Benq FP731. Weird Protection mode. Benq 48.L8202.A00

[no1knows]

Hello everyone,

I have a set of Benq FP731 monitors. Both of them had the classic problem of going dark after a second or two. Both have a Benq 48.L8202.A00 power board. I was able to fix one by replacing all the C5707s and a fuse. I did the same repair on the second one but it is giving me problems. The upper two (right side) transformers seem to be causing some sort of overload. But its not due to a transformer(s) itself.

This is the story so far: If I only plug in the lamp cable for the upper transformers (T753 and T751, to the RIGHT in the first image) the screen stays dark. If I plug the same lamps into the lower transformers the screen will light up for 2 seconds then go black. It will then light up again for 2 more seconds and then turn off and stay off. I unsoldered the transformers and swaped their locations. The problem does NOT follow the transformers. Something that runs the upper set of transformers is causing some sort of protection mode. I swaped the whole power supply all 4 lamps work fine. I also replaced ALL the large caps.

Items I have replaced are marked in the image. (caps have also been replaced) I also re-soidered all the known problem areas for this power board.

Benq 48.L8202.A00

Thanks.

[PlainBill]

Re: Benq FP731. Weird Protection mode. Benq 48.L8202.A00

Hello everyone,

I have a set of Benq FP731 monitors. Both of them had the classic problem of going dark after a second or two. Both have a Benq 48.L8202.A00 power board. I was able to fix one by replacing all the C5707s and a fuse. I did the same repair on the second one but it is giving me problems. The upper two (right side) transformers seem to be causing some sort of overload. But its not due to a transformer(s) itself.

This is the story so far: If I only plug in the lamp cable for the upper transformers (T753 and T751, to the RIGHT in the first image) the screen stays dark. If I plug the same lamps into the lower transformers the screen will light up for 2 seconds then go black. It will then light up again for 2 more seconds and then turn off and stay off. I unsoldered the transformers and swaped their locations. The problem does NOT follow the transformers. Something that runs the upper set of transformers is causing some sort of protection mode. I swaped the whole power supply all 4 lamps work fine. I also replaced ALL the large caps.

Items I have replaced are marked in the image. (caps have also been replaced) I also re-soidered all the known problem areas for this power board.

Benq 48.L8202.A00

Thanks.

First a few words on the way the inverter works: The logic card sends the BL_ON signal to the inverter. The inverter controller turns on the backlights at full brightness. After a short delay the controller monitors the voltage and current for each CCFL. If any CCFL is out of limits the entire backlight is shut down.

Now this inverter is a slightly unusual design. Instead of an oscillator in the inverter controller generating the drive signals, the transformers and the drive transistors themselves form the oscillator.

Possible causes are a bad (or reversed) transistor. The oscillator won't start if a transistor has too low a gain. Power to the transistors is switched by a FET, if the power is not turned on, no oscillation. A broken land or a solder short can cause this. You have demonstrated it isn't a transformer, but there is a lot more involved.

The synapses aren't firing tonight, but somewhere in this sub-forum is an extensive analysis of this design. It appears in a number of monitors, but I'd suggest looking at threads involving the Dell E172FPb or E173FPb. The alternative is to swap components from the good side to the bad side until it starts to work. Even that is no guarantee of success.

Many years ago I worked in the production test area of an electronics manufacturer. It was my habit that when things got slow or I was in a good (or bad) mood I would give a 'tough dog' card to a pair of techs and challenge them to fix it. One particular card had baffled everyone for several months. The card had perhaps 2 dozen 14 and 16 pin ICs on it. Every one of them had been replaced at least TWICE. Finally, some persistent tech decided to replace every IC AGAIN. And an eagle-eyed reworker spotted a tiny fracture in a land under one of the ICs.

PlainBill

[no1knows]

Re: Benq FP731. Weird Protection mode. Benq 48.L8202.A00

First a few words on the way the inverter works: The logic card sends the BL_ON signal to the inverter. The inverter controller turns on the backlights at full brightness. After a short delay the controller monitors the voltage and current for each CCFL. If any CCFL is out of limits the entire backlight is shut down.

Now this inverter is a slightly unusual design. Instead of an oscillator in the inverter controller generating the drive signals, the transformers and the drive transistors themselves form the oscillator.

Possible causes are a bad (or reversed) transistor. The oscillator won't start if a transistor has too low a gain. Power to the transistors is switched by a FET, if the power is not turned on, no oscillation. A broken land or a solder short can cause this. You have demonstrated it isn't a transformer, but there is a lot more involved.

The synapses aren't firing tonight, but somewhere in this sub-forum is an extensive analysis of this design. It appears in a number of monitors, but I'd suggest looking at threads involving the Dell E172FPb or E173FPb. The alternative is to swap components from the good side to the bad side until it starts to work. Even that is no guarantee of success.

Many years ago I worked in the production test area of an electronics manufacturer. It was my habit that when things got slow or I was in a good (or bad) mood I would give a 'tough dog' card to a pair of techs and challenge them to fix it. One particular card had baffled everyone for several months. The card had perhaps 2 dozen 14 and 16 pin ICs on it. Every one of them had been replaced at least TWICE. Finally, some persistent tech decided to replace every IC AGAIN. And an eagle-eyed reworker spotted a tiny fracture in a land under one of the ICs.

PlainBill

As always, you were right. I re-checked my work. At some point the wires going to the lamps failed. I re-tested using a set of known good lamps and found that both inverters work fine.