Reversed polarity cap killed PSU board

[mockingbird]

I recently recapped an Acer AL2017 LCD. The monitor worked a few hours but then started clicking which I erroneously attributed to a coil. The monitor died completely afterward and there's no power. I opened it up and surely enough, I soldered a Panasonic FC 10V 220uF capacitor the wrong way.

I'm not sure where to go from here. I replaced it, but there's still no power to the screen. Something is blown. Is there a fuse I should be looking for or is it maybe the starter cap or the main primary?

Also, the Panasonic cap that was in the wrong way bulged slightly and is probably not good anymore. The other caps look ok. Do you think I should trust them or should I check their ESR?

[retiredcaps]

Re: Reversed polarity cap killed PSU board

I'm not sure where to go from here. I replaced it, but there's still no power to the screen.

If there is no green LED power light, check to see if there is 5V DC on the connector from the power/inverter to the logic board.

If there is 5V DC present on the connector, test the voltage regulators to see if they are outputting the correct voltages.

[Toasty]

Re: Reversed polarity cap killed PSU board

Photo's of yours top and foil side, please. Model number of board?

There may be one or 2 fuses depending on the PSU model. They can be the regular cylinder type in ceramic or glass. There is a reddish colored "can style" that is popular, and sometimes a "pico" fuse that can appear as a small resistor or diode. All would -usually- be marked F something on the board.

No, the bulged cap is shite. In the bin with it. The rest of the caps are probably fine.

Find the fuse(s) and check it/them with an ohmmeter. The continuity check or lowest scale is perfect.

Toast

[mockingbird]

Re: Reversed polarity cap killed PSU board

I will post a picture later hopefully. But I wanted to ask you this... I tested all the picofuses, they all have continuity, but why do they have polarity? It should show continuity regardless.

If they are defective, should I just bridge it? It will be difficult to replace these little things. I would much rather install a regular fuse somewhere else on the board if I could.

[mockingbird]

Re: Reversed polarity cap killed PSU board

There are main fuses one of which is the black vertical cylindrical one near the plug which is 3.15a 250V. It shows continuity. The next is a glass horizontal fuse in the middle of the board. Also tests OK.

Here are some pictures someone posted in an different thread, so note that it is not my board, he hadn't recapped it yet. Very high resolution:

Top Of The Board
http://www.shadowhunt.com/images/badcaps/VG2021m-1.jpg - Two main fuses. One in the corner right near the power plug. Tests OK, and one in the middle of the board to the right of the four chips. Note: The two blue things to the left and right of the power plug... Either they're MOVs or resistors, forgive my ignorance. They do not show contiunity.

Bottom Of The Board
http://www.shadowhunt.com/images/badcaps/VG2021m-2.jpg

Two more he posted:
http://www.shadowhunt.com/images/badcaps/VG2021m-3.jpg
http://www.shadowhunt.com/images/badcaps/VG2021m-4.jpg

The large green Taicon capacitor to the right is the one I replaced with Panasonic FC but reversed the polarity.

Thanks for your help.

[Toasty]

Re: Reversed polarity cap killed PSU board

See attached photo of back with info on it.

Check the fuse, measure the 2 resistors, and use diode check on the dual diode pack. Can be common cathode or anode (see markings on it). Report diode pack numbers please.

Report findings.

Toast

[mockingbird]

Re: Reversed polarity cap killed PSU board

Fuse F102 tests good. Resistors R107, R108 show 103 Ohms (Don't I have to desolder them to test them?). Dual diode tests OK (I think), shows reading of 004. Markings on it are:

STPS10H100CFP
GK09P VW
CHN 643
And it has an ST Microelectronics logo on it.

[Th3_uN1Qu3]

Re: Reversed polarity cap killed PSU board

Fuse F102 tests good. Resistors R107, R108 show 103 Ohms (Don't I have to desolder them to test them?). Dual diode tests OK (I think), shows reading of 004.

That sounds like a short to me. If it reads 004 both ways around then it's definitely shorted.

[mockingbird]

Re: Reversed polarity cap killed PSU board

I don't have a diode tester, I'm using Ohms to measure it so a low reading should be ok. The other rectifier also tests the same.

edit: I also want to make a correction. I stated in my first post that the cap I reversed was 10V 220uF. I mean to say 10V 2200uF.

[mockingbird]

Re: Reversed polarity cap killed PSU board

Also, according to the datasheet the middle pin goes to the cathode. I read ~270 for the one you asked me to check and 310 for the one that's also screwed into the heatsink next to it. If I reverse the polarity, it just keeps climbing.

[Th3_uN1Qu3]

Re: Reversed polarity cap killed PSU board

In that case the diode is good.

[Toasty]

Re: Reversed polarity cap killed PSU board

2 separate circuits. The other is not the one that was "shorted" by the reversed cap.

Readings only for the one I asked please - 004 or .004 ?

DMM with no diode check? Okay.

Try to unsolder just the center leg of the diode pack and measure it. Make sure it is not touching the pad at all. If you can't do that, you'll need to unbolt and unsolder it to measure it. If you get the same readings, then it is shorted and needs to be replaced, and is the culprit. Is there a crack on the surface where it is marked with its' numbers?

If this is good, then the problem likely is on the other side of the transformer in the driving circuit.

R107 & 108 are good. They are 2 - 200Ω resistors in parallel, so their resistance is 1/2.

Toast

[mockingbird]

Re: Reversed polarity cap killed PSU board

On the lowest Ohm setting, I get 285 today. On two KOhms I get around 1.045. 20 Kohms gets me around 3.13. On 200Kohms, around 13.5. When I set the DMM to "2M" I get the aforementioned .004. I assume these numbers are meaningless, because I read that all a Ohm test on a diode can do is check for continuity in one direction and complete resistance in the other (Which shows it's doing its job).

I had to lift the traces to get this thing out (No worries, I'll bridge after). Here are the readings:

Speaker setting: 289
2K: 1.080
20K: 3.34
200K: 18.9

Reversing polarity shows no continuance. It looks like this diode tests good. Also there are no visible signs of failure on the diode. I am assuming you did not want me to test JUST the middle leg, correct?

[Toasty]

Re: Reversed polarity cap killed PSU board

"I had to lift the traces to get this thing out" wtf!?

[mockingbird]

Re: Reversed polarity cap killed PSU board

Don't worry, it's nothing. I took the entire thing out and when I was rocking it back and forth, the traces on the underneath came loose. It's difficult for me to pull things out which have more than two legs. It's no problem. Onward. Do you have any other suggestions?

Retiredcaps suggested I check the 5VDC output to the logic board. Thankfully it is marked. The third set of pins is +5V. I'm assuming one is positive and one negative. should I check if it is outputting +5v with the board plugged in? Should I measure the 5V from the PSU or from the logic board?

[mockingbird]

Re: Reversed polarity cap killed PSU board

Does this help? I found this in the service manual:

[mockingbird]

Re: Reversed polarity cap killed PSU board

Ok, I'm done with this one. Too much time, and I think I caused more damage when I was trying to measure +5V output and touched one of the other pins. Then I DID blow that "W" fuse underneath and now I get no reading.

In the rubbish bin this one goes.

[Toasty]

Re: Reversed polarity cap killed PSU board

Shame you're not in US, I'd ask you to send it to me.

OTOH: they are going for ~$50 with s/h on Ebay.

Toast

[mockingbird]

Re: Reversed polarity cap killed PSU board

If you pay shipping you can have it. I'd love to see you get it working again, and you can tell me what was wrong. I can ship Canada Post, wouldn't cost you more than $20, possibly a lot less.

[Toasty]

Re: Reversed polarity cap killed PSU board

Okay. PM sent.