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820-4924 no backlight

Jay

Jay

Member
By following the schematic best I could and with the help of two other posts on this forum:
- https://www.rossmanngroup.com/board...leshooting/19199-solved-820-4924-no-backlight
- https://www.rossmanngroup.com/board...leshooting/24554-solved-820-4924-no-backlight

I am stuck.

Unit POSTs, loads the OS (visible with flashlight) and passes all but one hardware test. Hardware test shows "there may be an issue with the display". Thanks Apple, helpful!

+ Tested with good 2015 display, also no backlight.
+ Measurements are taken with display connected and lid open.
+ I suspect minor liquid damage (small trace on bottom case) but board is clean, like new clean.

(Sorry, these are not in order, I kinda jumped all over the place)

R1430 = 3.34V
R8331 = 3.34V
U7700 = 3.34V pin 17 // 0V pin 2
Q7701 = 023.1mv slowly rising pin 5 // 0V all other pins
R7742 = 3.34V pin 2
PPVOUT_S0_LCDBKLT = 007.1mv on D7710, C7760-65, C8300, C8303 (these should read 53V correct?)
L7710 = 020.4mv slowly rising
Q7700 = 12.44V pin 3 // 12.6V pin 4 // 020.1 slowly rising all other pins
F7700 = 12.6V both pins
PPBUS_G3H = 12.6V
R5171 = 3.4V (SMC_LID)
J8300 = 008.2mv pin 1 // 5.1V pin 30 (connector looks good, all pins connected)
C7700 = 12.6V
R7701 = 12.6V
R7731/32 = 007.1mv with very slight variations up and down.

Any help appreciated :)
 
Jay

Jay

Member
Where would I find lp8550? Doesn't show on schematic or bv. I went over the entire board several times, it's pretty much like new.
 
Jay

Jay

Member
Continuity is ok from pins 3 and 4 to R7750-R7753 and probe points to U7700 (I see no means of measuring actual U7700 pins without removing chip).
 
Jay

Jay

Member
Ah yes... Diode mode. The one thing I've never used and am unable to figure out how to use. Maybe my meter doesn't do/support it? Click image for larger version Name: IMG_8661.JPG Views: 4 Size: 4.78 MB ID: 27394 Been meaning to ask how to do that. Only diode symbol I see is the setting I put it on to check continuity so all I get is beeping, no readout.
 
D

dukefawks

Administrator
Both of those measurements make no sense or there is something seriously wrong or are you measuring with power applied?
 
D

dukefawks

Administrator
I have no link. Probably in one of Louis's videos that you should just go and watch all of them. Google should also turn up enough hints.
 
Jay

Jay

Member
Alright measured again.
Diode mode on pin 3 = 0.000 - 0.001V
Diode mode on pin 4 = 0.001 - 0.002V
 
D

dukefawks

Administrator
I doubt that both pins would be shorted to GND.....possible but highly unlikely. Or are you measuring with power applied?
 
Jay

Jay

Member
1 - 1,171v
2 - 0L
3 - 0.857v
4 - 0.581v
5 - 0.543v
6 - 0.697v
7 - 0.696v
8 - 0.551v
9 - 0.764v
10 - 0.458v
 
J

jadao

Member
Not sure if ok, someone maybe can compare with another board, your diode mode on pin3 seems high.

R7750 measuring 0 ohms?
And resistance of R7751?
 
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