A1398 820-3787-A no backligh/image

[hgoncalves]

Hi.

I have an A1398 820-3787-A no without backlight/image.

I have USB activity, I've tried a known good lcd, and I've remaded the solders of U7701 but still have no image.

J8300:
PP5VR3V3_SW_LCD = 5V
PPVOUT_S0_LCDBKLT = 0V

BKLT_EN_R = 0V

R7742 has 5 ohm and I think it should be 0 ohm. Can it be it?

Shal I replace U7701?

Can you help?

 

[2informaticos]

Be sure to test with 2015 screen.
Older screen will not give you backlight.

Anyway, should get image first.
Stop digging into backlight circuit.

What is LCD_HPD level?

 

[hgoncalves]

I've just realized that this a 820-3787 model. Sorry for the error.

LCD_HPD = 0V

Q2010:
DP_IG_A_HPD_L = 1V
DP_INT_IG_HPD = 0V

 

[2informaticos]

Then be sure to use old screen model, not 2015.

Is not possible to get image (nor backlight of course), untill HPD signal goes high.
Do you get 5V at pins 28-30 of J8300?
Also be sure R8300 is good.

 

[hgoncalves]

J8300 pins 28-30 have 5V

R8300 seams to be OK, but has 1 ohm and should be 0 ohm I think.

Shall I replace U9600?

 

[2informaticos]

Once LCD gets 5V, it should set high LCD_HPD_CONN.
You have problems with LCD, or video cable/connector.

BTW, you should check that signal in the first few seconds, when machine chimes; for pulse at least.