LG 32LC7D No Backlight

[neilc6]

I repaired this model before with capacitor replacement but this one has a different power supply. I found one bulged 1000uF cap which I replaced.

TV powers on, flashing front green led goes steady green with no backlight.

On state skipping GND pins, all DCV
12V 12.25
3.4V 3.48
6V 6.05
19V 18.4

3.4_ON 1.52
6V 6.05
DIM 3.20
ST5V 5.09
I-B 1.72
ERR 0.03
I-C 0.07
ST5V 5.09
ON/OFF 1.98
ADC 4.98

Main filter cap 395V

Inverter board fuse - good, 24VDC on both sides

Inverter transformers pins 1-2 380-400 ohm, 1-3 780-800 ohms

Was going to replace all the caps but decided to test after replacing just a few including the bulged one.

Edit: found service manual on Elektrokanya. I-C is inverter control and I-B is inverter brightness. Added relevant schematic page.

[CapLeaker]

Re: LG 32LC7D No Backlight

sounds like the inverter isn't getting the turn on signal. I-C 0.07
I_C = Inverter control = BLon
BLon comes from the main board, goes through the PSU and over to the inverter board. Check and see if BLon is actually dead on the main board too.
If there is still I-C 0.07V on the main board, while the TV is on, then go through the test points for that Inverter Control on the main board.

[neilc6]

Re: LG 32LC7D No Backlight

Thanks capleaker. I-C is marked INV ON/OFF on the main board and is probably active high. Most likely bad bga connections on the cpu. TV is only worth $40 and a new main is $20 and it needs $10 worth of new caps. Need to decide I want to break even on it and save it from the landfill.

[CapLeaker]

Re: LG 32LC7D No Backlight

Look at these test points on the main board first. If there is indeed nothing coming from the main cpu. Try to unhook the BLon pin coming from the main board to the PSU. Then cheat a little using a 1k resistor and jumper maybe the on/off pin to I-C. Turn the tv on and see what happens. All you need is a valid active high more than 1.5V on the I-C pin.

[neilc6]

Re: LG 32LC7D No Backlight

Cut the I-C wire and put a 1K resistor from ON/OFF to I-C. Front led flashes green and goes steady as before and backlights come on but no video. At least this confirms the inverter, power supply and backlights are good.

The schematic isn't an exact match but does show how that pin is driven from the CPU.

[budm]

Re: LG 32LC7D No Backlight

Since you have backlights but no video, then it is has more than BL-ON control problem.
Do you get the 12V at T-CON fuse?
32" is not really worth much but for $20 main board, then it is not too bad.

[CapLeaker]

Re: LG 32LC7D No Backlight

most of the time if there is no BLon, the main IC is at fault.

[neilc6]

Re: LG 32LC7D No Backlight

Fixed with a new main board. Found one for $18 but will break even selling the TV after capacitor replacement.