820-01814-A No back light

[KC Micro Tronix]

Machine came in for no display. Quickly figured out it was no back light.
no evidence of water damage. Everything looks clean.


The back light enable is present, Back light voltage at the caps of the buck converters is 55V (PPVOUT_SO_LCDBKLT)
but there is no back light. Measured PPVOUT_S0_LCDBKLT on the plug to the LEDs on the display its self.

Tested with a tester display. Nothing.

What am I missing here? How do these work?

 

[2informaticos]

High voltage means no contact with LEDs strip inside LCD assembly.
LCD connector, video cables, or LEDs strip as suspects.
Did you test with known good assembly?

 

[KC Micro Tronix]

Yes I have tried two assemblies. I have also measured voltage on the display directly. 55v on both

The 55v measurement on the display was with the backlight unplugged.

 

[2informaticos]

Do you get 55V without LCD connected???

 

[KC Micro Tronix]

No. I use a LCD board to test with.. so it enables the back light so I can check it's output. Makes it easy when the LB is just on the bench.

 

[2informaticos]

"LCD connector, video cables, or LEDs strip as suspects."

 

[KC Micro Tronix]

LCD connector has been replaced. Tried two known good test screens.
ohm'ed out the PPVOUT_SO_LCDBKLT. Good signal through the cable.
with the LCD disconnected I diode test the LCD side of PPVOUT_SO_LCDBKLT and get 0.5v
Same as with the test screens. I pretty sure we don't have a screen or cable issue here.

Any other ideas? What did I miss?

 

[2informaticos]

You can't get 55V, unless no conection with LEDs strip!
Normal voltage is 45V approx...

 

[piernov]

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