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820-00165 backlight flickering

R

Ryanlen2

Member
Hi.

I have a 820-00165 with backlight flickering after liquid damage on and around LP85550. I have replaced LP8550 and all surrounding components.

BKL_EN - 2.78 V and up. Not steady.

BKL_PWM - 1.473v steady

PPHV_SOSW_LCDBKLT - 24V - 26V Fluctuates.

PP5V_S0 - 5V Steady

PP3V3_s0 - 3.3V Steady

Tried 2 LP8550's

Zero damage to LCD connector or surrounding area.

Thanks
 
Last edited:
J

JohnB8812

New member
Tried known good screen as well as DC-in board/flex? BKLT_EN should be steady not up and down. Replaced both resistors? LED driver soldered properly?
 
2informaticos

2informaticos

Administrator
Staff member
If you make a short D-S on Q7706 (pins 1-6), do you get stable backlight?
If yes, check for corrosion arround Q7706/07 and R7788/89; possibly need to remove C7782.
 
R

Ryanlen2

Member
Hello.

So far I tried 3 known good screens. 2 known good DC-IN boards and cables. All 6 LCD Return resistors are around 0 Ohm. Tried 3 lp8550's.

Put steady 3V on BKL_EN just in case. XW7720 is clean and not damaged at all. Continuity between pin 2 XW2770 and pin 2 D7701 strong.

Tried shorting pin 1 - 6 Q7706. Did blanket replacement on both lp8550 area and L7701 area,

Thanks
 
R

Ryanlen2

Member
JohnB8812 Yea, this blows man!! LOL
SMS did have some damage. I initially reflowed to get working.

2informaticos


U7701


Pin1 - 0
Pin2 - 0
Pin3 - .544
Pin4 - .515
Pin5 - .564
Pin6 - .415
Pin7 - .415
Pin8 - .544
Pin9 - .548
Pin10 .0
Pin11 -. 428
Pin12 - .547
Pin13 - .531
Pin14 - .340
Pin15 - .613
Pin17 - .542
Pin18 - .461
Pin19 - .463
Pin20 - .613
Pin21 - .610
Pin22 - .609
Pin23 - .608
Pin24 - . 0
Pin25 - .613

???????Thanks Guys!
 
2informaticos

2informaticos

Administrator
Staff member
SMC is not involved in backlight flickering.
Unless SMC_LID is not stable.

Do you note the same flickers with low and high backlight level?
Did you try with known good LCD assembly?
 
R

Ryanlen2

Member
https://photos.app.goo.gl/rTQoENVAgCwB5WZy9

https://photos.app.goo.gl/PAQYbsVUyYQuk1yd8


First link with original screen. Second link with entirely different display and body.

Quality gets way worse with some theatre light effects at lower backlight level and actually completely turned off at one point. I have tried 3 different LP8550's so far so I dont think its that. Unless the entire strip that i have is bad, lol!

Yea JohnB8812 , this blows lol. I usually am closed on Monday but came in just for this... Awesome :cool:.

Thanks again!
 
2informaticos

2informaticos

Administrator
Staff member
Reflow LP8550 and gently tap it on top, to see it dancing a bit.
This will assure all solders are good.
I had something like that with bad solders; got more flickers at low backlight level.
 
R

Ryanlen2

Member
Hi.

Reflowed LP8550 and replaced with LP8550 from donor as well. Replaced LCD connector just in case even though there was zero damage.
No louck. Think Im gonna throw in the towel on this.
 
2informaticos

2informaticos

Administrator
Staff member
Do few PRAM reset before.
Also remove R7704 and run a wire from C7704 to R7715.
If still flickers, remove R7731 and run another wire from R7715 to C7711.
In this way you force both backlight controls to maxim.
 
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