820-01814 No Backlight - 55v

[2informaticos]

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

 

[bls951]

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]

What is the backlight output voltage on the working board?

 

[bls951]

50v on a working board.

 

[bls951]

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.

 

[bls951]

From the VLED test point to backlight output on the MLB I do have continuity.

 

[2informaticos]

Try PRAM reset 2-3 times...

 

[bls951]

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]

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.