Yet another BenQ FP71G+

[Joe Black]

I have this BenQ which had classic issue with shorted C5707s.

So I replaced all 4 C5707s and corresponding capacitors (22k160), and resoldered everything, especially inverter components.

Now, it has classic 2 seconds to black: shows image, BL shuts off, LED stays green, and I can see image with a flashlight.

I have 15,8V passing PF 801, and reaching Q805 and Q812, but NOT reaching C5707s.

The only possible issue I can think of:
One FU 9024N was shorted (Q805), and since I couldn't grab one, I replaced it with FQU 11P06, so now I have one of each on board (Q805 and Q812)


(not my board, bud identical)

[stj]

Re: Yet another BenQ FP71G+

maybe the tubes are bad.

also check the blue ceramic output caps.

[Joe Black]

Re: Yet another BenQ FP71G+

Sure thing.
So, if the tube ignites for the 2 seconds, than shuts down, it still can be bad?

[budm]

Re: Yet another BenQ FP71G+

What resistance do you get on the PF 801? >1 Ohms?

'I have 15,8V passing PF 801, and reaching Q805 and Q812, but NOT reaching C5707s." So not reaching ALL 4 C5707?
The Voltage must have reached the C5707 (2 per Oscillator) Royer oscillators setup otherwise you will not get the lamps to even flash on for seconds.
You need test lamps to help narrowing down the problem.
https://www.badcaps.net/forum/showthread.php?t=19987
BTW, if Q805 shorted out, the the Inverter in that section will run at full lamp brightness.

[Joe Black]

Re: Yet another BenQ FP71G+

What resistance do you get on the PF 801? >1 Ohms?

In my case, PF801 is a jumper, I'll post my picture later.

The Voltage must have reached the C5707 (2 per Oscillator) Royer oscillators setup otherwise you will not get the lamps to even flash on for seconds..

As usual you are correct. I think voltage gets to C5707s and drops very fast.

You need test lamps to help narrowing down the problem.

Yes, that is a handy tool. Will make one and post results.

BTW, if Q805 shorted out, the the Inverter in that section will run at full lamp brightness.

I thought it fries the C5707 when shorted, since my burned a couple of times.
Guess something else fried them.

[Joe Black]

Re: Yet another BenQ FP71G+

After some playing with BenQ, i found out this:
If I lower the brightness enough, it will work normally and stay on forever.

If I put the brightness up (somewhere around 80 out of 100) it shuts down the BL.

Is this problem with some component not withstanding higher current?

What measurement could narrow it down?

[Th3_uN1Qu3]

Re: Yet another BenQ FP71G+

You either have something in the inverter dropping out or one of the wires to the CCFLs is weak and arcs at higher current. I would test the lamps with another inverter if possible.

[retiredcaps]

Re: Yet another BenQ FP71G+

After some playing with BenQ, i found out this:
If I lower the brightness enough, it will work normally and stay on forever.

If I put the brightness up (somewhere around 80 out of 100) it shuts down the BL.

I have the same problem with this monitor. If I lower the brightness down to 35, it stays on and works indefinitely. If I up the brightness, it goes 2 seconds to black.

Given it is only a 17 inch monitor and I have several spare, I just use it like so. The amount of effort and since I already ruined/broke a ccfl in a previous attempt, once this monitor dies, it goes to the recycle center. I'll save the power and logic board.

[Joe Black]

Re: Yet another BenQ FP71G+

Given it is only a 17 inch monitor and I have several spare, I just use it like so. The amount of effort and since I already ruined/broke a ccfl in a previous attempt, once this monitor dies, it goes to the recycle center. I'll save the power and logic board.

Totally reasonable. BUT, I just love to repair electronics.

If only I knew how to