9th Gen Dell Optiplex Micro PC motherboard repair: Short Circuit?

[Geeker007]

Hi All, I have a 9th Gen Dell Optiplex micro PC motherboard which doesn't power on. When I connect the motherboard with power adapter, it goes into safe mode and turns off. So from info I have gathered online, it looks like there is a short circuit on the motherboard. So can you guys help me with figuring out the issue?

I am new to electronics repair, so my queries could be naive but I am ready to learn.

So I have a multimeter and a solder with me. But I can try to get a variable voltage power supply with a thermal camera as well. From what I seen online, best way is to inject low voltage and check with component is getting hot. But I want to understand basics as well.

1. How to find power rail on mobo.
2. What else I can check with just multimeter? Like to check if inductors are grounded, if yes then there is short. Check if any capacitor is shorted? What else?
3. So if my multimeter is in continuity check mode, and I connect prongs to terminals of a capacitor it should not beep, right? If it is beeping there is a short?

I am attaching an image of mother board, this what I have checked as of now.

I put multimeter into 2000 resistance mode. Here are the values.

Blue Ones - 424 for two then 371 for others

White - 446

Green - 033

Yellow - 442

Orange - 451

Red - 432



Other things/measurements:

I took the voltage measurement on inductors immediately after connecting power, nothing came up, means power didn't reach till there? I checked the same thing on DC jack and it came 1.96V for while and immediately went back to .16V.

I checked short on DC jack, but ground and main is not shorted on it. I checked outer terminal with inner 3 terminals. So just to confirm, outer is ground and inner 3 are mains?



I got a hold of another laptop charger which is 19V but with different DC jack. So I would need to use DC jack of possibly culprit PSU. When I cut open the wire, it has three wires, black, white and blue. So that is live, neutral and blue is signal for which PSU is connected?
Can I simply connect black and white cables to laptop charger DC jack and keep blue wire open? Will that work?

Updated - So I went ahead and connected black and white wire. Inserted the jack and again my laptop charger also turned off immediately. So original PSU seems to be fine. I again checked short on input DC jack but no it's not shorted. There is a resistance coming when checking between outer ground and pin which between center pin and outer ground pin.
So I am again stuck. What else I can do? Could that DC jack from original power supply be bad?

Thanks, Sam

[dellxps15]

check resistance on that marked resistor to gnd. all coils are ok according to your reading.

[Geeker007]

So in continuity check mode, it is beeping. So this resistor is grounded. 00.4 value coming when checking resistance mode set at 200.

[dellxps15]

do you have lab psu ? you could inject 1 volt 3 amp and see what get hot spilling maybe some ipa or better with thermal camera.

[Geeker007]

Unfortunately I don't have thermal camera, but I can get a lab PSU and use 91% IPA, will that work?

[dellxps15]

yes it will work most of the time..... just spill some ipa close that dc connector where mosfet or caps are . if we had a boardview we could see that line where could possible go.

[SMDFlea]

yes it will work most of the time..... just spill some ipa close that dc connector where mosfet or caps are . if we had a boardview we could see that line where could possible go.

Schematic & boardview https://www.badcaps.net/forum/troubl...oard-schematic

[dellxps15]

tnx very much for the brd. what app can i use to open it ? openbv i use for mac doesnt work

[SMDFlea]

tnx very much for the brd. what app can i use to open it ? openbv i use for mac doesnt work

Allegro Free Physical Viewer, but there`s a problem, the file has a password , i tried it https://www.badcaps.net/forum/troubl...38#post1819438

[SMDFlea]

Allegro Free Physical Viewer, but there`s a problem, the file has a password , i tried it https://www.badcaps.net/forum/troubl...38#post1819438

Found a CAD boardview ,it opens ok with openboardview https://www.badcaps.net/forum/troubl...95#post3559795

[dellxps15]

it's hard to use it..... still some point give me nc or the bus stop somewhere.

ok give it a look.

+adp_19v

edit2: ok loks like it goes in every mosfet like other laptop. some go also to cpu mosfet so be carefull (i see u removed the cpu good job) so make good progress when injecting power.check if anything get hot.

[Geeker007]

Sorry for the delay in checking, was out of town. So I injected power but I don't see anything getting hot. Any other way of finding the short?

[Geeker007]

SMDFlea and dellxps15 So I was able to find the short. Thank you so much for your help. So it was a capacitor. I removed it and now short is gone. But I don’t have replacement capacitor.

Can I use the PC without that capacitor or I need to replace it. Can you please let me know its value if it is needed and I will try to find it on some donor board.

Attaching the picture of faulty capacitor highlighted in green circle.

[Geeker007]

I removed this one on the board. C178. I think it is 100uF/16V. So I don't have SMD capacitor. Will a normal Electrolytic Capacitor work here if I am able to find a one? I think this is in fan circuit. Please confirm.