[SOLVED]820-3662 No image no backlight

[smiba]

Q7706 / L7710 / Q7701 are all inactive because the system is probably not activating the backlight at all.

The system needs to "talk" to the display before it will activate the backlight, since there is no picture we can conclude the talking isn't going well
To verify this see how the BKLT_EN_R/EDP_IG_BKL_ON signal is doing. This signal is most likely low, so the backlight is not being enabled.

Since U8300's Pin 1 is LOW the display is not getting its 5V power signal, this is the issue we should look into.

How does the connector look like? Are all the pins making contact to the board?
Check if R8300 is alright

 

[Jay]

Thanks smiba,

R8300
Pin 1: 0V
Pin 2: 0V

J8300 looks good, all pins are connected properly.

To check BKLT_EN_R I need to measure pins on U7750 which I currently can't do with the meter probes I have. Should be able to check later this week though.
Using the probe point and R7756 to measure though gives me 0V

Measuring EDP_IG_BKL_ON on pin 1 of R7742 gives me 0V

 

[dukefawks]

First you need an image before even looking at the backlight circuit. Does L8300 get a pulse of 5V while powering on. Check it while powering on and it should go to 5V for a while. If it then drops off it means no LCD was detected.
Was this liquid damage? Tried an external screen?

 

[Jay]

No liquid damage reported or found. L8300 gets a split second pulse where the meter shows 0L but then 0V immediately after on both pins. An external screen receives no signal from either Thunderbolt port.

 

[TCRScircuit]

Does this even chime? Maybe you are addressing a no lcd issue when in reality you should be looking for a no brain issue.

 

[Jay]

It does chime yeah.

 

[smiba]

No video over DisplayPort (Thunderbolt) could indicate an issue with the CPU, I think even without display connected this should work, but I have not verified this myself.

Also to measure L8300 put your multimeter on Voltage mode and on a high enough range (One that can probe 5V). It shoudn't show OL as that would mean it peaks on a high voltage.

 

[dukefawks]

Inspect connector/cable and try a known good LCD assembly. 2012-14 all work for testing.

 

[Jay]

The meter figures out the range itself, it picks up 5V as well as 60V or 0.020 mV and displays it properly. That said I don't have years of experience using a meter so.. that's just my observations.
Would the Mac still chime if there's a CPU issue?

A known-good display was tried before it was shipped to me and it had the same problem. All I have is 13" 2013 displays to try, would that work? I know the techs at the shop this came from, they have the skill to properly troubleshoot so I don't doubt their KG equipment or methods. Once they believe it's board related they ship it to me, they don't tinker with it themselves. The connector and cable look good as new. The connector pins are all firmly connected to the board, the inside looks clean and ok as well.

 

[Jay]

I put the machine back together as I need some space for other repairs right now.
Aimed a bright LED light at the center of the screen and powered on. For a split second I see white lines on the screen, not visible without the LED light pointed at it. This is the same time as the chime is heard, maybe but barely before. Screenshot of a video recording I did:

 

[dukefawks]

Auto ranging meters are stupidly slow. No way to catch pulsing rails on these things.
First thing is still to test with a known good LCD. Just dig up a few cracked test LCDs. Also check for any knocked off caps/resistors in the data lines to the LCD.

 

[Jay]

KG display en route. Should get here same time as fine meter probes. To be continued.

 

[Jay]

KG display arrived, Mac is in perfect health. Lesson learned; test everything myself even if others have tested it already.
Thanks for the help along the way guys

(I don't know how to mark posts as solved, can an admin do so pls?)