DELL Optiplex 760 power supply issue

[jazzman72]

Hello friends,

I have an issue with this power supply. There is no output voltage on the standby section, the input section is fine, the bridge is fine and have stable 165 vdc on the bulk cap. While checking the stuff I saw IC501 along with the diode 532 (close to it) gets damaged (look at the picture, the one close to stb transformer). it's a 7 pin ic. I think this guy drives one of the mosfet attached to the heat sink. it looks as LinkSwitch-XT series IC but can't be sure as the top surface is destroyed. Any help would be appreciated. Please find attached.

[CapLeaker]

Re: DELL Optiplex 760 power supply issue

Usually the blown off lid pieces should be floating around inside the case. If you don’t have them, I suggest you carefully take that IC out and don’t damage it any further. Carefully look at it with a microscope and maybe we can piece together what was in there. Maybe there is a picture of it somewhere floating around too… other than that, we need a picture of the backside of the board and go by pin configuration of this IC.

[CapLeaker]

Re: DELL Optiplex 760 power supply issue

I looked a bit around and found a few pics of an Dell Optiplex 780 PSU and it uses a TNY280PN.
Psu model: H235P-00
I think the 760 and the 780 use the same PSU?

[jazzman72]

Re: DELL Optiplex 760 power supply issue

I ordered 10 of them already. Looking at the datasheet and pins soldered on the pcb. I think that's the right one. Thanks

[jazzman72]

Re: DELL Optiplex 760 power supply issue

Good afternoon,

I replaced the broken IC501 with a new one (TNY280PN). Now, I have 170vdc between source and drain and 13vdc between source and EN/UV pin. Does this 13v look normal? Not sure if the diode 502 is being replaced with compatible one since was burned out. Anyways, there is no switching coming out from this IC. I have no voltage on secondary side of the STBY transformer. Any clue what else to check?

@CapLeaker this PSU Model is: L235P-01

Also, I found the attached resistor open, it's right after the main cap on the positive track. If I am not mistaken it's 1 ohm, 1/2 watt. After replacement I measured 170 vdc between the source and drain.

[CapLeaker]

Re: DELL Optiplex 760 power supply issue

You say a damaged diode was replaced… what diode was in there first place, which diode are we talking about and what is it’s function in a circuit? Your pic in the first post is too dark to see anything. Resistor is 1ohm, I read brown , black, gold, gold.
I’d say your 13v must be quite a bit away from what should be on that enable pin.

[jazzman72]

Re: DELL Optiplex 760 power supply issue

Thanks bro,

this diode has nothing to do with EN/UV pin. See my drawing. I think there is something wrong with those three smd 1.5 megaOhm resistors. Sometimes I got 1.5 mega, sometimes 6, sometimes nothing (over limit). Also, is the brown smd component a resistor or capacitor?

[CapLeaker]

Re: DELL Optiplex 760 power supply issue

Take the resistors out and check them. However a higher ohm rating than spec suggests less voltage on the enable pin. Did you check that zener? What brown component are you talking about?

[jazzman72]

Re: DELL Optiplex 760 power supply issue

will do later this afternoon. will keep you posted.
All zeners are fine. I am talking about the brown one near to 4.7 (470) ohm smd resistor.

[CapLeaker]

Re: DELL Optiplex 760 power supply issue

Rereading this, I don’t think there is anything wrong with these resistors. I think you are charging up an capacitor. That’s why you can’t measure anything after a while. Makes sense, the 13v got to come somewhere. I hate these small form factor PSU’s. Start scraping away all that glue / silicone, maybe there are more parts covered in this stuff. Cording to the data sheet, you should have have like 2.2v on that enable pin not 13.

[jazzman72]

Re: DELL Optiplex 760 power supply issue

No. Resistors are fine but this very small voltage regulator diode Y4W (3 pin diode) near to EN/UV pin has no diode reading on it. its cathode (middle pin) is directly connected to the enable pin of IC. I tried all possible combinations. I am sure is dead. When it was removed from pcb, the measured voltage on En/Uv pin higher up to 36vdc. While waiting this from amazon can I install some zener (let's say with range from 2.2 to 15v) to test?

[CapLeaker]

Re: DELL Optiplex 760 power supply issue

Y4W marking is a 15v zener diode. If you can’t measure anything from it, she’s bad anyway.

[jazzman72]

Re: DELL Optiplex 760 power supply issue

Tested the circuit by installing a few random zeners. With the first one was measured 15v, the second was 36v but the third one got 9v. I think everything will be fine by using a proper zener. I tested again the original one and got no reading. How to test a zener with DC power supply to see what the upper threshold voltage is?

[CapLeaker]

Re: DELL Optiplex 760 power supply issue

A zener tests like a regular diode on your DMM. To test a zener on a psu, you need to have a cc cv bench psu. Dial in like 100ma and ramp the voltage up. Remember where the cathode is in order to make a proper measurement.

[jazzman72]

Re: DELL Optiplex 760 power supply issue

I ordered some 3.3v - 36v zener diode kit from aliexpress. As a hobbyist I should have them. Will keep you posted on the progress. Once again, thanks for your help.

[R_J]

Re: DELL Optiplex 760 power supply issue

That zener on the EN/UV pin should be 15v, If that voltage was going to 36v I suspect the optocoupler may be bad, it should be connected to the EN/UV pin to regulate the output voltage.
Maximum voltage on the EN/UV pin is listed as 9v. Typical EN/UV voltage is around 2v

[jazzman72]

Re: DELL Optiplex 760 power supply issue

Yes. That makes sense. Yesterday, I measured 0v between the negative rail of the main cap and anode of the new zener. Sounds like, this opto is shorted on its hot side otherwise it's not supposed to be 0v. Will take the side panel away to see where it is.

[CapLeaker]

Re: DELL Optiplex 760 power supply issue

Yeah, but you still got that open zener to take care off

[jazzman72]

Re: DELL Optiplex 760 power supply issue

Hello friends,

Isolated the side panel and checked all optocouplers, all were found in good condition then I did another test. On my testing PCB I soldered a brand new TNY280PN. The source pins were connected directly to the negative rail and the drain one to the positive rail of the main cap then I got 1.6v between the source and enable pin. Why is that 36v on the psu's IC then? I'm still waiting for zener diode kit. Please find attached.

[jazzman72]

Re: DELL Optiplex 760 power supply issue

Updates:: Replaced the existing IC from the pcb with a new one then I have measured 2.7v across the source and EN pin. Still have no switching but for sure there is some progress on it.