Optiplex power supply dead

[Luke Mawdesley]

I have 2 Dell Optiplex 790 SFFs that both have dead power supplies. When I plug them in I can hear a spark so they are at least drawing power. I've checked all the power rails and there's nothing whatsoever. The self test button on the back does nothing also. I took it apart, plugged it in and probed the back until the voltage stopped at a mosfet. Is the problem with the mosfet or something else? I de-soldered it and when turned on it stays closed for about 4 seconds before opening again (tested with a multi-meter). The mosfet is this: https://www.indiamart.com/proddetail...047252730.html . Any help is appreciated.

[CapLeaker]

Take wifeys hair dryer and warm the inside up a little then try to power up the PSU.

[Luke Mawdesley]

Take wifeys hair dryer and warm the inside up a little then try to power up the PSU.

Any specific part of it?

[CapLeaker]

Any specific part of it?

No. Just blow some warm air inside the PSU for like 30 seconds or so. Then turn on the PSU and see if it runs.

[Luke Mawdesley]

No. Just blow some warm air inside the PSU for like 30 seconds or so. Then turn on the PSU and see if it runs.

Just tried it and no difference. I should also mention that there are no visible signs of damage on the board and all the caps look fine.

[CapLeaker]

I guess we need straight shot pictures of the board,. Upload them using the attachment function and choose like a small or no thumbnail.
iI thought maybe the starter cap had dried out and that's why the PSU doesn't turn on when cold.

[Luke Mawdesley]

I guess we need straight shot pictures of the board,. Upload them using the attachment function and choose like a small or no thumbnail.
iI thought maybe the starter cap had dried out and that's why the PSU doesn't turn on when cold.

Here are some pics:

[SMDFlea]

Here are some pics:

What model number is on the PCB ,and on the sticker on the metal cover

[Luke Mawdesley]

What model number is on the PCB ,and on the sticker on the metal cover

Here

[CapLeaker]

Do you have 5VSB or is it totally dead?

[Luke Mawdesley]

Do you have 5VSB or is it totally dead?

No, its completely dead with no standby power.

[CapLeaker]

Is there a little chip starting with TNY xxx? May be an IC or TO220 style mounted to a heatsink with many (5) legs?
May be also a TOP xxx

[Luke Mawdesley]

Is there a little chip starting with TNY xxx? May be an IC or TO220 style mounted to a heatsink with many (5) legs?
May be also a TOP xxx

Yes, there's a TNY280PN. Its a small IC with 8 pins I think.

[CapLeaker]

That thing is doing your 5VSB. Look around and see if power is getting there. Be careful, you are on power mains! If you do get power there but it isn't doing anything, I'd change it out with a new one.
The 5VSB is always on. Also check the caps for this circuit. Next there has to be a rectifier diode for the 5VSB on the secondary side. Look that it isn’t shorted!

[Luke Mawdesley]

That thing is doing your 5VSB. Look around and see if power is getting there. Be careful, you are on power mains! If you do get power there but it isn't doing anything, I'd change it out with a new one.

I'll try this when I can. Where do I ground the multimeter though? Also how do I check the caps with a multimeter?

[CapLeaker]

Easy. Ground on the primary side is always the negative post of the main filter capacitor. You can’t really test capacitors with an DMM properly. You need an ESR meter for that. Maybe your DMM has a capacitance function, better than nothing.

[Luke Mawdesley]

Easy. Ground on the primary side is always the negative post of the main filter capacitor. You can't really test capacitors with an DMM properly. You need an ESR meter for that. Maybe your DMM has a capacitance function, better than nothing.

There was no power anywhere near the chip, however I found the issue somewhere else. I was looking more closely at where the voltage stopped and saw that 2 small capacitors similar to these had blown out and were cracked as well as a resistor that was blown. Should I try replacing these or is there another problem that might make them blow again?

Edit: The resistor is a 4.7 Ohms flame-proof resistor. The capacitors are completely destroyed so I can't tell anything about them.

[CapLeaker]

The flameproof resistor acts like a fuse, since it so low in ohm rating. And there is probably where your voltage to the 5VSB stops. The capacitors… they are looking kind of these shown in the picture? Be careful around there we might need to be able to identify them. What colour are they? Anything printed on there or are they burnt to crisp? Pictures? What kind of circuit are they in?

[Luke Mawdesley]

The flameproof resistor acts like a fuse, since it so low in ohm rating. And there is probably where your voltage to the 5VSB stops. The capacitors… they are looking kind of these shown in the picture? Be careful around there we might need to be able to identify them. What colour are they? Anything printed on there or are they burnt to crisp? Pictures? What kind of circuit are they in?

Here are some pics. There is no printing as far as I can see. Also there was power on both sides of the resistors and caps so not sure what that means.

[CapLeaker]

Read this thread and also check that MOSFET for short.
https://www.badcaps.net/forum/troubl...-7010-pc-tower