Seasonic X 850W not turning on

[MichR]

Hello everyone
I've got a Seasonic X power supply which does not turn on.
Here you can find pictures and info about it (it's a 750W unit, but PCB is basically the same).
So the symptoms are:
- with nothing connected:

• on PS_ON I get about 2.4V (it should be ok since WT7527's high-level input voltage is 1.8V)
• on +5VSB I measure 5.141V (ok)

- with PS_ON shorted to ground, +5VSB line falls from 5V to 0V just like if a capacitor is discharging, but it goes back to 5V instantaneously. PGON is always 0V; all the other lines (+12V, +3.3V, ...) are at 0V.

To me it looks like something is shorting +5VSB line, but I'm not totally sure.
I've checked all the components sorrounding the +5VSB line on the secondary side and primary side and found no shorts. I could not test the ESR of the caps on both transformer sides, but they look good (no bulges, correct capacity).

A little thing I noticed is this SMD ceramic capacitor (between +5VSB and GND) has fallen off from the PCB, but I don't think this is the issue here:


Thank you all in advance

[MichR]

Re: Seasonic X 850W not turning on

Just a little update.

I think there is a short between +12V and GND.
When I measure resistance with positive on +12V and negative on GND I always get 0 ohms; then when I flip multimeter clips it still measures 0 ohms, but after some seconds it slowly goes back to infinity.

Therefore I think it must be a polarized component, such as a diode on a MOS transistor. A careful inspection on the secondary revealed no diodes or MOS transistors (except those on the vertical PCB, which, however, are not directly connected to +12V).
So I decided to disconnect the front panel, but +12V keeps shorting.
At last, I must conclude that this problem is related to the +12V filter caps. However, I could not even remove one of them because of the large thermal pads sorrounding them. Also, the heatsink blocks every access to the capacitors.

So before going on with removing these caps, I would like to know if someone else had an issue like that.

[MichR]

Re: Seasonic X 850W not turning on

[...] (except those on the vertical PCB, which, however, are not directly connected to +12V).

Sorry for this stupid thing I said . They are indeed connected to +12V, and the fault it's probably there, since the sympton revealed with DMM seems to be a MOSFET that I charge/discharge when I invert the polarity of DMM's clips.

However I don't know if this behaviour is normal or not. I just can't access to the vertical board to see how these transistors are connected.

[MichR]

Re: Seasonic X 850W not turning on

Update:
Some folks over eevblog helped me out identifying a bad MOSFET I found on my power supply. For anyone's interested, here's the picture of it:


its reference designator is "U4" and its marking is "5014T". Apparently it's a SPN5001 (a 600V VDS N-channel MOSFET). More info here.

I replaced the two on my board with an equivalent BSS127.
Now +5VSB is super stable, but it doesn't end here. When shorting PS_ON to ground, fan starts but it stops after some seconds. Voltages on +3.3V, +5V and +12V all go to their nominal voltage but in less than a second they approach 0V. I can hear the thermistor relay switches for a fraction of second, it seems to turn on but then it turns back to the off state.

Apparently there's still a problem around the relay controller circuit and perhaps it is what caused the old MOSFET to turn bad.

[momaka]

Re: Seasonic X 850W not turning on

I replaced the two on my board with an equivalent BSS127.
Now +5VSB is super stable, but it doesn't end here. When shorting PS_ON to ground, fan starts but it stops after some seconds. Voltages on +3.3V, +5V and +12V all go to their nominal voltage but in less than a second they approach 0V. I can hear the thermistor relay switches for a fraction of second, it seems to turn on but then it turns back to the off state.

Make sure you are turning on the power supply with some kind of a minimum load, as some power supplies will turn off without that. If you have two or three old hard drives or perhaps an older motherboard you don't mind testing the PSU on, these would do as a load. Alternatively, 12V incandescent/halogen bulbs should also do (20-50 Watts each, and put one on each major rail - i.e. 3.3V, 5V, and 12V). If the PSU still does the same, the problem is likely elsewhere. In which case, it might be worth checking the voltage of the primary cap before and after you turn on the PSU (be extremely careful when you do that though - the primary side can be dangerous if you touch something with your bare hands!). With these newer PSUs, the APFC should be boosting the voltage on that cap to about 380V or more (typically). If you don't see the voltage change at all from when the PSU is in soft-off mode, then there may be a problem with the APFC circuit or its driving components.

Just a little update.

I think there is a short between +12V and GND.
When I measure resistance with positive on +12V and negative on GND I always get 0 ohms; then when I flip multimeter clips it still measures 0 ohms, but after some seconds it slowly goes back to infinity.

That's actually pretty typical when you measure the resistance between a rail and ground of any power supply with fairly large capacitance. With the caps discharged, you get a very low resistance for the first 2-3 seconds until the caps charge up. Then when you reverse your multimeter leads, the multimeter discharges the caps and charges them in the reverse direction. But while this is happening, the meter will read short-circuit or very very low resistance for a few seconds.

So I think what you are seeing there is normal.

[MichR]

Re: Seasonic X 850W not turning on

But while this is happening, the meter will read short-circuit or very very low resistance for a few seconds.

This reminds me of the study of an RC network. When R approaches 0 and a step voltage generator is used, the current tends towards a pulse which lasts longer the larger the capacitance.
I haven't studied it yet, but the ohm meter should work like a (step) voltage generator applied to the unknown resistor. The current is then measured and the resistor is then calculated with the Ohm's law.

Make sure you are turning on the power supply with some kind of a minimum load, as some power supplies will turn off without that.

I could not believe it was so simple! I connected the power supply to an old motherboard and it worked! Now I will test it deeply on my daily PC, but I do think the problem is solved.

In the further days I will upload here some schematics of this KM3 platform I drew to better understand the behavior. This may be helpful to someone else who has a similar PSU.

[MichR]

Re: Seasonic X 850W not turning on

As previously said, here you can find my partial schematics of the unit:
https://mega.nz/#!JJoyFK5L!LkH33C09s...ZBuAUOThSOIMxs

JPEGs are also attached here. If you download the entire archive you will find a .PSD (photoshop) file with the monitor board diagram whose routes can be hidden at will. First layer can be deleted (it's only there to reduce file size of about 10MB).

Note: in the PFC partial schematic, labels like MONITOR[X] mean that the branch is connected to the number X pin of the monitor board (with the same numbering as in the picture of the monitor bord). PFC[1] instead indicates the first pin of the PFC board, which is the nearest to resistor R205.
Also, note that I haven't double checked the schematics (it's probable I made some mistakes) nor I have measured the voltages, which I just assumed to be the nominal ones.

P.S: PSU is running fine since I reassembled it. Thank you very much @momaka for the last hint!

[momaka]

Re: Seasonic X 850W not turning on

P.S: PSU is running fine since I reassembled it. Thank you very much @momaka for the last hint!

You're welcome. Glad to hear it is running okay. Hope it stays that way too.

[webber77777]

Re: Seasonic X 850W not turning on

MichR
Please update your partial device diagrams: the link does not work

[MichR]

Re: Seasonic X 850W not turning on

MichR
Please update your partial device diagrams: the link does not work

Yes, sorry, I must have moved the file in another directory so it broke the link.
Here it is a new one: https://mega.nz/#!1RxkxY4L!LkH33C09s...ZBuAUOThSOIMxs

[webber77777]

Re: Seasonic X 850W not turning on

Good evening
Thank you for the files provided. Now I have a lot of Seasonic products at work - but there is no information at all.
One of the problems encountered is a short circuit on the 12 volt line. I assumed that one of the 4 field-effect transistors on the rectifier board was to blame. We take soldering irons and in four hands we solder the module. But the short was not there. It was to blame - polymer electrolytic capacitor. Which when defective - flew out of the room. But polymer - very durable capacitors.
Someone can tell - on which resource you can download schemes or service manuals on Seasonic power supplies?

[momaka]

Re: Seasonic X 850W not turning on

So which one of these capacitors was shorted - the blue "CapXon" branded ones or the "FP" (Nichicon) ones? Just curious, as this might be very useful information there regarding cheap polymer capacitor brands.

As for service manuals or schematics for this power supply... I don't think you'd find one anywhere, but I could be wrong - at least it doesn't seem like ATX PSU schematics for the popular good brands can be found. For cheapo generic half-bridge PSUs - yes, plenty available.

[stj]

Re: Seasonic X 850W not turning on

one of the capxons has a dent in the top,
i'v seen this type of damage before and always wondered if it could be a problem.

[momaka]

Re: Seasonic X 850W not turning on

one of the capxons has a dent in the top,

Can you point it out? I don't see it at all. I've seen caps (both solid and wet electrolytic) get their cases trashed a lot more and still work fine.

[stj]

Re: Seasonic X 850W not turning on

upper right blue cap in 1st(left) picture

[webber77777]

Re: Seasonic X 850W not turning on

The blue marked CapXon was shorted in the upper right corner. He gave a short circuit on the line of 12 volts. In the most inconvenient place in the entire power supply. When burned, it scattered completely. If the photos are preserved - lay out. As for the capacitors - I had a similar failure with the Xilence power supply, but there, at least from the factory, the polarity was reversed during installation. I was looking for schemes, but I did not find anything for this company. All intuitive repairs

[Hitto]

Re: Seasonic X 850W not turning on

Was it a dead short or very low resistance? A bit afraid using polymers because of their failure mode (a power supply shuts itself but into an amplifier there is a cascading failure).

[Aeron]

Re: Seasonic X 850W not turning on

Update:
Some folks over eevblog helped me out identifying a bad MOSFET I found on my power supply. For anyone's interested, here's the picture of it:


its reference designator is "U4" and its marking is "5014T". Apparently it's a SPN5001 (a 600V VDS N-channel MOSFET). More info here.

I replaced the two on my board with an equivalent BSS127.
Now +5VSB is super stable, but it doesn't end here. When shorting PS_ON to ground, fan starts but it stops after some seconds. Voltages on +3.3V, +5V and +12V all go to their nominal voltage but in less than a second they approach 0V. I can hear the thermistor relay switches for a fraction of second, it seems to turn on but then it turns back to the off state.

Apparently there's still a problem around the relay controller circuit and perhaps it is what caused the old MOSFET to turn bad.

Sir please help me i have the i same issue with my asus prime 650w platinum

[squall_leonhart]

Re: Seasonic X 850W not turning on

i believe mine has failed in the same way as ops,

though the failure mechanism was unusual, i put the system into S3 sleep to reset a stuck winbond sensor and when i woke the machine up it had the wrong cpu/qpi voltages,

slept the machine another time to see if it would correct that ended with all board LEDs going out, system non powerable.

theres a faint acrid smell emanating from it, i don't know if its a 5vsb failure or overall 12v since these are DC-DC for voltages other than 12.

recently had a prime gx delivered for another project so i just repurposed that, KM3 and Primes use the same pinouts so i just swapped psu's out leaving cables alone.

[DynaxSC]

Re: Seasonic X 850W not turning on

Hi, try to get a scope and look how the output voltages (beside the 5V standby) behave after PSON signal, the best is to use a 4-channel scope, and see -12V, 3,3V and 12V same time. The PSU monitors all the output voltages, and if one does not come in a certain, relatively short time, it stops the PSU (a little bit too intelligent, but it protects the devices connected). I had once an very hard issue to find (on a similar Seasonic Prime Platinum PSU), it was a very small board behind the board with the Weltrend chip, it is the -12V generator. There was a Zener diode shorting sporadically, what swithed off the PSU during operation. It took me several days to localize this error, but i had a second working PSU to compare - just swithed the modules between the PSU's to narrow down the issue. This board has also a small one chip 3A VRM, it can also fail (had this issue an another PSU). I'm not sure your PSU has this small board, can't determine this for sure from the pictures attached, anyhow the Welltrend board seems to be the same (I believe you design does not have this separate -12V baord, a pinline is missing in this place on the backside). Anyhow there must be something generating the -12V. A further PSU I had, there was the Welltrend chip failing, after replacing it, the PSU was OK again.

BTW from my experience Seasonics do not need to have a minimal load to start, but don't know about this specific PSU.