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820-3662 (mbp 15)

J

jamiehorn1

Member
Hi all,

I have a 2014 MBP with no backlight, image is present with torch under screen.

Backlight Output = 12v
KG Display has been tested.

I'll be honest with no backlight, I generally replace most common component failures instead of testing as it's just quicker.

But this one doesn't want to play ball.

Components changed:

- FB Resistors (R7708/9)
- LED Driver (U7701)
- LCD Connector
- Both mosfets (Q7706 incl. resistors for v. divider) + (Q7701)

The odd thing is that on the SW lines I get 0v so Q7701 mosfet is not opening. R7703 is not blown, checking the voltage at pins 1 and 2 of LED driver I get 0.01v.

Any ideas on why that might be?

There is also no short on backlight output.

Thanks
Jamie
 
2informaticos

2informaticos

Administrator
Staff member
"The odd thing is that on the SW lines I get 0v so Q7701 mosfet is not opening."
When Q7701 is opened, it will pull down PPBUS_S0_LCDBKLT_PWR_SW.
So it will have 0V on both D and S in that moment...
However, Q7701 is not triggered if you have 12V DC on L7710/D7701.

Do a PRAM reset first.
Can you see any activity on BKLT_SCL/SDA lines (with o-scope) when pressing backlight adjust keys?

If you have 0V at BKLT_SD means U7701 receive information to control backlight somehow.
Check all resistor/traces from its area.
You can also try changing U7750.
 
J

JohnB8812

New member
What do you get in diode mode when you measure Backlight output? Sounds like possibly an issue with the feedback or switch trace.
 
J

jamiehorn1

Member
Hi both,

I have tried a PRAM reset but not since replacing the components so will try that later on and report back.
I also replaced U7750 with no luck.
SCL/SDA lines were pulsing when I checked but will double check later on.

Diode mode on backlight output is 0.750 which I believe is normal. I checked continuity on both FB and SW lines, both okay.

I'll have another look at the U7750 area and see if anything is reading incorrectly.

Thanks!
 
J

jamiehorn1

Member
I changed U7701 twice because I thought the same but no luck there either.

I did however forget to mention when I first replaced Q7706 and plugged in, pin 1 on the LCD connector completely fried along with Q7706. After this I obviously replaced the LCD connector and Q7706. It hasn't done that since but I'm not sure if that gives any hints?
 
2informaticos

2informaticos

Administrator
Staff member
Did you try the new (compatible) LCD after that?
Did you solder correct Q7701 (N-channel)?

Post voltage at pins 15, 16, 17 of U7701.
R7740 is OK?
 
J

jamiehorn1

Member
Gave up on this one in the end, I plugged the original LCD back in and it fried pin 1 (again!). I would have needed to change the LCD connector again too as that melted inside. Time to move on as I've run out of donor boards for this.

Thanks everyone!
 
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