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[SOLVED]820-3209 sent back - no backlight again

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ytesfay80

ytesfay80

Member
Okay I made this topic a few weeks ago and I fixed the backlight and sent it out. The customer has sent this back saying the backlight will only turn on after like 10 or times of turning the machine on and off. I have the machine back and there is no backlight and I can see when I flash a light on the screen.

This was the previous topic I created https://www.rossmanngroup.com/board...-820-3209-no-backlight-no-feedback-continuity

The backlight voltage from LCD connector shows from 7.49v to 8.36v. In diode mode Pin 2 of D9701 and C9799 show 0.599v.

To fix this board I scratched a little next to A5 feedback ball put some solder there and ran a wire to Pin 2 of D9701.

Should I try putting a new LP8550 on the board?
 
D

dukefawks

Administrator
If LP8550 was not replaced there is probably corrosion under it. Replace and make sure feedback trace is good.
 
TCRScircuit

TCRScircuit

Super Moderator
Staff member
Intermittent back light is most likely the LP8550.... Hopefully you replaced it and did'nt reuse the old one like Louis said. If you did replace, put a new one on and see if it fixes as it could have been a defective chip right from the reel.
 
ytesfay80

ytesfay80

Member
I replaced the LP8550 after the liquid damage with a new one.
I just replaced it again and I can see it is soldered correctly and the feedback ball looks good.
Before I put the chip on there was continuity from the feedback pad to its traces that I scratched so I just put the new LP8550 on the board and I ran a wire from the trace of the A5 fb ball to Pin 2 of D9701. When I plug it in and measure pins 3/4 of LCD Connector I still get 8.35 and diode output is 0.591v.

Also, BKL_EN on its probe point measured around 2.78v.

Update: I believe the issue here is BKL_PWM. I checked the voltage on it and it was showing around 0.14v to 0.80v.
I then checked the resistance with no power and it showed around 1 Mega ohm so I replaced R9704/C9704. Now it is showing 0v so I ran a wire from BKL_EN(R9715) to BKL_PWM(R9704/C9704) then both of them showed 0v and in diode mode I get a 0v drop to ground on Pin 2 of R9704.

New measurements:
Diode mode (D9701) - 0.52v
R9704(BKL_PWM) - 0v
Voltage mode Pins 3/4 LCD connector - 0v
L9701 (1/2) - 0v
U9000(Pin 1) - 3.21v
 
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ytesfay80

ytesfay80

Member
Yes. my new measurements were with LCD connected and screen open which is also how I got 3.2v on Pin 1 of the U9000 chip.

Resistance to ground on R9704 shows around 0.5ohms
 
ytesfay80

ytesfay80

Member
How else can I fix this? I had already replaced R9704 and C9704 from a donor. Could the U1800 Chip be pulling this down or the LP8550 again?
 
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TCRScircuit

TCRScircuit

Super Moderator
Staff member
Is there signs of liquid near the PCH? The LP8550 is 2 bucks. Its worth a try as a diagnosis step. I doubt the laptop would boot if the PCH was screwed up.
 
ytesfay80

ytesfay80

Member
There doesn't look to be liquid damage by the pch.
I removed the LP8550 and the short was still there was so I removed R9704 and C9704 and the short was gone. I will put a new LP8550 and replace those and see if that works.
 
ytesfay80

ytesfay80

Member
I'm not sure which component was causing the short but before I removed them there was a short to ground on both sides of R9704 and C9704. Now there is no short.
With the LP8550 and R9704/C9704 removed from the board Pin 1 of R9704 shows 0L in diode mode and Pin 2 shows 0.55v in diode mode.
 
TCRScircuit

TCRScircuit

Super Moderator
Staff member
What is the voltage on LCD_BKLT_PWM? I believe the PCH generates that signal, If it is missing, my guess would be a bad or shorted PCH. Did you ultrasonic the board? If there is corrosion under the PCH that could explain this. When you measure voltage, use the probe point on the board and see what you get. But before jumping to the PCH, double check the entire board and see if there are any signs of damage such as burned or red probe points.
 
ytesfay80

ytesfay80

Member
Okay so I put another new LP8550 and replaced R9704/C9704. I plugged in the screen and measured BKL_PWM and it was back at 3.24v but I still had 0v on Backlight output.
BKL_EN was 0v which is why I figure there is no voltage on backlight output. BKL_EN measured 0.54v in diode mode.

R9731/R9715 weren't measuring the proper resistance so I just replaced both of them but I still get 0v on BKL_EN and on Pins 3/4 LCD connector.

The probe point (Probe 957) that goes to R9731 was barely visible so I tried to add some solder to it and hopefully bring it back but now it's completely gone. Is that probe point critical to where I need to run a wire for it?

Resistance to GND - BKL_EN - 100k ohms
 
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ytesfay80

ytesfay80

Member
There was no voltage on any pins of Q9706 and there was voltage on Pins 2/5 of Q9707 so I checked the fuse and it was blown. I replaced the fuse and now the backlight is working.

Thanks for help guys.
 
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