LG 47LG50 No Backlight

[neilc6]

Initially, I found a blown fuse on the main AC input (ceramic 8A/250V). After replacement, the tv powers on with no backlight. The front LED flashes red and then turns to a very faint, barely visible blue on state. The backlights flash for a millisecond.

Inspected caps on inverter boards and none are bulging. Caps on PS are non-bulging as well though there are some Samwa's there.

In on-state, big filter caps at 371 VDC

Left Inverter Connector
24V = 25.0
BRI = 1.7
ON/OFF = 3.4
PWM-DIM = 2.8
ERROR = 0

Right Inverter Connector
24V = 24.9
BRI = 1.7
ON/OFF = 3.4

Main board Connector
BRI = 1.7
PWRON = 2.8
ERROR = 3.3 *****Problem here???***
PWM-DIM = 2.8
ON/OFF = 3.3
ACD = 5.0
5V = 5.1
12V = 12.5
16.5V = 16.7

Symptom is similar to this thread https://www.badcaps.net/forum/showthread.php?t=51391

I have touched up the joints at transformers T101, T102, MOSFET Q110, Q111, Caps C114, C125 with no improvement.

I suspect ps board (error signal, blown fuse) or inverter boards.

[budm]

Re: LG 47LG50 No Backlight

No blown fuse on either one of the inverter boards?

[neilc6]

Re: LG 47LG50 No Backlight

There are 4 white fuses on each inverter board. They all tested ok.

[dick_barton]

Re: LG 47LG50 No Backlight

Are there any brown scorch marks on the underside of the inverter board?

[budm]

Re: LG 47LG50 No Backlight

There are 4 white fuses on each inverter board. They all tested ok.

How about the small black fuse F1 with '15' (0.75A) printed on top?
Look at both boards for the black fuse.

https://www.badcaps.net/forum/attach...3&d=1508868265

[neilc6]

Re: LG 47LG50 No Backlight

Found the "15" black fuse on the master and it tests ok. Can't find one on the slave board.

Back of the boards shows slight browning behing top and bottom transformers.

[budm]

Re: LG 47LG50 No Backlight

Check the resistance of the High Voltage winding of all transformers to see if they are within 3% from each other, especially the one that has heat marking on the bottom side of the board.

[neilc6]

Re: LG 47LG50 No Backlight

Using the test points and everything disconnected. Starting from top to bottom. In Kohms.

Master - 5.72, 5.70, 5.71, 5.72, 5.71, 5.71, 5.72, 5.74, 5.72, 5.74
Slave - 5.71, 5.70, 5.74, 5.72, 5.73, 5.71, 5.71, 5.74, 5.73, 5.73

There are some black marks on connector #3 and #4 from the top on the slave board. There is continuity from the transformer pin to the connector pin. Pic attached.

[vinceroger69]

Re: LG 47LG50 No Backlight

have you got a ccfl tube tester as maybe a tube failed

[neilc6]

Re: LG 47LG50 No Backlight

have you got a ccfl tube tester as maybe a tube failed

Don't have a tube tester. If everthing points to a failed tube, then I would scrap the TV and sell the boards. Would a failed tube blow the main fuse??

[vinceroger69]

Re: LG 47LG50 No Backlight

sorry i didnt notice the blown mains fuse part, im not honestly sure maybe budm will have some more tests you can try

[dick_barton]

Re: LG 47LG50 No Backlight

If the fuse on the power supply board had not blown then I would ignore the fact that the 8A fuse had blown in the mains lead. Perhaps it's not the original lead for the set.

[dick_barton]

Re: LG 47LG50 No Backlight

Using the test points and everything disconnected. Starting from top to bottom. In Kohms.

Master - 5.72, 5.70, 5.71, 5.72, 5.71, 5.71, 5.72, 5.74, 5.72, 5.74
Slave - 5.71, 5.70, 5.74, 5.72, 5.73, 5.71, 5.71, 5.74, 5.73, 5.73

There are some black marks on connector #3 and #4 from the top on the slave board. There is continuity from the transformer pin to the connector pin. Pic attached.

I would say you have found your fault. That white connector is in a terrible state and I suspect the connection plug and socket may also have burnt or badly corroded pins.

[neilc6]

Re: LG 47LG50 No Backlight

The fuse that blew was on the PS board, right beside the AC input connector.

[dick_barton]

Re: LG 47LG50 No Backlight

The fuse that blew was on the PS board, right beside the AC input connector.

The power supply has a ceramic 3.15A anti-surge fuse so where does 8A come into this?

[neilc6]

Re: LG 47LG50 No Backlight

I would say you have found your fault. That white connector is in a terrible state and I suspect the connection plug and socket may also have burnt or badly corroded pins.

The top part of the plastic is black but the pin underneath looks fine and there is still continuity from the transformer output pin to the connector pin.

[dick_barton]

Re: LG 47LG50 No Backlight

But what do the plug pins looking like. You definitely have an issue with that connector.

[neilc6]

Re: LG 47LG50 No Backlight

The socket pin looks ok. The plug has a scorch on top. Here are some more pics.

[dick_barton]

Re: LG 47LG50 No Backlight

Personally I would cut the plug off and remove the socket from the pcb board and solder the pink and white wires directly to the circuit board.
Are any of the other CCFL connectors / plugs showing similar signs?

[neilc6]

Re: LG 47LG50 No Backlight

There are no other connectors with burn marks. I did pop everything back together and it is still the same. Slight backlight flicker when turning on. The fuse which was blown is F100 on the PS board rated at 8A/250V beside the AC input. Pic enclosed.