Dell PSU H240AS

[midibob]

Hi all,

I've been asked to look at one of these that was dead.
On opening I've found that R21 (1R) was burnt and open cct and IC21 had exploded (TNY280).
The snubber cap (C25) is original and will need upgrading to a 2KV one, as has been suggested by others.
All the main FETs and diodes appear to be OK as does the SMD components. I'm unable to find a schematic for this model to delve any deeper at present. I've removed the high voltage cap and it measures 130uF instead of 150uF although it's ESR is still very low. No bulging or leaking caps are evident but that's no guarantee they are OK of course.
I'm probably going to re-cap it anyway to get rid of all those 'elite' caps.

Does anyone have a schematic so I can investigate why IC21 blew up. I'm not comfortable just swapping parts and crossing fingers.

Cheers
Bob

[stj]

Re: Dell PSU H240AS

pretty safe to say that if there was a schem we would have found it - given how these like to go boom.

[midibob]

Re: Dell PSU H240AS

Yes, that's what I was afraid off.
It's probably going to be a case of new bits then firing it up on the rheostat and see what happens.
Oddly enough nobody has mentioned having the fault of IC21 blowing up, plenty of the main FET and snubber network plus the collateral damage that goes with it.
I'm sure I'm not the first to have it?

[stj]

Re: Dell PSU H240AS

get the datasheet on the i.c. - chances are the psu users the example schematic.

btw, when you reassemble it - make sure the fan works.
i had a few where the wire broke under the insulation from all the pulling around at the pointless crimp terminals.
replaced the crimp-pins with a regular 2.54mm fan connector on those.

[midibob]

Re: Dell PSU H240AS

Yep, that's a good idea about the fan. I did notice the wires might start to fracture with the fan flopping around.

Horrible design to work with and as you've probably guessed it's not a commercial repair so I don't mind spending my spare time on it.

[PeteS in CA]

Re: Dell PSU H240AS

Look for a small moderate value (maybe 22uF-100uF) electrolytic cap in the circuit around that Standby regulator. It is a part that is stressed some, and can leave the Standby regulator in a shut-down-and-retry loop that can cause it to get hot. A shorted rectifier on one of the outputs could cause ugly things.

[stj]

Re: Dell PSU H240AS

the problem with the standby supply is there is a lot of silicone poured over that area.
a lot of digging needs to be done - often literally!

[midibob]

Re: Dell PSU H240AS

Thanks for the replies guys.
I've now removed all the caps (after a lot of digging) and they appear to test fine both in value and ESR. Probably still re-cap anyway being as they are all out.
All mosfets and power diodes removed and they all look good too.

I think it's going to be a case of rebuild and see what happens.

[midibob]

Re: Dell PSU H240AS

Well, the plot thickens.

Firstly I noticed this...

C6 isn't on it's pads. Is this a mod or has it slipped during soldering?
I'm still trying to draw the cct out but it looks like this is on the feedback cct from the opto.

Also I've come a cross a thread about TNY280's failing for no reason in large numbers.
https://ac-dc.power.com/forum/low-po...4/tny-problem/

Those two horrible words are cropping up again together ie conterfeit and China. Let's hope not.

[stj]

Re: Dell PSU H240AS

most parts from china arent conterfeit, they are surplus from factory production lines.

[midibob]

Re: Dell PSU H240AS

Time for an update on this one.

Once the TNY280 was replaced along with it's series flameproof resistor and rebuilt, power was applied via a 40W bulb. The bulb would pulse bright and dim but the PSU wouldn't stay running. At least it looked like nothing was going to blow up. Replacing the bulb with a 100W bulb and it then ran OK with all output voltages correct.

With full power applied everything looked good and it powerd the PC up OK. I'll let it run for a while to make sure it's stable but it's a mystery why the chip exploded?

I've now got another 3 to look at so it'll be interesting to see if it's the same issue.

[stj]

Re: Dell PSU H240AS

getting interesting.

[momaka]

Re: Dell PSU H240AS

I wonder if it's the snubber network causing this. Try different loads on the 5VSB. If it blows up again, the snubber definitely might have something to do with that.

[midibob]

Re: Dell PSU H240AS

I have changed out the 1kv cap on the snubber for a 2kv as suggested in other threads so we'll see what happens after it's been running for a while.

The next one on the bench also had the 1R resistor burnt and although the TNY280 hadn't exploded this time it was measuring about 900 Ohms across most of the pins.
With a new resistor and IC it fired up and again appears to run OK. The snubber cap was upgraded too.

The last one was different and this fired up but shutdown approx 7 secs later? The series lamp also glowed slightly with no load whereas the other two didn't. Could be something on the secondary side that's going amiss so further investigation is needed. All the secondary voltages do appear to be correct though?

[momaka]

Re: Dell PSU H240AS

The last one was different and this fired up but shutdown approx 7 secs later? The series lamp also glowed slightly with no load whereas the other two didn't. Could be something on the secondary side that's going amiss so further investigation is needed. All the secondary voltages do appear to be correct though?

Sounds like something is eating power when it shouldn't.

I'd check the snubber diode first. If it is shorted, it will conduct both during the TNY IC's On and Off cycles, which can in turn make the TNY IC overheat and go into thermal shutdown protection.

[stj]

Re: Dell PSU H240AS

check ceramic caps too.

[midibob]

Re: Dell PSU H240AS

Will do. Thanks chaps. I'll report back when I've checked it out.