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PPVOUT_S0_KBDBKLT voltage low

bjf

bjf

Member
Does anyone know if the PPVOUT_S0_KBDBKLT line on an 820-3536 board supposed to be in the 40V range? On a cracked-screen, non-spill board that had no other issues I have about 15.9V on PPVOUT_S0_KBDBKLT. No keyboard backlight. Tried SMC and PRAM resets. The keyboard backlight level meter is going up and down normally on the screen. Photometer is working normally. Trying to find a test backlight.
 
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dukefawks

Administrator
No idea, but it is only 4 LEDs or so. 16V does not seem too low, it should vary with turning it up and down.
 
bjf

bjf

Member
Yeah, seems adequate but I don't know what it's supposed to be. Schematic says 40V. It doesn't change when turning it up or down.
 
bjf

bjf

Member
Ok, so nothing changes using a test backlight. Strange. Boost circuit is working, at least to some extent, but no backlight.
 
bjf

bjf

Member
.485V on both sides of R7747. Keyboard backlight plugged in. Booted into system. Nothing else plugged in.

R7720 and R7721 both start around .85V and work their way up. It just measured at 1.6V after staying around .9V for a while. The voltage is always seemingly unstable too. It bounces around wherever it is. Not much, maybe plus or minus .25V.

No voltage on R7733.

Customer would like keyboard backlight fixed also. No sign of spill. Board looks clean and undamaged. Not sure why this failed.
 
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dukefawks

Administrator
Well there is no voltage to the PWM drive for the backlight, R7747. Only things on this bus are SMC and backlight chip. I wouldn't touch this any further.
 
bjf

bjf

Member
What should a PWM voltage be for something like this? I have a good 820-3536 board and I'm measuring .055V with it booted and keyboard backlight on. The .485V I measured on the bad board is weirdly much higher. Not sure what to make of this.
 
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dukefawks

Administrator
I don't know. There in nothing to this circuit besides the SMC and backlight chip. If you want to mess with it and take the risk start with the backlight chip. I know I wouldn't touch this.
 
bjf

bjf

Member
Yeah, I tried replacing U7700 after my last entry. No change. I''ll have to move on from this. Thanks.
 
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