820-00840-A no backlight

[mbcontrol]

This board came with water damage arround Q8400 and no backlight. Picture is present and it boots into the login-screen. The board was in terrible condition. I think it was a refurbished board from apple. the customer promised me that he bought an refurbished macbook and there was no repair. There was flux an solderballs an bad solder joints all over the board. So i made a video to safe the hints and did first an ultrasonic-cleaning. Normaly i make ultrasonic after repair, but at this board you could not see anything - i never saw something like this before.

I cleaned the water damage an changed, over hours of troubleshooting, the following parts:
U8400 (new chip), U8400 (from donor), R8450, R8451, R8452, R8453, R8432, C8412, C8413, C8414, C8475, Q8400, C8400, R8401, R8402. C8476, C8477, C5451, C5452, R8400, F8400

Other hints:
Backlight keyboard works and is dimmable with function keys.
PRAM-Reset don't work - tried many times with int. keyboard.
Display-Backlight always tested with original display in macbook and known good display from us.
I don't see any data signals on the oszi - only the dc voltage with 5.1v.
R8450-R8453 was good, but i changed it with new ones (you never know).

Here are the measurements:
I2C_BKLT_SCL = 5.1v (no data-signal on oszi)
I2C_BKLT_SDA = 5.1v (no data-signal on oszi)
ISNS_LCDBKLT_P = 13.061v
ISNS_LCDBKLT_N = 13.061v
PP5V_S0_BKLT_D = 5.109v
LCDBKLT_FET_DRV_R = 0v
LCDBKLT_SW = ca.115mV
PPVIN_SW_LCDBKLT_SW = ca.240mV
BKLT_SD = 12.879v
BKLT_EN_R = 3.3057v
PP5V_S0_BKLT_A = 5.1096v
BKLT_SENSE_OUT = 0v
LCDBKLT_FB = ca.250mV
PPVOUT_S0_LCDBKLT = ca.282mV
LCDBKLT_TB_XWR = ca.281mV

 

[2informaticos]

Check EDP_BKLT_PWM; changes pressing backlight buttons?

Check diode mode at backlight output, w/ and w/o LCD connected.
J8500 wasn't affected by liquid spill?

 

[mbcontrol]

EDP_BKLT_PWM changes by pressing buttons in this range: 0.17v to 3.297v
J8500 wasn't affected by liquid spill.
Diode mode PPVOUT_S0_LCDBKLT with LCD connected: 1.1646v
Diode mode PPVOUT_S0_LCDBKLT without LCD connected: 1.1704v

 

[2informaticos]

So high?
Did you check with red probe at ground?

If PWM works, then you should have activity on I2C backlight bus too.
If not, there is a problem with LCD assembly, video connector, or U8400.
Did you try another small video cable with other LCD assembly?

 

[mbcontrol]

Yes, red probe at ground, black probe at PPVOUT_S0_LCDBKLT (for example at C8464 Pin1)
What value did you expect?

I also changed now Q8401, C8460/2/3/5/6/7/8 and U2890 (had corrosion before ultrasonic).
I did a reflow of J8500 and measured every pin with the continuity tester.
What do you mean with "small video cable"? The Cable from J8500 to the display? I tested with 3-4 cables - known good ones and new ones - every one at both displays i have (original from the mb and an known good test-display.

I told you i changed U8400 two times (new chip and from donor). What do you think, shall i change it one more time?

Is it possible that the displays are not compatible? Or is there only one type existing for the A1708?

 

[2informaticos]

You may have corroded traces, or components in backlight area.
As I said, backlight control comes from LCD assembly, through I2C bus.
If you see PWM activity from PCH, then the problem is between LCD and U8400.