[SOLVED]820-3437 - No Backlight.

gcain

New member
Liquid damaged all around the LED Driver.

I replaced it with a fresh LP8550, the old one looked like it had mud under it. Still no backlight.

Where should I be looking next?

R7780 is good.

Q7706 has 8v on 3 & 4 but nothing on 1,2,5 and 6.
 

jadao

Member
Because pin 3 of Q7706 is high (8v) it will not allow 8v on pins 1,2,5 and 6. Pin 3 should be lower than pin 4.

In order to have pin 3 lower than pin 4 the voltage divider (R7788,R7789) must operate by providing it a path to gnd.

Check first voltages on EDP_BKLT_EN and BKLT_PLT_RST_L if they are present further troubleshoot must be done.

Assuming you checked that image is present.
 

gcain

New member
BKLT_PLT_RST_L = 3.3
EDP_BKLT_EN = 3.3

R7788 & R7789 measure correct.

Image is present.
 

SMMRepair

Member
What's your diode reading on backlight output? Might be a feedback problem or a poorly soldered LP8550. Obviously check the backlight fuse as well.
 

gcain

New member
Fuse is good.

R7731::1 is about 50mV

Where should I measure backlight output?

Diode mode on D7701 = 0.507
 

PhilipC

New member
did you check R7331 and R7715? you could check continuity from A5 on u7701 to D7701 pin 2 but you'd need to remove u7701 first obviously
 

gcain

New member
R7715 is giving me low readings. I'll replace it to be sure.
R7731 is correct at 200k.

I have already replaced U7701. Should have thought to check while I was doing that... Maybe I should replace it again.
I'll try a few things first then do that if nothing else works.
 

larossmann

Administrator
Staff member
Liquid damaged all around the LED Driver.

I replaced it with a fresh LP8550, the old one looked like it had mud under it. Still no backlight.

Where should I be looking next?

R7780 is good.

Q7706 has 8v on 3 & 4 but nothing on 1,2,5 and 6.

BKLT_PLT_RST_L = 3.3
EDP_BKLT_EN = 3.3

R7788 & R7789 measure correct.

Image is present.

We cannot trouble any of the backlight boost circuit if it is not yet getting power. Q7706 is what supplies power to the backlight boost circuit, and it is outputting nothing. This must be dealt with before we touch the LED driver or anything else "after" Q7706.

P channel mosfets here will open when the voltage on the gate is lower than the voltage on the source. Q7706 needs less voltage on pin 3(gate) than what is on pin 4(source) to open. If pin 3 and pin 4 have the same voltage on them it will not open.

R7788 takes pin 4's 8.4v and puts it on pin 3. R7789 turns R7788 into a VOLTAGE DIVIDER so that the voltage on pi n3 is LOWER than the voltage on pin 4 which is required for this Q7706 to send voltage to output(pins 1/2/5/6)

If the voltage on pin 3 of Q7706 is the same as the voltage on pin 4, 1 of two things is happening.

Q7707 is not giving R7789 the path to ground it needs so that it can act as a voltage divider. This can occur from either Q7707 being bad, or LCD_BKLT_EN/BKLT_PLT_RST_L not being present.

Q7706 has a short circuit between pins and 3. This can occur if Q770 goes bad or if C7782 short circuits and turns into a wire.

If BLKT_PLT_RST_L is 3.3v and EDP_BKLT_EN are 3.3v, then that rules out those signals. So what you are left with is either a bad Q7707, bad R7789, C7782 being shorted, or Q7706 being shorted.

You said R7789 measures correctly, so this leaves Q7707 being bad, C7782 being shorted/bad, or Q7706 being bad.

Without power applied, what is the resistance between pins 4 and 3 of Q7706?

If you short R7789 pin 2 to ground, do you get a backlight?

There is no point in visiting U7701 or the BKL_EN resistors before getting power flowing through Q7706 to the circuit. BKL_EN resistors R7731 and R7715 get their power FROM q7706 so if q7706 is not opening there is nothing to even troubleshoot yet.
 

gcain

New member
Without power applied, what is the resistance between pins 4 and 3 of Q7706?

0.1 ohms.

If you short R7789 pin 2 to ground, do you get a backlight?



Oh No.

It seems I fixed the problem but blew the fuse somehow. Promise I checked it before I started working on it.

Thanks Louis. Mark solved please.
 

gcain

New member
When doing a final test to return to customer I noticed that the backlight is out again.

In fact I have two of them now.

BKLT on LVDS is measuring 8.59v. Is this the LED Driver or feed back?
Both of these had the LED Driver replaced.
 
Top