SparkyC
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Aug 28, 2018, 11:51 AM
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LG 42LN540V backlights not turning on
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Hi everyone,
I'm currently working on this TV and its somewhat stumped me (but I'm not giving up!)
The TV backlights won't power up. I can see the screen menus etc and LG logo if I shine a bright light on to it.
All LED strips test fine with absolutely no blown LEDs using a PSU set to 27 volts on each LED string.
Already have replaced power board and main board. Still no backlights. Measured P201 connector when TV is powered on and DRV-ON is slightly low at 2.93V and the 24V pins measure 23.3 on P201.
At L801 LED connector, when TV is powered up, voltages are Pin1=218 to 234V, P2=141-159V, P3=141-159V, P4=0
MAP3202 PWM driver IC tests OK on the pins according to datasheet. I put old power board (which I thought was defective) and exactly the same symptoms appear - no backlight - voltages at P201 identical to replaced power board. Only part I haven't replaced is the T Con board. But as I have a good (but non lit) picture, can this really be causing the LEDs not to light?
Any pointers where to look next would be very much appreciated. I am going to test continuity to the LED connector behind the screen later this week (most likely today). I was thinking this might be the cause. Not sure why I'm not getting 3V on DRV-ON at P201 and why 24V pins are low (at 23.3V). I didn't notice a fuse or something similar on the PCB connector strip which the LED strings connect to.
When my friend presented the TV to me, they said it gradually got darker and then went out completely. When I first tested LEDs, about 6 were blown across 3 strings. All strings were then replaced.
I have the service manual for the TV but this doesn't go into much detail about LED driver testing procedures (using jumper wires etc). I think I need the LG training manual for that.
Any advice would be appreciated :-)
Many thanks everyone,
SparkyC
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!! Turns out it was a broken pin on the Led connector under the screen. Soldered a bridge wire directly to board from the plug and all lit up! Time to sell the spare board on eBay!