Power supply doesn't work on just one motherboard (stumped)

[Dannyx]

Good day folks. Little brain-teaser for us all: I've got this small E-machines desktop which doesn't turn on. I tried a different power supply and it works, so at first I was convinced the PSU is at fault. Normally, I wouldn't have even given it second though: I'd just leave the working PSU in it, pack it up and that's it, end of story. Trouble is, its original PSU (the faulty one) is a dedicated model for this case which doesn't accept regular PSUs - it's a slim-mini-type sort of thing. It's a Lite-ON pe-5221-08 in case that's important. It seemed I'd have no choice but to repair this one, so I started taking various measurements and trying out different combinations....to sum it all up, the weird part about the whole story is this: this slim PSU doesn't work on the Mini-pc, but works on every other motherboard I plugged it into.....WTH ? The actual problem is that the 5vSB line drops when I plug the 20p connector into the motherboard, but ONLY the motherboard that's on the E-machines case ! Remember: this board does successfully boot with another PSU ! :| I thought maybe the 5VSb bus starts failing under load, so I decided to test that theory and the way I've done that is I took a 12v car lamp, plugged it into the 5Vsb pin and tried it out: the PSU fired up just fine (I guess that's a pretty big load to carry, so if it can handle that, then it most certainly can power the MB in standby). It's definitely not a MB issue either because the board works with other PSUs.....what's going on here ?! I even went the distance and took the PSU apart and started changing capacitors thinking it's a 5Vsb issue, but it's very stable and works just fine on other motherboards....hell, I can even load the hell out of it with those car bulbs I got and it doesn't break a sweat....what are your thoughts on this, as I'm quite simply stuck ! :|

[stj]

Re: Power supply doesn't work on just one motherboard (stumped)

first, check the e-machines psu caps - they have a rep for murdering the motherboard chipset.

second - i wouldnt put it past them to "do a DELL" and change the connector pinout slightly!!!'

[Dannyx]

Re: Power supply doesn't work on just one motherboard (stumped)

first, check the e-machines psu caps - they have a rep for murdering the motherboard chipset.

second - i wouldnt put it past them to "do a DELL" and change the connector pinout slightly!!!'

Already did the cap part. Regarding the connector: no, it's a standard 20p connector, plus it wouldn't explain how the motherboard fires up with any other PSU...

[Khron]

Re: Power supply doesn't work on just one motherboard (stumped)

What're the odds there's some "powergood"-signal timing issues?

As per the standard, that's supposed to toggle within not-sure-how-many milliseconds, and that particular PSU is just slower than the others perhaps? Just speculating here...

[Dannyx]

Re: Power supply doesn't work on just one motherboard (stumped)

What're the odds there's some "powergood"-signal timing issues?

As per the standard, that's supposed to toggle within not-sure-how-many milliseconds, and that particular PSU is just slower than the others perhaps? Just speculating here...

My PSU testers show 250ms, which is theoretically in spec (I believe it's between 0.1 and 0.5 s)...I too thought of that at first, but it turns out it's not.

[PeteS in CA]

Re: Power supply doesn't work on just one motherboard (stumped)

Maybe a poor wire crimp on the +5VSB wire? Or a contact that has been spread a little? Being plugged into a different MB with a different wire angle might accidentally close the intermittent circuit. Power it up and try moving the +5VSB wire around a bit near the ATX connector.

[Dannyx]

Re: Power supply doesn't work on just one motherboard (stumped)

Maybe a poor wire crimp on the +5VSB wire? Or a contact that has been spread a little? Being plugged into a different MB with a different wire angle might accidentally close the intermittent circuit. Power it up and try moving the +5VSB wire around a bit near the ATX connector.

I can most definitely say it's not a connection issue - that's just it: the abnormal behaviour of the 5vSB line begins the moment it IS connected to the board, so if anything, if there were a bad connection between the two, I should read a stable 5v, but instead, I know the purple pin made contact with the slot on the motherboard because 5vSB drops to 0 :| Forgot to mention that it's like the PSU is "struggling", like it's under a very heavy load or it's "fighting" with a short: it makes a very faint clicking sound every second or so, which is in time with the 5vSB bus trying to go up. Instead it only goes to around 1v then back to 0 - another click, another retry (1v then back to 0 and so on), but again, all of this happens ONLY on this one motherboard :|

[kc8adu]

Re: Power supply doesn't work on just one motherboard (stumped)

bad caps on +5stby.

[CapLeaker]

Re: Power supply doesn't work on just one motherboard (stumped)

bad caps on +5stby.

He did a test with a car lamp on the 5v rail?

[momaka]

Re: Power supply doesn't work on just one motherboard (stumped)

I even went the distance and took the PSU apart and started changing capacitors thinking it's a 5Vsb issue, but it's very stable and works just fine on other motherboards....

So did you change the output caps on the 5VSB? What about any small caps related to the 5VSB?

hell, I can even load the hell out of it with those car bulbs I got and it doesn't break a sweat....

That's a really good test. It's how I test the 5VSB circuit on all of my repaired PSUs.

But to better understand how the 5VSB of a PSU performs, you need to do a bit more work with this.
First, check how much current your car bulbs/lamps are drawing. Preferably, you should use enough bulbs so that they load the PSU's 5VSB circuit very close to the maximum stated on its label (typically 2 to 3 Amps).
Also, make sure to connect the load (the light bulbs) to the 5VSB circuit before plugging in the PSU. Then plug in the PSU and see if the 5VSB actually starts. It is possible that the 5VSB is marginal and can start fine without any load but can't when fully loaded and cold-plugged.

the abnormal behaviour of the 5vSB line begins the moment it IS connected to the board, so if anything, if there were a bad connection between the two, I should read a stable 5v, but instead, I know the purple pin made contact with the slot on the motherboard because 5vSB drops to 0 :| Forgot to mention that it's like the PSU is "struggling", like it's under a very heavy load or it's "fighting" with a short: it makes a very faint clicking sound every second or so, which is in time with the 5vSB bus trying to go up. Instead it only goes to around 1v then back to 0 - another click, another retry (1v then back to 0 and so on), but again, all of this happens ONLY on this one motherboard :|

So what about the 5VSB of the working PSU then? Does it appear lower than normal when connected to that e-Machines motherboard?

What I am trying to figure out is if the e-Machines motherboard is faulty (i.e. drawing too much current that the original PSU cannot supply but the working PSU can) or if the original PSU's 5VSB circuit is faulty.

[Dannyx]

Re: Power supply doesn't work on just one motherboard (stumped)

Ok, let's answer some questions:
Caps: I replaced what I believe to be every cap related to the 5vSB circuit (even some small ones on the primary side).
Load test: I did the cold-start test for the same reason you pointed out and it starts ok. True, I might not have gone full-out with the load, because I only have some 24v bulbs on hand, so they are underpowered. I haven't tried hooking two of them up at the same time, but I did measure the current draw of one of the larger ones I had and it was around 450mA, which is too little now that I think about it....that could be a reason.
Other PSUs and motherboards: other PSUs maintain a steady 5Vsb when connected to the E-machines motherboard, which we can most likely deem as non-faulty because it's able to start up and display video and all that. Conversely, the PSU we're talking about manages to turn on other motherboards, so that's what's particularly strange. It's like it refuses to run on this one PC....
Other thing I noticed: the voltage on the main reservoir cap in the primary is a bit low for my taste: around 364v when the PSU is on...PFC doesn't seem to be working too well, or perhaps that's acceptable and unrelated to our issue.

[Dannyx]

Re: Power supply doesn't work on just one motherboard (stumped)

Here are some picture of the supply, front and back, just out of curiosity. I know it may not be particularly handy, but I'm just going to put it out there.

[Khron]

Re: Power supply doesn't work on just one motherboard (stumped)

Yes, you've replaced the caps, but none of those seem to be from brands i trust. Ok, that big green Taicon(?) might be ok, those Ltecs might not be dead yet, but the rest... Kinda make me shudder.

Let me guess, those are all recovered / scavenged from other power supplies?

On second thought, are these photos from before the re-cap, or after?

[Dannyx]

Re: Power supply doesn't work on just one motherboard (stumped)

Yes, you've replaced the caps, but none of those seem to be from brands i trust. Ok, that big green Taicon(?) might be ok, those Ltecs might not be dead yet, but the rest... Kinda make me shudder.

Let me guess, those are all recovered / scavenged from other power supplies?

On second thought, are these photos from before the re-cap, or after?

Actually the pictures were taken BEFORE I started messing around with it The caps I put in are new. They may not be brand names, but they SHOULD work....I'm not exactly interested in this thing lasting a life time - I just want it to turn on enough to return it to the guy who brought it in

[CapLeaker]

Re: Power supply doesn't work on just one motherboard (stumped)

450ma isn't enough to test the 5V rail with.

[Dannyx]

Re: Power supply doesn't work on just one motherboard (stumped)

The saga of weird things about this whole repair project continues: I put the power supply back in and it magically worked...again, WTF ? :| Don't you just love it when things sporadically fix themselves with no clue as to what you did ? Anyway, I've had it on for a few hours now and it's been ok. It's worth mentioning that prior to doing this, I took a 20p connector which I chopped off a PSU long ago, plugged that into the motherboard instead and then jammed each of its wires into the 20p connector of the "faulty" PSU one by one, starting with the 5vsb line, which ironically remained stable this time, so I continued to plug in the remaining voltages, including the PG and PS_ON wires too and I managed to switch on the PC. Still, whenever I tried plugging the connector straight into the MoBo like you'd normally do, I'd get the same issue........for a while, because after inserting and removing the 20p connector several times, it somehow worked......I still don't know what I did, but it doesn't really matter at this point

[Khron]

Re: Power supply doesn't work on just one motherboard (stumped)

Oxidized contacts would be the first logical explanation

[Dannyx]

Re: Power supply doesn't work on just one motherboard (stumped)

Oxidized contacts would be the first logical explanation

Yeah, but it still doesn't explain why the 5 volts dropped to 0 beforehand.....

[ChaosLegionnaire]

Re: Power supply doesn't work on just one motherboard (stumped)

like the previous poster said, oxidized contacts can also cause poor/intermittent contact with the multimeter probes so it dropped to 0v due to absent contact with the metal conductor.

[kc8adu]

Re: Power supply doesn't work on just one motherboard (stumped)

it is possible you plugged it enough to wipe the contacts.
but i bet this will rear its ugly head again soon.shortly after you declare it "fixed" and return it.