BenQ FP71G+ inverter problem (possibly).

[Helios]

My BenQ FP71G seems to work perfectly, that is,when not connected to PC, green light up and after a few moments turns to amber EXCEPT that the panel remains totally black. I can sometimes detect a very faint white ghost when the light goes from green to amber. When I connect the monitor to the PC, same behaviour; my PC log (I am using Linux) tells me that the PC recognizes the monitor and no video errors afterwards.

I opened the back panel and I realized that I have the 48.L8302.A30 board. I disconnected the connectors, four of them, to the backlights and when I power on the monitor, my voltmeter gives me 0 V DC or AC on these connectors. So it seems that it is an inverter problem. Now, I inspected carefully the board, there is no sign of damaged components.

What would I have check to point me to the real source of this problem.

Thanks for your attention.


Helios

[sabre504]

Re: BenQ FP71G+ inverter problem (possibly).

Transistors in RED check for shorts

Transformers in BLUE resolder all legs on all 4

Pico fuse in GREEN check and replace

Then retry your board

But you should upload picture of your board back and front so others can see it as there maybe something else wrong that can be spotted

[PlainBill]

Re: BenQ FP71G+ inverter problem (possibly).

My BenQ FP71G seems to work perfectly, that is,when not connected to PC, green light up and after a few moments turns to amber EXCEPT that the panel remains totally black. I can sometimes detect a very faint white ghost when the light goes from green to amber. When I connect the monitor to the PC, same behaviour; my PC log (I am using Linux) tells me that the PC recognizes the monitor and no video errors afterwards.

I opened the back panel and I realized that I have the 48.L8302.A30 board. I disconnected the connectors, four of them, to the backlights and when I power on the monitor, my voltmeter gives me 0 V DC or AC on these connectors. So it seems that it is an inverter problem. Now, I inspected carefully the board, there is no sign of damaged components.

What would I have check to point me to the real source of this problem.

Thanks for your attention.


Helios

The "very faint white ghost" may indicate the inverter is starting up, then shutting down due to an error condition - possibly a broken wire to a CCFL. Or there could be other causes.

Also, it is not recommended you try to measure the voltage out of the inverter. It can run well over 1000 volts at about 40 Khz. While the current is low enough it probably won't kill you, the DMM may not be so lucky.

We don't have a library of pictures of all (or any) revisions of power supply boards. Standard practice is to ask the person to supply the pictures. We prefer one of the top - taken from directly above the board - and another of the bottom. Try to get the resolution as close to the maximum of 2000 x 2000 as practical. Attach them using 'Manage Attachments', below the text entry area. Also note the links in my signature for other hints on taking the pictures.

PlainBill

[Helios]

Re: BenQ FP71G+ inverter problem (possibly).

We don't have a library of pictures of all (or any) revisions of power supply boards. Standard practice is to ask the person to supply the pictures...
PlainBill

Sorry, I am new on the site, forgive me. I uploaded the two pics of the board.

This morning I checked again in a black room, and now for a very brief moment I see the BenQ logo when powering up the monitor. If I power down the monitor with the switch (LED green/amber), and at once power on, I cannot discern the logo. But if I wait some time (20 seconds and more), I see the logo again.

To check for the transistors, do I have to desolder them from the board ? Some with the pico fuse ( I guess that it is the PF801 on my board.)


Helios

[sabre504]

Re: BenQ FP71G+ inverter problem (possibly).

Ok do you have a multimeter and know how to use it .

These parts do not have to be removed to test

Depending on multimeter you have will determine which tests will be advised

[Helios]

Re: BenQ FP71G+ inverter problem (possibly).

Ok do you have a multimeter and know how to use it

I have one and know how to use it. I saw a thread how to test transistors in circuit for shorts and the pico fuse , and another for checking transformers. Will be doing that and come back later.

Thanks


Helios

[petersdrz]

Re: BenQ FP71G+ inverter problem (possibly).

sabre504 is absolutly correct, but i would just like to add if the fuse at pf801 is blown thats usually because one or both the fet's are shorted located at q805 & q812 they must be replaced as a pair even if one is ok as with the 2sc5707 transistors you must replace all four, you can get the full kit including the fuse for about $6 USD (plus postage) on ebay, I live in Australia but have my parts sent from the U.S (cheaper) I can post a link to the kit on ebay if you want. Also this problem can sometimes be caused by secondary caps but only in about 10% of cases (they can also be purchased as a kit)

[Helios]

Re: BenQ FP71G+ inverter problem (possibly).

The pf801 is indeed blown. I can't detect shorts for the Q805 and Q812. But both the Q808 and Q809 are shorted (between BE and CE); Q815 and Q816 seem to test good, that is no shorts.

So sabre504 I think I would need your ebay link, if you don't mind.


Helios

[sabre504]

Re: BenQ FP71G+ inverter problem (possibly).

Well depending on what part of the world you are in depends on where you get a kit from .

But if you search on ebay put in 2sc5707 and it will bring up kits if you are in the UK i would recomend using semtech 1 a supplier i use a lot and trust there are probably loads more but that one i know well
Item number: 290306640904

[Helios]

Re: BenQ FP71G+ inverter problem (possibly).

Sorry, I should have said that I am in Canada (Quebec).


Helios

[sabre504]

Re: BenQ FP71G+ inverter problem (possibly).

Item num 320717660332 is the closest i can find to you .

But Retiredcaps or one of the other members may know somewhere better or another supplier

[Helios]

Re: BenQ FP71G+ inverter problem (possibly).

Just received the kit from coreweave; 7$ CND with free shipping. Replaced the transistors and the fuse, and all is just fine now!


Thanks a lot guys!

Helios

[3DiM]

Re: BenQ FP71G+ inverter problem (possibly).

Hello everyone!
I'm new guy here.

I have a problem with this kind of model (Benq FP71G+)

My problem it was: the green led (button ON/OFF) it was ON but the image nothing.
Ok, I open it and I saw 4 swollen capacitors and 2 transistors C5707 are shorted.

I changed:

4 Transistors C5707
2 Capacitors 0.22nF/120V
all electrolytic capacitors from the board
and the fuse 3A


My problem is:

when i plug the cable supply, my display remain white, after that when I push the power button i see the logo benq and check the signal....

what i must check for the first stage (with the white display)

I hope to give me some advice.

Thanks!

[3DiM]

Re: BenQ FP71G+ inverter problem (possibly).

This is my photos

[retiredcaps]

Re: BenQ FP71G+ inverter problem (possibly).

I changed:

4 Transistors C5707
2 Capacitors 0.22nF/120V
all electrolytic capacitors from the board
and the fuse 3A

Resolder all your inverter transformer pins.

My problem is:

when i plug the cable supply, my display remain white, after that when I push the power button i see the logo benq and check the signal....

Clean and reseat all your internal cables.

[Rtech]

Re: BenQ FP71G+ inverter problem (possibly).

There are 2 other transistors which physically look the same as the 5707's, they are probably FUs ??,the one associated with the failed 5707's you replaced, has probably gone short cct as well.

[Th3_uN1Qu3]

Re: BenQ FP71G+ inverter problem (possibly).

Basically your inverter does not turn off anymore hence the white screen when you plug it in. I know someone with the exact same problem. You'll have to trace the power on signal going to the inverter. Do you have the service manual?

[Rtech]

Re: BenQ FP71G+ inverter problem (possibly).

Basically your inverter does not turn off anymore hence the white screen when you plug it in. I know someone with the exact same problem. You'll have to trace the power on signal going to the inverter. Do you have the service manual?

He has implied that after the white screen,pressing on button brings normal operation... so will probably be the mostfet indicated, it is a common fault occuring quite regularly when eiher pair of 5705s has to be changed.

[budm]

Re: BenQ FP71G+ inverter problem (possibly).

There are two MOSFET's that supply the variable DC (depends on the brightness) to the Royer Oscillator output circuits, I can see one of them as Q812, another one I can not see (it is on the top center of the board), they are driven by the PWM IC. check those two MOSFET's to see if they are shorted which will cause the Output drive circuits to be on all the time.

[Nocta]

Re: BenQ FP71G+ inverter problem (possibly).

Are you sure the ribbon cable is well connected? I mean, are you sure that's the way and it's not connected the other way about it?