Benq FP71E/Q7C4 intermittent backlight

[gypsy]

The backlight on this Benq FP71E (or Q7C4) sometimes works fine for hours, then suddenly dips in brightness for about a second, followed by the backlight going off completely; other times, upon startup, the Benq logo shows dimly and the backlight cuts out immediately. The screen remains on/does not go into standby. Usually, powering it off and giving it a few good whacks on the backside fixes it (the problem does not get resolved without the whacks), not sure what that could mean. I've taken out the power board and none of the caps look bulged, should I just recap them all? Could there be another issue?

Caps are:
- two browns on the northwest side are Elite 1000uF 16V
- all other browns are Elite 470uF 35V
- two small black/blue on northeast side are Jamicon 47uF 50V
- two smaller greens on west side are Lelon 25V 100uF
- two even smaller greens on west and east side are Lelon 1 uF 50V
- one green on the east side is Lelon 0.22uF 50V
- small black/blue near two greens on southeast side is Jamicon 10uF 50V
- three larger black/blues on southeast side are Jamicon 470uF 16V
- giant Elite 120uF 450V

Pics here and backside (roughly 1MB each).

Thanks =)

[gypsy]

Noticed some gunk around Q815, Q816 and Q808, Q809 (in between each pair of transformers) on the backside.. seem to be C5707 transistors, could that be an issue as well?

Also, missed the two fatter green Lelon 1000uF 10V at C712 and C709.

[PlainBill]

Originally Posted by gypsy

Noticed some gunk around Q815, Q816 and Q808, Q809 (in between each pair of transformers) on the backside.. seem to be C5707 transistors, could that be an issue as well?

Also, missed the two fatter green Lelon 1000uF 10V at C712 and C709.

The symptoms scream of an intermittent connection, either in the inverter itself or in the wiring to the CCFLs. I'd start by resoldering the transformers in the inverter section, as well as any other points that don't look good.

As far as recapping, speaking euphemistically the present caps have a less than stellar reputation. It is unlikely they are the cause of the problem, so while you have the monitor open it would be a good opportunity to replace them with high quality caps.

PlainBill

[gypsy]

Resoldered most caps, transformers as you said, and the 4 transistors in question (after cleaning the board up a bit with 99% isopropyl alcohol), works fine and consistent now =)

Thanks for the advice, once again. Second monitor you've helped me fix =D