LN46A750r1F no backlight

[paulstef]

Hi I 'm looking for a service manual or schematic or similar for this TV.

All voltages, 5V, 12V etc are there but no backlight. From the mainboard there is a signal coming which is called PWM_OUT, I'm assuming this to be the signal; that controls the backlight intensity? The signal is only 100mV or so with a 50% duty cycle or so.

From the PSU then goes a cable containing signals FB and LD.
FB sounds like feedback but what is LD? FB stays 0V all the time, LD is 0V when TV is off and just under 10V when TV is on.

[budm]

Re: LN46A750r1F no backlight

100mVDC? PWM for the dimming is 0~5V pulse around 200~300 Hz so at the 50% pulse width, the average DC will be 2.5VDC, minimum pulse with to keep the backlights on is 10% (max is 90%). so 100mVDC will cause the backlights to stay off.

LD = lamp detect (if lamp is not connected or will not stay lit), 10~11VDC = OK, 0V = BAD, more likely one of the lamp is bad.
FB = Feed back AC Voltage (mV)range for maintaining constant brightness.
See these videos:
https://www.youtube.com/watch?v=oE6K...ature=youtu.be
https://www.youtube.com/watch?v=8oZnSleucJ0

[paulstef]

Re: LN46A750r1F no backlight

Ok, disconnected PWM_OUT and left it floating, backlight turns on and picture is also there. Seems to be a problem of the circuitry driving the PWM_OUT signal on the mainboard.. :-/

[paulstef]

Re: LN46A750r1F no backlight

LD = lamp detect (if lamp is not connected or will not stay lit), 10~11VDC = OK, 0V = BAD, more likely one of the lamp is bad.
FB = Feed back AC Voltage (mV)range for maintaining constant brightness.

Thanks for that useful info!

[budm]

Re: LN46A750r1F no backlight

Good, lucky you do not have lamp problem.

[paulstef]

Re: LN46A750r1F no backlight

Just in case anybody else has the problem.

The marked capacitor had a weird almost-short, it's the bypass capacitor for the IC right next to it. The PWM_OUT signal goes through that chip.

Problem solved.

[budm]

Re: LN46A750r1F no backlight

Great, thank you very much for finding the root cause, that MLCC (Multi Layer Ceramic Capacitor) failure mode is shorts or very low resistance.
I put the link here:
https://www.badcaps.net/forum/showth...039#post573039