HP 1755 Benq Inverter Board

**Toasty** · 10-12-2009, 06:58 PM

Re: HP 1755 Benq Inverter Board

Welcome eastcoastjoe!

There are several Benq threads on the forums. Some research may reveal an answer.

You may have a bad transformer. If it shorted, it likely took out the driver transistors and the chip you're seeing sounds like a surface mounted MOSFET. Inspect the transformers. Do any look like they got hot, causing discoloring of the board? Check their soldering too, as it's a common problem.

Check the resistance of the transformers from pin to pin. Compare each reading to the other transformer(s) and see if there is any variation between them.

Check those caps. Just because they aren't bulged or leaking means nothing.

What test equipment do you have?

Clear pics of top and bottom help.

Toast
Sounds like

**Krankshaft** · 10-12-2009, 08:00 PM

Re: HP 1755 Benq Inverter Board

Yesterday I just found a guy selling the OEM Benq inverter transformers (part number 19.26039.001) for these inverters for 5 bucks each on e-bay so there's no excuse to chuck these anymore.

The transformers rarely go but if you happen to be the unlucky one at least now there's a source.

The SMT chip is the backlight on/off MOSFET. If it burn't up then most likely one of the C5707s shorted and for some reason the inverter fuse didn't blow fast enough this C5707 short sourced a buttload of current through the backlight on FET and it fried.

On my monitor the 1955 I fixed back in late 07 that I'm using now when the C5707 went short circuit the fuse blew and it also took out the FET. Just a little bubble in the package though no spectacular magic smoke show. Because the inverter fuse blew quickly (it's a fast blow fuse) and not at a time lag.

Post some pics (top and bottom of the board) I have the part numbers for the SMT part if you need it.

I'd be happy to circle the parts to check on the board.

I've helped others revive these HP Benq builds before they're really not that hard to fix.

Be sure to check for shorts on the tiny transformers as Toasty said.

**dtechguy** · 12-02-2009, 01:02 PM

Re: HP 1755 Benq Inverter Board

Hello Krankshaft. Thanks for your many posts on the BENQ 48.L1A02.A00 boards. You seem to have lots of experience with them! I myself am trying to repair the same power board from my broken HP 1955 LCD monitor. I am experiencing the same problem as in this post. I have replaced all 4 of the C5707 transistors (2 tested bad) on the board. I hooked the monitor back up. It worked fine for about 2 hours, and then smoke appeared, and it stopped working. I took the monitor back apart and found that 2 of the C5705 transistors are blown once again. I Replaced the SMD MOSFET on the reverse side and replaced the C5705 transistors once again. This time, the transistors smoked and blew right away! So, I have something that seems to be shorting them out. I did re-solder the contacts on all 4 of the transformers (19.26050.001) previously with no effect on the isue. I tested the two fuses at both PF801 and F601 with a continuity check, and they are both passing a current so don't seem to be blown. I did an Ohm check on the 4 transformers (19.26050.001) and found an inconsistency with one transformer on the inverter side that is blowing the C5707 transistors. This transformer is reading about 186 Ohms from the pins on one side (1 - 6) to the other side (7, 8)

My question is, would you suggest replacing this transformer at this point? I am not sure as to what else is shorting and frying the C5707 transistors. The only other thing I can think of is the capacitors. Please let me know if anyone has an opinion on this matter. Thanks!

**Krankshaft** · 12-03-2009, 07:50 PM

Re: HP 1755 Benq Inverter Board

At the first blowout I would have suspected a bad transformer since a bad CCFL would have just prompted a shutdown of the inverter without any parts failures.

A few shorted turns may yield a working circuit for a short while but will eventually take out the driver transistors. Probably by drawing excess current and slowly cooking them.

You probably originally had a few shorted turns that went into a full blown failure which is why the drivers fried instantly the next time.

Without a ring tester yes a resistance check will yield results if the transformer is in bad enough condition. If one resistance is way off from the others then that's the bad transformer.

Also if you can trace this transformers primary back to the 2 driver FETs that shorted then you have another confirmation.

**dtechguy** · 12-04-2009, 03:05 PM

Re: HP 1755 Benq Inverter Board

Thanks for the reply! I took your advice and replaced the transformer with a known working one. Unfortunatley, this still did not resolve the issue! The monitor is now flashing on for a second, and then flashing off repeatedly. So, it seems to be cycling on and off. There is also a few audible clicks coming from an unknown component on the power board.

After a few minutes of this "on and off" with audible clicks, the C5707 transistors popped and fried once again! What else would you suggest at this point? I do not know what keeps on shorting the transistors out! Any other parts I should try replacing? Thanks for your help!

**Toasty** · 12-04-2009, 03:24 PM

Re: HP 1755 Benq Inverter Board

Replacing the transformer and transistors without first verifying the caps IMO is the problem. Do the caps first. At least verify they're good/bad or just flat out replace them.

What about the burned chip & traces?

Still waiting for some pics of damage.

**dtechguy** · 12-09-2009, 04:23 PM

Re: HP 1755 Benq Inverter Board

Hi Toasty, thanks for the reply! I will try to take some pics of the board and post them up soon. It seems that all caps look fine physically (no leaks or buldges) But as I have read from you guys in the forum, this doesn't mean that all are functioning! I have a multimeter with a "Capacitence Check" function. It measures low capacitence levels ( < 1 micro farad I believe) I ran this on the plastic "box" capacitors in between the C5707 transistors as well as the blue disc capacitors downstream from the secondary transformers right before the CFL bulb plug. These caps seem to check out fine. I have no other test equipment besides my multimeter.

My question is, should I just go ahead and start replacing every single electrolytic capacitor on the board? Or, is there any way I can narrow down the culprit that is frying the C5707 transistors? Maybe some of the capacitors that fail more often? I am just hoping that I can find the faulty cap without having to replace every single cap on the board.

Thanks for your help!

**Toasty** · 12-10-2009, 02:37 AM

Re: HP 1755 Benq Inverter Board

"I have no other test equipment besides my multimeter."

Based on that statement, your question -

"Or, is there any way I can narrow down the culprit that is frying the C5707 transistors?"

Makes the answer to this one rather obvious, don't you think?

"My question is, should I just go ahead and start replacing every single electrolytic capacitor on the board?"

The "audible clicks" sound(s) is probably the power supply cycling/trying to start. Most likely due to a bad cap or two or 12...

Without an ESR meter, there is no way for you to test them. They may test fine on capacitance checkers, but fail on the ESR meter.

If your board looks like this one from Krankshaft's post of 2007:

https://www.badcaps.net/forum/attach...achmentid=6511

You have ~12 caps to purchase and replace. Cost should be under $20. Those and the 5707's of course, and maybe the 4431's?

You made a statement about finding "an inconsistency with one transformer" ... "is reading about 186 Ohms from the pins on one side ... to the other".

-Compared to what reading(s) on the other transformer?
-Does the replacement one now give matching readings compared to the transformer that you did -not- replace?

Toast

**dtechguy** · 12-15-2009, 03:38 PM

Re: HP 1755 Benq Inverter Board

Hi Toasty! I have tried your advice. I replaced all electrolytic capacitors on the board. It was running fine for a few days straight (I was doing an extended burn-in test on it) It worked without problems, I thought I had it fixed. BUT, I changed the brightness of the backlight as it was only set to 60% When I changed it to up around 90% or so, the C5707 transistors blew again! So, it seems that the same problem still exists.

Should I now look into replacing the plastic "box" capacitors and blue capacitors? I don't know what else could be constantly frying this board! I am about ready to give up! Any suggestion would be appreciated.

Thanks!

**Toasty** · 12-15-2009, 04:05 PM

Re: HP 1755 Benq Inverter Board

-ALL- of them blew? Or just one set?

You didn't answer the question regarding the trafo 'inconsistency' in my previous post.

Toast

**Acorlin** · 05-07-2012, 11:20 AM

Re: HP 1755 Benq Inverter Board

I also have a problem with the board pictured in--
https://www.badcaps.net/forum/attach...achmentid=6511

On the reverse side of this board is an SMD part. 8 legs marked "Q-812"
It is marked as follows.

4431
AM
L53C

To the left of the "A" on the second line is what looks like a scroll
and to the right of the "M" is a Delta symbol (triangle)

Can anyone tell me what this part is? There are two of them on the reverse of this board.
Thanks
Alan
AA6DW

**retiredcaps** · 05-07-2012, 01:38 PM

Re: HP 1755 Benq Inverter Board

Originally posted by Acorlin

4431

I believe it is AO4431

http://www.ebay.com/itm/320890581440

**budm** · 05-07-2012, 01:59 PM

Re: HP 1755 Benq Inverter Board

Did you check the value and the leakage resistance of the Royer Oicillator tuning caps ( the gray rectangular caps or blue epxoy dip by the transformers)?
You can read this thread.
https://www.badcaps.net/forum/showthread.php?t=19878

HP 1755 Benq Inverter Board

HP 1755 Benq Inverter Board

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