psu ac240as-01 dell Optiplex 7010 pc tower

[stj]

Re: psu ac240as-01 dell Optiplex 7010 pc tower

your seeing something connected to the resistor
if they dont look burned they are probably o.k.
trace the gate pin of the mosfet and see what is driving it - thats the main thing to check.
if it's a chip then dont be surprised to find a hole in it.

[matser]

Re: psu ac240as-01 dell Optiplex 7010 pc tower

I also do some meassuring with the working PSU (which have the same shorts)

[matser]

Re: psu ac240as-01 dell Optiplex 7010 pc tower

Sorry stj, I didn't saw your quick answer. Which resistor do you mean? How could the resistors be ok, when showing a short instead the right Ω? The chips are looking good.

[stj]

Re: psu ac240as-01 dell Optiplex 7010 pc tower

the resistors are connected to the rest of the circuit,
so when you meter them your seeing the circuit too.
to test a suspect resistor you have to lift one end first - unless it's surface-mount.
with surface-mount if they fail you can see a burn on them under the print.

[matser]

Re: psu ac240as-01 dell Optiplex 7010 pc tower

Of course I've lifted them on one side before meassuring (is also shown on the fotos). They are also lifted while meassuring the shorts I mentioned.

[stj]

Re: psu ac240as-01 dell Optiplex 7010 pc tower

r46 = 4.7ohms
so your within range

when you see a 3rd band is gold or silver it's a divider and your looking at very low resistances.

62.9ohms - blue-red-white-gold

[matser]

Re: psu ac240as-01 dell Optiplex 7010 pc tower

Today the mosfet arrived, I soldered it in, after switching on, the new mosfet also is gone and shorted

But now I want to see how the cat jumps and ordered 2 more of this mosfets ;-)

[stj]

Re: psu ac240as-01 dell Optiplex 7010 pc tower

so you didnt find the other bad bits.
trace the gate-pin back to it's source - 100% that chip or transistor is toast

[matser]

Re: psu ac240as-01 dell Optiplex 7010 pc tower

No not until now. My plan is to solder in a new mosfet and then I'll compare the bad PSU with a running PSU and do some meassuring. Optical I didn't find a burned chip.

[stj]

Re: psu ac240as-01 dell Optiplex 7010 pc tower

1st rule of switching psu design,
if you dont find all the bad parts before you power up, you blow everything again

[matser]

Re: psu ac240as-01 dell Optiplex 7010 pc tower

I have soldered in a new mosfet now and compared the bad with a good PSU with some meassuring, but can‘t find any big differences. I don‘t want to burn the new mosfet again and I‘m wondering what to check next.

[stj]

Re: psu ac240as-01 dell Optiplex 7010 pc tower

what is driving the gate pin?

[matser]

Re: psu ac240as-01 dell Optiplex 7010 pc tower

Sorry, but I‘m not sure how to find the driver. I checked also IC TNY280PN because it was mentioned in a thread as faulty at a similar PSU, but it shows the same values as on the good PSU (of course soldered out).

[stj]

Re: psu ac240as-01 dell Optiplex 7010 pc tower

the TNY is for the standby voltage

[stj]

Re: psu ac240as-01 dell Optiplex 7010 pc tower

check pin 11 of the cm6800 - see if it's shorted to ground or + and if not,
find out how it gets to the gate pin of the fet.

the other chip - cm03x, i have no idea what the hell that does or how!

[matser]

Re: psu ac240as-01 dell Optiplex 7010 pc tower

Thanks for the info stj, no short between PWM out and Gnd, Vcc or Vdc, every meassuring shows nearly the same values on bad and good PSU (meassuring in diode mode, IC CM6800 soldered in).

Also meassuring on CM03X shows no relevant differences with bad and good PSU.

Am I right, that the problem must be on the primary side of the PSU?

[stj]

Re: psu ac240as-01 dell Optiplex 7010 pc tower

yes, if the secondary was shorted the psu would just shutdown

[matser]

Re: psu ac240as-01 dell Optiplex 7010 pc tower

With meassuring I didn't find any relevant differences between the bad and the good PSU?! Could it be, that this white glue on some components gets conductive after some years? I remember that I have this in the past on a PSU of a Samsung monitor.

[stj]

Re: psu ac240as-01 dell Optiplex 7010 pc tower

the glue only causes problems if it turns brown
follow the pwm pin and see if it goes to any transistors to reach the fet.

[matser]

Re: psu ac240as-01 dell Optiplex 7010 pc tower

@glue: everything is white, so should also be ok.

I meassured once again between PVM out (CM6800), gate of blown (now new) mosfet and also all pins of the 4 transistors I found (Q100, Q101, Q102 and Q103). Every meassuring shows nearly the same values (max. 10Ω difference) between good and bad PSU. No direct connection, the lowest resistance is between PVM out and gate (40Ω - same on good and bad PSU!).

It's frustrating to find no differences at meassuring between good and bad PSU. I think I meassured all the pins on primary side to ground - nothing

I'm nearby to give up and try once again to switch on the PSU (I have another good mosfet, if it blows again).

What do you think about driving the primary side of the PSU with low voltage from an extern PSU? I think about putting in 2-5V DC at the rectifier +/- to check if there gets something hot or the extern PSU switches because of a short to ground.