Dell Optiplex 990 psu H240AS-00

[Gabriel]

We have these fail at work from time to time, had to do a teardown.

All caps are Ltec and and Elite.
Don't know if the N-channel mosfet STW12NK90Z failed first or the cap.

[kasfamily]

Re: Dell Optiplex 990 psu H240AS-00

you should check Q51 and R52 and replace C52

[SteveNielsen]

Re: Dell Optiplex 990 psu H240AS-00

(R25 not R52) I'd be surprised if all three parts aren't shot and probably the transistor took them down. Solid state devices have short failures way more often than resistors and small capacitors do.

[kasfamily]

Re: Dell Optiplex 990 psu H240AS-00

Sorry, it is R25

[SteveNielsen]

Re: Dell Optiplex 990 psu H240AS-00

At first my brain read it R52 as well.

[kasfamily]

Re: Dell Optiplex 990 psu H240AS-00

Hi SteveNielsen ! and i also read upside down

[SteveReyes]

Re: Dell Optiplex 990 psu H240AS-00

Any way to get a schematic and part numbers for these components? I'm having a rash of bad power supplies. I'll gladly buy what I need.

[stj]

Re: Dell Optiplex 990 psu H240AS-00

there is a newer version of that supply with PFC that does exactly the same thing,
the cause is the snubber cap overheating.

you will need to check the fet, the snubber cap infront of it (it needs to be 2kv, but the original is 1kv) the fuse, the fuseable resistor to it's right,
some fast rectifier diodes to it's left,
and the driver circuitry for the fet.
fet is W12NK90Z from ST
the rectifier diodes can be replaced by UF4007

and check the fan while your there.

[seanc]

Re: Dell Optiplex 990 psu H240AS-00

This isn't good news :/ I've got quite a few Optiplex 790 and 9010 USFF out there. I presume the PSU is similar, if not the same.

[stj]

Re: Dell Optiplex 990 psu H240AS-00

i think 790 is the newer one with pfc.
if it is, you will find a vertical pcb behind the fet heatsink that contains the driver circuits and a pair of electrolytics.

if those psu's fail, the 8pin driver chip will blow along with a few resistors and diodes etc.
also there are a few smd resistors under the main board that will burn.
you need to meter *everything* on the small board, and all resistors on the underside of the mainboard.

i said in another thread some time ago - these psu's are shit!
if you have working ones - upgrade the ceramic cap infront of the fet to 2kv or even 3kv before it fails!

[SteveReyes]

Re: Dell Optiplex 990 psu H240AS-00

Thanks. Are there any specific ceramic cap requirements? I was leaning toward .01uf 3kv ceramic. My originals are crispy and I can't see any writing on them at all.

I do appreciate all of your input!

[stj]

Re: Dell Optiplex 990 psu H240AS-00

let me make a call later, a friend has some spares so i can ask him the value on them.

[Rashididrus]

Re: Dell Optiplex 990 psu H240AS-00

Hi.. I also have problem with the same PSU. Can I know the value of the ceramic capasitor since I can't see clearly. What i see is 681k, 1kv.. Is it right? Appreaciate your help. Thanks.

[Doc_u]

Re: Dell Optiplex 990 psu H240AS-00

Just had one of these go bad.

I can confirm that the cap is marked 681K (top line) and 1KV. (bottom line).

The resistor appears Pink in color, bands are Red, Violet, Silver, Gold.

Is there anything special I should know when replacing this?
Does anyone know the power rating for this part?
(I'm assuming it's 1 or 2 watts).

Any chance the FET is not bad?

[lotec25]

Re: Dell Optiplex 990 psu H240AS-00

I had one of these go bad on me, same issue as above. What a hunk of crap. Lucky the listed value is here for the Ceramic Cap as mine popped into pieces.

[lotec25]

Re: Dell Optiplex 990 psu H240AS-00

Here is a pic of the cap in pieces

[darkose]

Re: Dell Optiplex 990 psu H240AS-00

H240AS-01 version has two identical capacitors at C52 and C11.

Problem is still the same.

[hed190]

Re: Dell Optiplex 990 psu H240AS-00

Hey guys,

I have 3 of these PSU to repair and i have the resistor blown, the transitor mosfet shorted and the fuse blown too, the 681k 1kv Capacitor still good but ill upgrade it to 2kv, and i will try to repair the other components, anyone know what dissipation is that resistor? 1watt? 2 watt? the value is 0,27 Ohm right?

[Mandessa]

Re: Dell Optiplex 990 psu H240AS-00

Hello hed190 I live in the south-east asia country,I have an experience to share with for the H240AS-00 PSU from Dell. For the problem you've asked ,You should also check some components on the board locate on P3xxA Connector behind the Q51(STW12NK09Z). From my experience,components on this board usually damage after the short. I use 470pf 2KV or 680pf 2KV (Blue one) instead of the old one.For R25,I use any value from 0.1 ohm to 0.5 ohm 1W wire wound or metal film type.

[nikgegr]

Re: Dell Optiplex 990 psu H240AS-00

Hi Guys same problem here also resistor R55 is blown anyone knows the specs for this ?