Benq FP71G+ Backlight (different PCB)

[dhea1]

Hi all,
Nice forum i found on google. Glad to join here.


I have a problem with my Benq FP71G+ Q7T4, batch date May 2006.

When i power it up, it's only lite up for 2 second, then off.
When i plugged it into working computer, the picture only appear 2 seconds also.

I have different board that already discussed here.
- power led indicator : OFF.

what could be the problem?






the circled components are LR3410


Best Regards,
Dee

[sam_sam_sam]

Re: Benq FP71G+ Backlight (different PCB)

Here what I would check is if you have (a ) (or some ) spare CLFs tube laying around and see which outputs
from the inverter part of the board are working and which one do not and you will have some where to start from

You ether have a bad CLF tube or a problem with one side of the inverter board

I would check LR3410 to see if they are shorted that would also be a good place to start as well

Also look for fuse and see if they are open

[dhea1]

Re: Benq FP71G+ Backlight (different PCB)

Hi,
thanks for the prompt reply, Sam.
I have resolder all of them. Now when plugged into PC, the power led stay green. But no change on the backlight. Still disappear after 2 seconds.

Is it possible to substitude the LR3410 with ordinary FET (without dumper diode inside?)


Best Regards,
Dess

[PlainBill]

Re: Benq FP71G+ Backlight (different PCB)

Hi,
thanks for the prompt reply, Sam.
I have resolder all of them. Now when plugged into PC, the power led stay green. But no change on the backlight. Still disappear after 2 seconds.

Is it possible to substitude the LR3410 with ordinary FET (without dumper diode inside?)


Best Regards,
Dess

Not necessary. Those two transistors drive both transformers. If either were bad the backlights would not come on.

If you have a spare CCFL, or an LCD panel with compatible CCFL connectors, hook that up to this inverter. If the 'test' CCFLs stay on, the problem is not the inverter.

Another approach is to disconnect the lower pair of CCFLs and see if the backlights come on. Then reconnect the lower pair and disconnect the upper pair, and see if those backlights come on. If one pair does not come on, there is a problem. By swapping connections between upper and lower pairs, you can see if the problem is a transformer or a CCFL.

PlainBill

[jetadm123]

Re: Benq FP71G+ Backlight (different PCB)

The inverter transformers (T801 and T802) are also a problem with this board. With the monitor unplugged and your meter set to resistance, try checking the secondary of each inverter transformer. Looks like the secondary pins are marked 7 and 8 on each transformer. The two readings should not differ by more than 3%.

Also, are those the original caps on your board? Bad caps can also be the cause of your problem.