820-3437 backlight fashes

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a 820-3437 The voltage on the back light booster D7701 gradually drops from 33v to 8v.
the logic board turns on after 2 second turns of and than turn on ans stays on.
any suggestion?

 

[2informaticos]

Check XW7720 and FB solder under U7701...
R5490 is OK?

 

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Dear maybe it's a silly question but there is no XW7720 on the board between D7701 and L7701.

 

[2informaticos]

Is just a trace, near L7701.
It gets corroded many times...

 

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Thanks for your replay on R5490 1 and 2 have 8.6v but 3and 4 nothing

 

[2informaticos]

Did you change U7701?
Be sure it is properly soldered...

 

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no sir i haven't replace the U7701 yet
Should I do it?
it lights up when it chimes and than the voltage on D7701 gradually drops to 8.5 which is my ppbus-G3h.