[SOLVED]820-3437 missing backlight

2informaticos

Administrator
Staff member
There is pulse width modulation (PWM) signal, not pure DC voltage.
Voltmeter in DC range will make an integration of that signal and show some kind of medium value; depending of the duty cycle.
Check resistance to ground on both pins of D7701.
Liquid signs on that area?
 

MDY

New member
Of course I dont have short on D7701 as LCDBKLT - 8.57V = PPBUS means -> No short
I did PRAM reset
No liquide damge board.
 

dukefawks

Administrator
PWM seems too low. Remove R7704 and connect pin 3 directly to 3V somewhere and see if BKL works at full brightness. If not the LP8550 is suspect
 

MDY

New member
PWM now is 3.2V and also replaced LP8550 and still no backlight.

Compared with another board isen4, isen5, isen6 are 0L to ground in diode mode, so I miss these 3 RETURNS.
Is that enought to miss backlight?
 

dukefawks

Administrator
That could only happen if R7720-22 are bad or the trace. Most likely is a badly soldered LP8550 of course.
 
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