Benq FP767 PS caps

[splaz]

The monitor had started to turn off every now and then for no reason, I actually thought at the time maybe it was video card related as I'd mucked around with drivers for it only recently.

Then the screen though would spend the majority of the time off and would only every now and then turn on... until over the last few days it wouldn't turn on at all... maybe a bit of blinking from the led and that's about it.

Opened her up and took out the power supply board, 2 bulged caps.



The brown one is a 1000uF/35v Elite.

Black one is a 1000uF/16v Elite.


Probably from different series, there are two more of the brown caps, look intact to me but I'll replace them as a precaution, same goes for another of the black cap.

There are a few Lelons on the board, I know they're known to be bad but none appear in anyway to be comprised I guess you'd say. Besides I don't have the right values to easily replace them with quality caps... to give you an idea, if I went to a local supplier... they'd sell me Lelon.

So they'll stay for now.

From my parts stash I'll use 1000uF/35v Panasonic FM to replace them, see if that fixes things. If not, might have to investigate further.

[Krankshaft]

A great majority of the time the failure you describe will be remedied by capacitor replacement.

My HP 1955 (Benq build) that I repaired for myself (and that I'm using now) had Elites on it they were ok (monitor had a shorted inverter transistor and a shorted On/Off MOSFET) but I replaced them anyways with Panasonic FCs so that I wouldn't have to work on it twice.

To tell you the truth I haven't come across a single inverter board that I couldn't fix.

They are WAYYY simpler then ATX PSUs .

[splaz]

Success.

Monitor is now working fine. I replaced two Elite brand I found that had vents, 470uF/16v off the top of my head with 470uF/35V Panasonic FM. There were smaller ones all over the place but didn't seem worth the effort to go all out.

I'd have some recap pics but the camera's battery is flat...

Oh and thanks again Krankshaft, you're quite the source of information.

[maverick530]

I have the exact same problem with my BenQ-built HP 1955 Power Inverter Board. The monitor does not power on at all.

Below are two pictures of my board. Notice the two burned marks on the board that I've circled in white.

I've done a bit of research on this forum already and have come across a few threads that are helpful:

http://www.badcaps.net/forum/showthread.php?t=5031
http://www.badcaps.net/forum/showthread.php?t=6374

However, I'm new at this--so far my experience with fixing monitors has been limited to swapping parts, but now I want & need to actually FIX these boards as nobody sells working power boards.

My biggest question is, you say you've never come across a board like this that can't be fixed, but can this type of problem really be fixed without having to do any work on the under side of the board?
It seems like replacing anything on the back of the board is impossible since the parts are so small and practically "printed" onto the board.
Same thing goes for the PCB's themselves. What if there is a problem INSIDE the printed circuit board?

Lastly, can someone show me (by circling on my picture) what exactly would need to be replaced? A picture speaks a thousand words. Thanks so much in advance,

[Bobdee]

Looks like you have been overlooked here Maverick, dont know why,
Post and let us know if you havent fixed problem, you probably wont have to touch those small components on underside of board to fix this one

[maverick530]

I still haven't fixed the problem; I haven't touched the board since I received no posts. That is encouraging that you think I won't have to touch the back of the board, but I'm still not positive where to begin.

[PlainBill]

Originally Posted by maverick530

I still haven't fixed the problem; I haven't touched the board since I received no posts. That is encouraging that you think I won't have to touch the back of the board, but I'm still not positive where to begin.

I'd like to help, but need more information. First of all, a picture of the back of the board would be helpful. I'd like to see what caused those two hot spots on the board.

The second piece of information is what is your level of experience? And what equipment do you have available? A DVM, soldering iron, and wire cutters are the minimum tools required.

A first step is to use a DVM and Carefully! check the voltage across C605 with the power cord plugged in and the power switch on.

PlainBill

[maverick530]

Originally Posted by PlainBill

I'd like to help, but need more information. First of all, a picture of the back of the board would be helpful. I'd like to see what caused those two hot spots on the board.
PlainBill

Here's a picture of the back of the board. I can't see any burns on this side of the board, but this should help to see what might have caused the burns on the front side.

Originally Posted by PlainBill

The second piece of information is what is your level of experience? And what equipment do you have available? A DVM, soldering iron, and wire cutters are the minimum tools required.
PlainBill

I'm new at this, but really want to learn. I'm ok with messing up a few boards in the pursuit of learning how to fix these. I have access to a DVM, soldering iron (I need to get a hotter iron, and a better tip--any suggs where to go?) and wire cutters, yes.

Originally Posted by PlainBill

A first step is to use a DVM and Carefully! check the voltage across C605 with the power cord plugged in and the power switch on.
PlainBill

Hope to test the power to this board as suggested this weekend.

[PlainBill]

Originally Posted by maverick530

Here's a picture of the back of the board. I can't see any burns on this side of the board, but this should help to see what might have caused the burns on the front side.

Hope to test the power to this board as suggested this weekend.

Huh!!! It looks like the heat was caused by the smd resistors in that area.

You might take a look at IC701 on the back; that doesn't look healthy. And as an item of curiosity, what are the numbers on the 16 pin IC on the back of the board?

PlainBill

[maverick530]

Originally Posted by PlainBill

A first step is to use a DVM and Carefully! check the voltage across C605 with the power cord plugged in and the power switch on.

PlainBill

What do you mean by "Carefully"? Careful I don't touch the high-voltage and shock myself? Or careful because there is a wrong way to probe and I might break something (either my meter or my board)? If so, what settings do I set my DMM for and what probe do I place where? Ex. do I touch the black wire on the negative pin of cap?

[Krankshaft]

Wouldn't contact with the output of the inverter circuit give you an RF burn due to the high frequency output?

[Bobdee]

sorry messed up

[jpdoe]

Great walkthrough, PlainBill.

I've always wondered which is the easiest way to test the bridge rectifier.
From reading these forums, I've gathered that it's a good idea to desolder one terminal of a diode in order to test the bridge.
If we number the diodes on the next figure from 1 to 4, from top to bottom, which terminal should I desolder?

Thanks

[Bobdee]

Great posting Bill thanks

[PlainBill]

Originally Posted by jpdoe

Great walkthrough, PlainBill.

I've always wondered which is the easiest way to test the bridge rectifier.
From reading these forums, I've gathered that it's a good idea to desolder one terminal of a diode in order to test the bridge.
If we number the diodes on the next figure from 1 to 4, from top to bottom, which terminal should I desolder?

Thanks

In the case of the bridge in this type of power supply it isn't necessary. There is no input transformer to confuse the issue. Just do it with the cord unplugged. Measure from each terminal to each other in turn. If you see a short between any two terminals there is a problem. Then it becomes a matter of determining if the problem is the bridge or the power FET. Since it's less messy to remove the bridge, I usually do that IF NECESSARY. Even with a fuse in the circuit, a shorted diode or a shorted FET will often have visual damage.

PlainBill

[maverick530]

I re-tested the board and I miss-spoke: I DO get a GREEN LIGHT on the front panel and a small weezing noise when the monitor is "turned off" from the front panel.

What does this change?

I will test the voltages across the mother cap and the cables that go to the logic board as soon as I can. Thanks,

[PlainBill]

Originally Posted by jpdoe

Great walkthrough, PlainBill.

I've always wondered which is the easiest way to test the bridge rectifier.
From reading these forums, I've gathered that it's a good idea to desolder one terminal of a diode in order to test the bridge.
If we number the diodes on the next figure from 1 to 4, from top to bottom, which terminal should I desolder?

Thanks

To correct my earlier answer, don't bother unsoldering any leads for the first test. Measure across the center pair of leads. These are the AC input to the bridge. If they are shorted it's either a shorted bridge or another problem on the AC side. Then measure across the outer pair. This is the DC output. You may see an increasing resistance as the filter capacitor charges.

PlainBill

[chotzeny]

[IMG]BENQ LCD FP767-12 Q7C3[/IMG]

Image of trouble shhoting the Power Board
and concept of replace parts...

[EGuevarae]

Originally Posted by Bobdee

Looks like you have been overlooked here Maverick, dont know why,
Post and let us know if you havent fixed problem, you probably wont have to touch those small components on underside of board to fix this one

I didn't see this one. I normally browse only the Computer Display area.
Well, there's not a single cap bulged in the picture (as far as I can see), so either you replaced them already, they are OK, or at least they LOOK ok.
First thing first : Are you getting any AC input (fuse in the yellow/red square is not open)
Also, check if you can get voltage readings in the pico fuse (yellow) and in the bridge that should have been a pico fuse too. If the caps are all the originally installed, post the info on them, just so the info can be referred too for later reference.

[kc8adu]

moved to get better exposure.
just do the usuall benq fix.
5706/7's, fu9624,solderjoints on transformers,open fuses,and lets not forget bad caps.