[solved] 820-3115 no backlight after liquid damage

Status
Not open for further replies.

spakhnyuk

New member
The board was liquid damaged

I wasn't getting voltage at the fuse, replaced U9701 and got voltage. Now it appears the circuit is not being boosted to drive the backlight.
I also replaced 0910


I'm getting 14.3V on PPVOUT_SW_LCDBKLT on the LVDS connector but no backlight.
I'm guessing there is no boost?
Diode mode measurement on PPVOUT_SW_LCDBKLT is 0.608
SW via is blown?
How do check the SW traces?
 

larossmann

Administrator
Staff member
You have teeny tiny kinda sorta boost. What voltage do you get on BKL_PWM? If you shove PP3V3_S0 on BKL_PWM, do you have backlight? Actually, let's not get ahead here, answer question #1 first before question #2 :)
 

larossmann

Administrator
Staff member
This is odd. BKL_EN 2.7v?

You check the SW trace by removing the LP8550 and testing continuity of the two switch pads to pin 2 of L9701. It sucks, a waste of an lp8550. :(

but that shouldn't cause this low 14.3v. Is it consistent 14.3v, or is it a spike that then slowly diminishes to 1.2v over time? The latter can mean issue between Q9706 and L9701 - this is a common path to corrode, and that is what will happen when the via inside the board blows.

Keep the meter there and tell me if it is a consistent 14.3v on backlight output, and also, BKL_EN voltage please! :)
 

spakhnyuk

New member
Removed the LP8550, I have continuity from PPBUS_SW_LCDBKLT_PWR_SW (pin 6 and 7) to pin L9701.
Does that mean the SW trace is good?
 

larossmann

Administrator
Staff member
What aprendiz said. Next is feedback missing.

12.6v means no boosting.No break in the line, but no boosting.

It would help to see what this looks like.
 

larossmann

Administrator
Staff member
Push the chip down onto the board hard before applying heat,to make sure it is pressed on all the right pads. If it is floating on one side it won't go on properly. Using flux?
 

spakhnyuk

New member
Yes, using Amtech 559. I removed the LP8550 yesterday and replaced it and the problem was solved. Backlight now works! I'm waiting on a 4mm nozzle for the my Weller WHA900. Reflow with Chinese air stations sucks!

Thanks Louis and aprendiz!
 

larossmann

Administrator
Staff member
I'm gonna close this before duke says what he uses as a hot air station and you guys get any ideas. :p ;)

I'm glad this worked!!
 
Status
Not open for further replies.
Top