Re: LG LCD LED TV No Standby Light Won't Turn On Pwr-On Relay Clicks On Then Off

[Fixin Stuff]

Underside of mainboard

[sean0118]

So with the power-on forced using my 'switch', I got:
- 12.5VDC on the 12V pins of the output connector
- 20.7VDC on the 20V pins of the output connector

I also tested the LED driver section of the power supply board, and I got:
- 4.9VDC at the 5.7V pin on the LED driver board
- At all the Vxx pins on the LED driver board, I got between 13VDC and up to 17VDC

At the LED output connectors, I got:
- 115VDC at the grey and red cables
- Between 15VDC and 17VDC at the other cables

Okay so the 3.5/12/20V rails are all ok, so that's good.

I'm pretty sure the 5.7V pin is actually 5.2V? Based on the photos in this thread:
https://www.badcaps.net/forum/showthread.php?t=52635

13-17V is probably okay for the Vxx pins, but not 100% sure. Assuming you mean V3a-, V3B- etc, these are all the cathode return lines for the LED strips. It probably depends on whether the backlights are on or off.

The red and grey cables (V1+, V2+, V3+, V4+) are meant to be 136V based on the photo you posted earlier. In the bottom left corner there's jumper next to the model number which lists 136V 0.66A output. Maybe the 115V jumps to 136V once the backlights are on?


Relay RL101 would be part of the power supplies overload protection, so if it's clicking off it's because the mainboard or backlights are drawing too much current. But because you are measuring all the power supply rails while the mainboard is connected I'm starting to think it might be the backlights.

If you have the mainboard connected, but the backlights disconnected, does it still shutdown? If it doesn't shine a torch at the screen and see if you can see the logo etc.


The missing standby light is a mystery, does it come from one of the mainboard connectors? If so, which one?

[shakalnokturn]

The relay is there to enable power to the main supply through PFC.
Having the LEDs on would be better as a load test, that was the point in forcing the supply active.
The PSU seems to have a serial bus for communication with the mainboard, I assume that this is mainly related to fault detection on the LEDs but could be wrong. I also assume this should trigger an error blink with the shutdown.

Standby led comes from the mainboard microcontroller, is not just a standby pilot, it's also used for blinking error codes, so indeed failure to light at all is a bad sign for the mainboard.

On the mainboard you could check the various low supply voltages (usually between 1.2 and 3.3V) for CPU, DDR whatever... They are easy to spot on linear regulators like IC8800 others should be present on electrolytic capacitors or ferrite "beads".
Things are pointing towards the mainboard by now.

[sean0118]

Having the LEDs on would be better as a load test, that was the point in forcing the supply active.

oh, but I thought it was shutting down before the backlights came on? Or are the backlights on as soon as the 115V is present?

Standby led comes from the mainboard microcontroller, is not just a standby pilot, it's also used for blinking error codes, so indeed failure to light at all is a bad sign for the mainboard.

Okay that is good to know, I agree it's a bad sign for the mainboard.

[Fixin Stuff]

Hi gents. Thanks for the comments shakalnokturn and Sean. I just did some more testing based on your thoughts.

I tried powering up the TV with the backlight cables disconnected. Made no difference. The relay still clicked on and off immediately.

I then did some tests on the mainboard with the power supply forced on as per my previous post. Results were:
- 20v, 12v and 3.5v are all present at mainboard connector, AND at the 3x electrolytic capacitors immediately next to the main incoming connector. You can tell which capacitor corresponds to which voltage rail from the voltage ratings on the caps.
- I was also able to find 3.5v and 12v a little bit away from these caps doing a bit of testing, but couldn't find another 20v reference. I've attached a pic showing some areas of the board which seemed to be dead/powered off highlighted in red.
- The connector on the left side of the mainboard (P1895) seemed to have just a few millivolts registering to GND on the pins. This goes to the small board at the bottom of the TV off-centre towards the mainboard side. Not sure what this is for?
- IC2600 had essentially 0v to GND on all pins
- IC8800 had essentially 0v to GND on all pins
- The connector P8800 at the bottom is for the speakers, and had 1.4v-1.5v present on all pins to GND.
- Connector P8200 next to it which goes out to the IR receiver/power buttons/standby LED board had 3.5v on a bunch of pins, and perhaps even 12v on one (can't remember 100%, didn't take notes and tested dozens of points  )
- I also tested the IR receiver/power buttons/standby LED board itself (pictures of the board below). This had 3.3v - 3.5v on at lest a couple of one pins on each connector. Both electrolytic caps had 3.3v - 3.5v. I took these pics the other day, but I haven't actually tested the LED itself or the rest of this board apart from the live testing I did tonight. Was interesting that it seems to have lots of power but not even an LED. As mentioned previously, it seems the LED is an 'all systems go' indicator for the whole TV, so there's obviously something wrong upstream which is preventing it from being switched on.

Not sure if the 20v is pretty much immediately chopped up and reduced once it goes into the mainboard, but there definitely seem to be some areas of the mainboard which are 'dead'/not turning on.

When I get a chance, I'll take another look at the schematic and see if I can figure out why it's not powering up. Might do some testing of it powered off too and see if there are any obvious shorts/open circuits etc.

Feel free to let me know any other thoughts/suggestions