Right, so a client brought in this 32" LG TV a couple days ago, with the symptom of the picture being very dim, and allegedly dimming even further within 5-10mins.
What i've tested and/or discovered so far:
- the voltages produced by the bias generator IC on the TCON all seem to be stable and agreeing with datasheet values (AVDD, VGH, VGL etc)
- LED+ voltage is about 145v, LED- (which goes into the drain of an N-channel MOSFET) around 45v
- PWM-dimming signal seemingly stuck at 35% duty-cycle, regardless of the menu setting (no change, whether at 0 or 100)
After a quick consult with Th3_uN1Qu3, i traced the PWM signal back from the connector on the logic board, through a series resistor, back to the main processor itself. As advised, i pulled the PWM signal up to 3.3v, for testing purposes. Further discoveries:
- no change in brightness at all
- interestingly enough though, the LED- voltage was now 0v (as one might expect, for full backlight brightness)
Attached you'll find a photo of the TV in a dark room, taken with a Canon EOS 550D, f/4.0 aperture and 1/5sec exposure. And it's (still) dim as hell.
The LCD panel itself seems to be made by Heesung, model no. HC320DXN, and interestingly enough, the TCON's "spread" across the bottom edge of the panel and says Sharp on it (with their specific font).
Is this yet another case of crapped-out LEDs? But if one or more had fried, how come at least some of them still work?
A quick googling of the panel produced a few links with allegedly spare LED strips for this model - 19 diffusers visible, and with a 145v supply, that should mean two LEDs under each one (145 / 19 = 7.6; something over 3v per LED is expected).
What i've tested and/or discovered so far:
- the voltages produced by the bias generator IC on the TCON all seem to be stable and agreeing with datasheet values (AVDD, VGH, VGL etc)
- LED+ voltage is about 145v, LED- (which goes into the drain of an N-channel MOSFET) around 45v
- PWM-dimming signal seemingly stuck at 35% duty-cycle, regardless of the menu setting (no change, whether at 0 or 100)
After a quick consult with Th3_uN1Qu3, i traced the PWM signal back from the connector on the logic board, through a series resistor, back to the main processor itself. As advised, i pulled the PWM signal up to 3.3v, for testing purposes. Further discoveries:
- no change in brightness at all
- interestingly enough though, the LED- voltage was now 0v (as one might expect, for full backlight brightness)
Attached you'll find a photo of the TV in a dark room, taken with a Canon EOS 550D, f/4.0 aperture and 1/5sec exposure. And it's (still) dim as hell.
The LCD panel itself seems to be made by Heesung, model no. HC320DXN, and interestingly enough, the TCON's "spread" across the bottom edge of the panel and says Sharp on it (with their specific font).
Is this yet another case of crapped-out LEDs? But if one or more had fried, how come at least some of them still work?
A quick googling of the panel produced a few links with allegedly spare LED strips for this model - 19 diffusers visible, and with a 145v supply, that should mean two LEDs under each one (145 / 19 = 7.6; something over 3v per LED is expected).
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