LG LCD TV power supply - Page 1
Forum section: electronics / repair

author: EK701
author_rank: Contributor
timestamp: February 22, 2015, 07:45:45 am
content: Trying to troubleshoot an LG LCD TV power supply. In the following photo, you will see the bottom connector along the right edge. That connector is supposed to have 3.5v, 12v, and 24v present, but I'm only seeing the 3.5v. I don't see any bulged or leaking caps and nothing else appears to be burnt or otherwise damaged. I don't have a schematic, so any pointers are appreciated.Top side:Bottom side:

author: miguelvp
author_rank: Super Contributor
timestamp: February 22, 2015, 08:15:59 am
content: I see at least 3 fuses, did you check them?

author: wraper
author_rank: Unknown
timestamp: February 22, 2015, 08:41:06 am
content: It does have two transformers, therefore main and standby power supplies. Main power supply will turn on only if receives the signal to do so. So even if it is completely OK, you won't get other voltages unless CPU board sends turn on signal. Read description of TP201 connector on the bottom side of the PCB.

author: Psi
author_rank: Super Contributor
timestamp: February 22, 2015, 10:48:42 am
content: You see that pin which is labeled "PWR-ON", this is the pin which turns on the main psu voltages.Its usually active low. So it will have maybe 3.3V on it and will turn on if you pull this voltage down to ground.A 1k resistor is usually all that's needed, and much safer than trying to short it to GND directly.If there's zero volts on this pin already, try putting 3.3V onto it through a 1k resistor in case its active high.That DRV-ON pin may also have something to do with it. Maybe that turns on the LCD backlight part of the psu separately.

author: EK701
author_rank: Contributor
timestamp: February 22, 2015, 04:11:34 pm
content: The three fuses are good. The symptoms are when AC power is applied, the tv never turns on. The indicator LED comes on red, then turns white, then goes back to red. The black relay on the high voltage side clicks. When I push the reset button on the CPU board, it sounds like the relay drops out, then reconnects, so it appears there is some communication between the CPU board and the power board.

author: sdev2004
author_rank: Contributor
timestamp: March 28, 2015, 01:30:15 am
content: What does the standby voltage read? have you tried to connect PWR-ON to ground with a 1K resistor? that should turn the PSU in the "ON" state and from then you need to check the voltages. voltages coming out of the PSU (to the mainboard) should read something like 3V, 5V, 12V, 15V. report back with the voltages. Also check the voltages from the inverter (with the inverter unplugged) assuming its a separate board you should see 24V on that line. Example if there's 12 pins 5 would be ground and 5 would be 24V the other 2, one of them would be a signal lead. If all tests come back OK you can rule out the PSU and turn to the mainboard.

author: EK701
author_rank: Contributor
timestamp: March 28, 2015, 03:03:48 am
content: I ended up just replacing the entire power supply board and that resolved the problem. I never did figure out what was wrong with the power supply board.

author: Psi
author_rank: Super Contributor
timestamp: March 28, 2015, 11:02:03 am
content: A proper DSLR camera and a proper flash (pointing at the roof to illuminate the entire room )

author: sdev2004
author_rank: Contributor
timestamp: March 28, 2015, 01:07:04 pm
content: Good to hear you got it sorted