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820-01814 No Backlight - 55v

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bls951

New member
I have an 820-01814 with no backlight. The board is very clean and has no sign of liquid or anything else that is obvious.

There is 55v on PPVOUT_S0_LCDBKLT - I understand that means it's not detecting a load/LED strings from the LCD. I have changed the LCD cable with a known LCD cable, a known good working LCD, and I changed the LCD connector despite it looking good. I also checked for continuity between J8500 and D8410 and it's there.

I'm fresh out of ideas and was looking for another opinion in case I'm missing something or if someone had the same issue and maybe a solution.

Thanks for your help.
 
2informaticos

2informaticos

Administrator
Staff member
Did you test with known good and compatible LCD assembly?
Be aware of new chinese flex cables; some of them may be bad.
 
B

bls951

New member
Yes it’s an original cable. I have a complete working a1990 that I took the cable from and installed the logic board into the housing. I’m also able to verify the lcd cable and lcd for the board I’m having issues with works with a known good logic board.
 
2informaticos

2informaticos

Administrator
Staff member
Compare diode mode readings (to ground) between both boards, for all EDP_INT_ data lines.
There may be a problem with the image, not the backlight; possibly bad U9850.

5V difference is not too much.
Check the max voltage for the good board with brightness adjusted to max.

Do you get external video?
 
B

bls951

New member
Bad board

EDP_INT_AUX_N - .393
EDP_INT_AUX_P - .394
EDP_INT_ML_P<0> - .335
EDP_INT_ML_N<0> - .333
EDP_INT_ML_N<1> - .334
EDP_INT_ML_P<1> - .333
EDP_INT_ML_N<2> - .335
EDP_INT_ML_P<2> - .333
EDP_INT_ML_N<3> - .335
EDP_INT_ML_P<3> - .333

Good Board

EDP_INT_AUX_N - .394
EDP_INT_AUX_P - .394
EDP_INT_ML_P<0> - .346
EDP_INT_ML_N<0> - .346
EDP_INT_ML_N<1> - .347
EDP_INT_ML_P<1> - .345
EDP_INT_ML_N<2> - .347
EDP_INT_ML_P<2> - .347
EDP_INT_ML_N<3> - .348
EDP_INT_ML_P<3> - .345



50v is the max on a working board.

Yes I do have image, internally and externally.
 
B

bls951

New member
I tried a pram reset without success.

but what did work for me was putting the MacBook into dfu. From there, since I didn’t have to worry about data, I restored it. After the restore was finished I had backlight again.

A really strange issue but hopefully this will help someone else.

thanks for your help. I really appreciate it.
 
2informaticos

2informaticos

Administrator
Staff member
This makes sense.
We suspected no connectivity with LEDs strip at the beginning.
But in such case should get higher voltage, not 55V I think.
Thanks for solution feedback.
 
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