Gateway LP2407 Power Board dead

[bryanchan]

Hi Guys,
I went dumpster diving yesterday and came home with two large LCD monitors, one of which was the LP2407. I pulled the power board and found a bunch of caps bulging so I went ahead and replaced them. After plugging it in, I still got nothing. I looked in some other threads and based on the info there, have come to determine that there's no voltage coming out of the board at JP4. Everything pin shows no voltage. I checked the fuse and it seems fine. I was wondering where I should start looking.

It looks like someone has already replace R874 and R872. I have replaced C884, C865, C870, C869, C866, C864.

Are there some test points in between that would help me narrow down the culprit?

Thanks in advance!

[bryanchan]

Re: Gateway LP2407 Power Board dead

By the way, I do get 159V at the points across C854.

[bryanchan]

Re: Gateway LP2407 Power Board dead

Now that I look at it, T851, the transformer in the top left of the board (component side pic) looks to have an open secondary winding. Is this typical?

[bryanchan]

Re: Gateway LP2407 Power Board dead

Okay, so this is what I've got.

Red to Green: .5 ohms
Green to Blue: 2.3M
Blue to Purple: Open
Purple to Red: Open

That should confirm that the transformer is dead, right? If so, does anyone know where I can get a replacement?

It has this written on the side.
CHK-078
DN 0639 A

Thanks in advance again.

[budm]

Re: Gateway LP2407 Power Board dead

T851 is the PFC Voltage booster inductor, it only has one winding, if the PFC Voltage booster is working you will get about 400VDC on the two legs of the main filter cap.
We need to find out which pins are what for that white connector that goes to the logic board.
What dcV do you get at each pin of that white connector? All show 0V?
At this point you have to have at least the standby power supply working, without that nothing else will matter at this point.
"It looks like someone has already replace R874 and R872" The pictures are too out of focus to see.
That PCB with 11 pins sitting vertically is the heart of the power supply, it probably contain circuit for the PFC and the circuit for the standby/main supply.

[bryanchan]

Re: Gateway LP2407 Power Board dead

Yes, all of them show 0VDC. So, since I'm getting only 159V at the main filter cap, I have something wrong upstream of that, I would presume? I'm heading out to dinner, so I won't be back to reply for a couple hours.

[bryanchan]

Re: Gateway LP2407 Power Board dead

I based that on the flux residue on the backside of the board. Maybe this pic will be more in focus. I had the aperture set too low.

[retiredcaps]

Re: Gateway LP2407 Power Board dead

If 5V standby is missing, check that little stand up board just above the large filter capacitor. edit: Oops, I see budm edited his post to say the same thing.

Look at the pwc IC on that board, find its datasheet and see which pin is VCC. The voltage on VCC probably has to be in the 16V range for it to start.

edit: PS. Listening to George Strait's The Cowboy Rides Away CD right now.

[bryanchan]

Re: Gateway LP2407 Power Board dead

I measured from Gnd to VDD and it measured 15.98VDC. From the SG6902 datasheet minimum is 15V and typical is 16, so I'm well within the requirements. Are y'all suggesting that the PFC chip is bad?

[budm]

Re: Gateway LP2407 Power Board dead

The PFC does not have to be functioning for the standby power supply to work.
Check R853 (Source resistor for power MOSFET Q850 which drive the power transformer T850) to see if it shows < 1 Ohm.

[bryanchan]

Re: Gateway LP2407 Power Board dead

In circuit, it measures .3 Ohms.

[budm]

Re: Gateway LP2407 Power Board dead

OK, just run the power supply board by itself without having it connected to other boards and see if you will get any DCV on the connectors.

[bryanchan]

Re: Gateway LP2407 Power Board dead

Oh, I should have mentioned that too. The board has been disconnected for all of these tests/measurements.

[budm]

Re: Gateway LP2407 Power Board dead

Well, it will need more indepth troubleshooting start by tracing out the circuit to get some Voltage test points.
I see two small brown electrolytics cap next to the Vertical board, they are more likely to be for startup/running caps. You can try heating around that area with hair dryer to see if the standby power supply will start up. The hard part is that vertical board which is hard to probe and trace out the connections.

[bryanchan]

Re: Gateway LP2407 Power Board dead

I just noticed a discolored spot, the resistor has a brown/black/brown stripe which should be a 100 ohm resistor and it measures as such. Would this be indicative of any known problems? Should i just replace all the rest of the caps or is this something bigger than just a capacitor problem?

I also noticed on the inverter(?) board that the two caps on there are leaking as well. I know this shouldn't effect standby power with the power board disconnected. Just thought that'd be something good to know.

Thanks again for your help!

[selldoor]

Re: Gateway LP2407 Power Board dead

Are you aware that the caps have polarity? or perhaps someone pre working it did not?

In your picture in post 7 the cap in the bottom left corner is/appears to be the wrong way round. If so perhaps check all the others.

[bryanchan]

Re: Gateway LP2407 Power Board dead

Selldoor, you are right, that cap is in backwards. I was aware they have polarity, but I mistakenly put it in backwards. Thanks for that catch.

[selldoor]

Re: Gateway LP2407 Power Board dead

It will probably need replacing then got any spares.