Does it work when you unplug the case fan? Maybe you triggered current/overvoltage protection/whatever it's called on the PSU. I'd open the PSU if it doesn't turn on and look for damage.

From the description, it sounds to me like the PSU has bad capacitors.

Case fan is shorted or you PSU just happened to die at that time, probably bad caps. Try a different PSU and do not connect the case fan. Hopefully the mobo boots up. If it does then try again with the old PSU but no case fan. If no boot then PSU. I would check the fan with a DVM and make sure no leads to ground were less than a few 100 ohms....

Thanks for the advice so far everyone. I am leaning towards it coincidentally going out while testing the fan. It was the first time I booted it today, so I'm hoping it is just caps.

I'll take it out and do a visual inspection, and then I'll test as you said, Horun.

Side note, how do you guys get the mobo connector off usually? I am always worried about damaging the board or connectors! The ATX ones come off easy, but these AT ones are insane...

Mattyice1994 wrote on 2020-08-03, 02:37:

Side note, how do you guys get the mobo connector off usually? I am always worried about damaging the board or connectors! The ATX ones come off easy, but these AT ones are insane...

I'm probably gonna get yelled at by someone for suggesting this.....BUT:

If you look at the power connectors. You should notice some little stubby bits going across the connectors that help keep them locked firm.

Just saw them off with a knife (carefully). That'l make removing and inserting the power connectors MUCH MUCH easier. The connector will still stay in place properly. You might just need to push it in a second time just to make sure it's 100%.

I do this to all my AT power cables. Hasn't failed me yet 😜

SodaSuccubus wrote on 2020-08-03, 03:03:

I'm probably gonna get yelled at by someone for suggesting this.....BUT: […]
Show full quote

Mattyice1994 wrote on 2020-08-03, 02:37:

Side note, how do you guys get the mobo connector off usually? I am always worried about damaging the board or connectors! The ATX ones come off easy, but these AT ones are insane...

I'm probably gonna get yelled at by someone for suggesting this.....BUT:

If you look at the power connectors. You should notice some little stubby bits going across the connectors that help keep them locked firm.

Just saw them off with a knife (carefully). That'l make removing and inserting the power connectors MUCH MUCH easier. The connector will still make proper contactt. You might just need to push it in a second time just to make sure it's 100%.

I do this to all my AT power cables. Hasn't failed me yet.

This sounds very practical... I might end up doing this.. I appreciate the advice!

Also.. I opened up the PSU, nothing jumps out as odd.. Nothing looks bulged, nothing looks like it leaked..

I'm a noob when it comes to diagnosing and testing electronics, but I do have a good digital multimeter, can anyone point me to what I could start doing if I want to diagnose this thing properly? I'd rather start learning some electrical engineering so I can restore things at a more fundamental level!

How about providing the brand and model number of the PSU along with some high resolution photos of its innards ? You may not see anything wrong, but someone with more experience might see something of interest and point you in the right direction . I will likely not be that person (not my field of expertise), but several people here are quite familiar with such things .

Another point to consider is that your PSU might not even be worth repairing . Some PSUs, old or new, have very little in the way of security features and may be prone to failing in a way that can kill or at least damage PC components .

Hissing noise of an AT power supply is a very typical sign for a short circuit at the output. While other causes are possible, the first thing I would do in this case is checking the PSU and mainboard for shorts on all power rails.

You can easily do it with your digital multimeter. Put it into continuity tester mode (that's the one where the multimeter beeps if the probes are connected). Often this mode is shared with diode test mode or a low resistance (ohms) range, and you might need to push a mode select button in addition to turn the rotary switch to the right position. If you made sure continuity test works (by just holding the probes together and checking for the beep), connect the black lead anywhere to the PC case (if the PC is still properly mounted), or into one of the holes with a black lead on the Molex connector used to power 5,25" drives. Check for the connection of the black connector of your multimeter by touching the contact of a black wire on a different connector. If there is no beep, something is not connected as it should be. If it beeps, you are ready for the actual test: Keep the black lead where it is, and use the red lead to touch all the other power supply outputs, especially +12V and +5V (the red and yellow cables). A short beep (typically up to 0.3s) is perfectly fine and indicates the charging process of the capacitors in the supply, on the mainboard and possibly on other connected hardware as well. A continous beep for more than one second shows a short circuit. If you find a short circuit, remove components until the short is gone.

This test for short circuits is completely safe for you if the computer is turned off or (better) unplugged from mains, and you don't tuch anything inside the power supply. If you find a short circuit inside the power supply (the continuity tester keeps beeping between a power rail and ground with nothing connected to the power supply), that's the moment where you need to start probing inside the supply, and that's also the moment where things might get dangerous, especially if you are inexperienced with AT power supply trouble shooting. The very least you should do: Never probe inside the supply with the mains plug connected. After disconnecting mains, turn on the power switch for some seconds before you start probing (to discharge the 300+ volts on the primary capacitors). I'm not telling you that everything is safe for sure if you work this way, but it will mitigate the danger by a great amount.

If you don't find a short circuit, the next most likely cause of the problem is that the capacitors on the +5V line failed, and the line works so badly that overvoltage protection kicks in and creates an artificial short circuit inside the power supply to protect the computer from damage. This kind of protection circuit is called "crowbar circuit" and present in most mid-tier and upper-tier AT power supplies. The artificial short-circuit self-resets as soon as you unplug or turn off the power supply, so you will not be able to measure it in the first test I suggested.

darry wrote on 2020-08-03, 03:46:

How about providing the brand and model number of the PSU along with some high resolution photos of its innards ? You may not see anything wrong, but someone with more experience might see something of interest and point you in the right direction . I will likely not be that person (not my field of expertise), but several people here are quite familiar with such things .

Another point to consider is that your PSU might not even be worth repairing . Some PSUs, old or new, have very little in the way of security features and may be prone to failing in a way that can kill or at least damage PC components .

Hey Darry, thanks for your post. You're right, I think it would be better to get some other eyes looking at this thing. I'm going to put a link on this post, as well as make a new thread specifically asking for help looking into it.

Also, I agree... perhaps it isn't worth repairing.. I'd honestly like the chance to learn and repair it though, if others agree it might be possible to diagnose and replace parts.

Here's a link to an Imgur album of picture of my PSU. I'm going to also create a new thread - I'll edit this post with a link for the new thread shortly.

PSU pictures: https://imgur.com/a/7Qv7NxA

Thank you everyone for your insights so far!

mkarcher wrote on 2020-08-03, 07:14:

Hissing noise of an AT power supply is a very typical sign for a short circuit at the output. While other causes are possible, t […]
Show full quote

Hissing noise of an AT power supply is a very typical sign for a short circuit at the output. While other causes are possible, the first thing I would do in this case is checking the PSU and mainboard for shorts on all power rails.

You can easily do it with your digital multimeter. Put it into continuity tester mode (that's the one where the multimeter beeps if the probes are connected). Often this mode is shared with diode test mode or a low resistance (ohms) range, and you might need to push a mode select button in addition to turn the rotary switch to the right position. If you made sure continuity test works (by just holding the probes together and checking for the beep), connect the black lead anywhere to the PC case (if the PC is still properly mounted), or into one of the holes with a black lead on the Molex connector used to power 5,25" drives. Check for the connection of the black connector of your multimeter by touching the contact of a black wire on a different connector. If there is no beep, something is not connected as it should be. If it beeps, you are ready for the actual test: Keep the black lead where it is, and use the red lead to touch all the other power supply outputs, especially +12V and +5V (the red and yellow cables). A short beep (typically up to 0.3s) is perfectly fine and indicates the charging process of the capacitors in the supply, on the mainboard and possibly on other connected hardware as well. A continous beep for more than one second shows a short circuit. If you find a short circuit, remove components until the short is gone.

This test for short circuits is completely safe for you if the computer is turned off or (better) unplugged from mains, and you don't tuch anything inside the power supply. If you find a short circuit inside the power supply (the continuity tester keeps beeping between a power rail and ground with nothing connected to the power supply), that's the moment where you need to start probing inside the supply, and that's also the moment where things might get dangerous, especially if you are inexperienced with AT power supply trouble shooting. The very least you should do: Never probe inside the supply with the mains plug connected. After disconnecting mains, turn on the power switch for some seconds before you start probing (to discharge the 300+ volts on the primary capacitors). I'm not telling you that everything is safe for sure if you work this way, but it will mitigate the danger by a great amount.

If you don't find a short circuit, the next most likely cause of the problem is that the capacitors on the +5V line failed, and the line works so badly that overvoltage protection kicks in and creates an artificial short circuit inside the power supply to protect the computer from damage. This kind of protection circuit is called "crowbar circuit" and present in most mid-tier and upper-tier AT power supplies. The artificial short-circuit self-resets as soon as you unplug or turn off the power supply, so you will not be able to measure it in the first test I suggested.

Mkarcher, thanks a million for this post! I'm going to read through this again carefully tomorrow and give it a shot. I definitely hear you, and I won't assume that anything is safe - ESPECIALLY if I am going to probe the inside of the PSU. I'll make sure to take the precautions you mention (at the least). I appreciate the time you took on this post, it really helped me understand what might be going on, and I think I can start figuring this out.

As frustrating as it is to have this PSU die, I'm really glad to be going down this 486 rabbit-hole! It's fun tinkering and getting things to work!

This unit is very nice, but fuhjyyu caps
on secondary side should be ripped away
as soon as possible, they are rated very
high on badcaps.net ...

https://badcaps.net/forum/showthread.php?t=47274

I would clean this PSU very well and then
replace those crap capacitors.

Can you take a look at the voltage selector?

Mattyice1994 wrote on 2020-08-04, 06:38:

PSU pictures: https://imgur.com/a/7Qv7NxA

Wait a second: In the last picture, you have a good image of that 16-pin control chip (most likely a TL494 or a clone like the KA7500), and it seems to have a crack straight through the whole chip. If that's the case, some catastrophic failure severely overvolted that chip such that it "exploded" and tore its case appart. If that's the case, and you are not already an experienced AT power supply rebuilder wanting to top off your experience with a complicated case, I recommend to just ditch the supply. It looks like an run-of-the-mill AT power supply without any special bells and whistles, so I don't see an objective reason to fix it. If you have emotional connections to that computer or enjoy tinkering on that supply more than using the computer it was connected to, go for it, though. You better have access to an oscilloscope to check the regulation circuit using external DC power before plugging it back into mains. If you are feeling lucky, you might just swap the chip, but as they usually blow for a reason (broken components around the chip), it is very likely that the replacement chip will die the same way when you power up the supply again.

A broken (literally!) TL494 is also able to create this hissing noise, just like a short circuit. The primary cause of the hissing noise is not a short circuit, but a non-operating TL494. A short circuit overloads the supply, so it is unable to properly power the TL494 chip, and this is the most likely reason for it to not operate. If it's blown, it will not operate too, causing the same symptoms.

Yeah, unfortunately that chip seems to be cracked. It's a shame, because it's a nice unit (built by Sparkle/FSP) but unless you're comfortable desoldering and replacing chips I wouldn't recommend attempting to repair. Besides, other components may have also failed, and you will also need a full recap on the secondary side (due to the Fuhjyyu caps).

The PWM ic looks to me like there are several strands of spider web on it rather than cracked. If the PWM ic is bad or blown, I don't think there will be any hissing as there is nothing driving the MOSFETs which drive the transformer. The hissing is probably due to the transformer working extra hard due to excessive output current drawn.

@Mattyice1994 you can try wiping the ic (with power cord unplugged of course) to see if its spider web or indeed a crack.

A simple test you can perform with a true RMS multimeter (with enough bandwidth): measure AC voltage on the +5V and +12V outputs to get a figure for ripple.

kalohimal wrote on 2020-08-05, 15:49:

The PWM ic looks to me like there are several strands of spider web on it rather than cracked. If the PWM ic is bad or blown, I don't think there will be any hissing as there is nothing driving the MOSFETs which drive the transformer.

@Mattyice1994 you can try wiping the ic (with power cord unplugged of course) to see if its spider web or indeed a crack.

After reporting the supposed crack, I looked at the picture in detail. It seems that it is a thread of hot-glue gunk and not a crack in the chip. At manufacturing, they pulled the glue gun from the adjustment pot to somewhere on the output side and deposited this thread over the PWM chip.

There are two sources to drive the primary transistors: Either the PWM chip drives them (and if it does, they run cleanly at super-sonic frequencies, so the supply should be silent to human ears), or the primary side oscillates as free-running oscillator, just like an electronic lamp ballast. I'm quite confident the hissing noise only occurs in free-running oscillation. This second mode is needed to work in TL494-based AT power supplies, because the TL494 is powered from the secondary side so the supply can not start up without oscillation. Take a look at http://danyk.cz/s_atx01a.png: The base drive transformer has 5 "windings" (acually 3 windings, with two of them center tapped). If you remove the left part of the base-drive transformer and the components that drive this left side (the two 2SC945 transistors, the five 1N4148 diodes and so on), the reminiscent part of the primary side circuit quite closely resembles http://www.pavouk.org/hw/lamp/luxar11w.png. In the AT supply, the primary of the main transformer and the 2.5µF capacitor form a tank circuit and the middle winding of the base-drive transformer creates feedback to drive the transistor bases from the momentary tank circuit current. I did not analyse whether the oscillation frequency is based on the resonant frequency of the tank or is based on saturation of the base-drive transformer that causes base drive to break down, but in either case, the primary side is able to oscillate on its own without any power at the PWM chip.

Edit: replaced "MOSFETs" by "primary transistors". AT power supplies use bipolar transistors for the half-bridge, not MOSFETs.

I read through all of your posts and the analysis, thank you everyone so far. This is truly an awesome community. I've been a bit busy, but I will write a better post tonight. So, I'll double check by opening it up, but it seems that the IC isn't cracked, that's good to hear.

Just to clarify, the sound almost sounds like something "decompressing" or "relieving" stress when I power it off. It is like a slight hiss coupled with a slight crackle. I imagine it as components releasing some sort of energy or stress in a way that shouldn't occur.

Verysaving, I appreciate you identifying the bad caps - looks like I should plan to replace those either way (only if you all think this is salvageable).

Mkarcher, the details you provide shed some great light on this, I need to re-read through your latest post to understand better. Thank you!

So far, do you guys think this thing might just need some caps replaced? Or something more serious?

I know that Verysaving also wanted me to get a better pic of the voltage reg, I'll try to do that tonight also.

Yeah... it's a good PSU. I'm willing to get a different one for sure, but if it isn't too technical of a repair, I'd be absolutely willing to do some component replacement (especially if it means knowing that it is good to go).

I have a Delta that I originally bought, but silly me got one that looks to be oriented for a desktop style case...bummer.

I'm trying to avoid the ATX to AT situation... Maybe I'm being stubborn, but I have a feeling (from things I read, too) that newer PSUs may not be setup to run the kind of components that the 90s PCs typically had, at least in an ideal world. I want to do whatever it takes to preserve the components by providing reliable, clean power.

Well, considering the caps are Fuhjyyu, the probability of those having failed by this point is extremely high (they are known to fail and even leak on unused hardware! It happened to me...).

So I'd say replacing them could be a first step in diagnosing the problem. You don't really need to replace the two big caps on the primary side, those are not known to fail very often except on catastrophic situations, besides they're Japanese Rubycons so you're good.

But be aware that the actual cause of the problem may be something else... usually it's easy to find replacements for every component in a PSU except for the transformers, but it may take more time troubleshooting than it's worth.