[SOLVED] 820 - 4924 no backlight

[G.Beard]

Hello, I'm having trouble with no backlight. Screen shows an image with no entery sign.

Voltages:
0v on pin 1 eDP
5v pin 3 & 4 eDP
Q7700 Drain = 12v, Source = 0v (Q7700 test good, changed it anyway, no luck)
U7700 VDD pin 5 = 5v, VDDA pin 18 = 5v


Diode mode:
Measured open loop from pin 1 eDP to ground.
eDP pin 3 to 4 = 0.7v
D7710 = OK

Continuity test:
No shorts to ground found
Pin 1 to cathode D7710 = OK
Cathode D7710 to R7731 = OK
Anode D7710 to Q7700 source = OK


Only thing I can think is LP8548 / U7700 is bad?

 

[aprendiz]

Voltage on pin 30 from J8300???

 

[G.Beard]

Voltage on pin 30 from J8300???

5v with conector plugged

 

[G.Beard]

Ok, so replaced U7700. Now have 52V on pin 1 but still no backlight and and after about a minute the voltage drops to 0. Image is there (no entery sign) but still no backlight.
Any pointers?

 

[larossmann]

52v means it's not making its way to the LCD. bad LCD cable, or bad connector on the LCD end. it is creating the voltage but since it doesn't "see" the load of the screen the voltage is higher than usual. Think about how old phone systems worked - when the phone was on the hook it'd be 48 or 50 volts, when it would be off the hook it would be 6 volts or something.. point being, when there is a "load" from a complete circuit you'll see a lower voltage. backlight on any of these systems should never exceed 35-39v. If it does, the screen light isn't plugged in or is totally fucked.

 

[G.Beard]

I get you. Under load ≈ 35v, thanks..... The voltage drop off after a minute or so is the LCD driver IC performing it's intended function? (probably only deserves an answer if I'm wrong). Thanks man I'll try the cable. Not sure I'm wanting to risk testing this with a good screen though.

 

[larossmann]

voltage drop off after a minute is probably the machine noticing no activity and turning the backlight off.

 

[G.Beard]

Thanks man. Every success to you!.... Now for that cable.

 

[G.Beard]

All fully working