the back of that PCB, preferably wash it as well - tap water is good enough but let it dry in room temperature for a few days. You can try some gentle scrubbing, maybe that white stuff will go away but probably not (it might look gone when wet but will reappear when dry). Do not use any aggresive cleaners, you'll only damage the parts and/or the PCB.

The good thing about that PSU is that it works, as-is. Cleaning it is a good idea but putting money into it (new parts) is not. Unless you can't get anything else. This one has no input filter, poor heatsinks, I see no surge limiter, primary caps look to be undersized, and the PCB is not great as I've mentioned. The secondary caps are not bulging but could be dry. 2 out of 5 is the best I can give it.

Athena Power still makes some AT psu's. Not the greatest quality, but far better than your rusted one at least.

Hey, I am not the author of the topic 😀 My PSUs are DUSTY not RUSTY 😀 They are in pretty good shape, I just want to be sure I did everything I could for them.