LG, Model 47LG50, not coming ON

[tictac]

SYMPTOMS: At standby the front LED is solid red. When turned ON, it flashes brighter red twice, then goes out. From the inside of the TV, I can see the light change to blue but it is barely visible on the front of the TV. There is no picture and no sound.

TESTS DONE: Visually all the boards look normal, no burnt marks, no fat caps, no smoke. All cable connections were re-seated but made no difference.

With power unplugged, using "diode test" on a meter, all diodes and bridge rectifier tested OK. Although the Dual-Diode packages showed continuity between the outer terminals (I assumed that was normal). None of the MOSFET gate-drain legs showed shorted. Two fuses tested OK also.

With the TV turned ON, the measured voltages were as follows:
AC on bridge rectifier=124 VAC
DC on bridge rectifier=122 VDC (should this not be higher?)
5V output at connectors=5 VDC
12V output at connectors=0 VDC
16.5V output at connectors=0 VDC
24V output at connectors=3 VDC

Attached is a photo of the PS Board. Could someone please advise me on how to proceed. If it's a power supply problem I'm fine with replacing parts, but I'm not sure how to find the bad part. Thank you in advance.

[budm]

Re: LG, Model 47LG50, not coming ON

'DC on bridge rectifier=122 VDC (should this not be higher?)'
Is that what you are getting measured between the two legs of the main filter cap?
We need the dcv readings of each pin of the connector with the name of each pin so we can see what your are missing.

[tictac]

Re: LG, Model 47LG50, not coming ON

Previously, I measured the 122 VDC directly on the PLUS & MINUS terminals of a bridge rectifier. Today, I measured the two large caps near the bridge rectifier and they were 384 VDC each. Following are the DC voltages on the 3 connectors on the PS board as you requested.

P201 (to main board)
LABEL DC Voltage
16.5 0
GND 0
12V 0.01
GND 0
5V 5.11
5V 5.11
GND 0
GND 0
ACD 0
ON/OFF 2.8
PMW-DIM 1.16
N.C. 0

P205 (to inverter on right side)
ON/OFF 3.02
BRI 1.61
GND 0
GND 0
GND 0
GND 0
GND 0
24V 0.37
24V 0.37
24V 0.37
24V 0.37
24V 0.37

P204 (to inverter on left)
ERROR 0
PWM-DIM 1.35
ON/OFF 3.02
BRI 1.61
GND 0
GND 0
GND 0
GND 0
GND 0
24v 0.37
24v 0.37
24v 0.37
24v 0.37
24v 0.37

Thanks for your quick reply, Budm.

[tictac]

Re: LG, Model 47LG50, not coming ON

Budm,
The voltage readings in the previous post are poorly formatted. On each line the connector label is followed by a space and then the DC Voltage. When you get a chance, could you please give me an idea of what to check next.
Thank you.

[tictac]

Re: LG, Model 47LG50, not coming ON

'DC on bridge rectifier=122 VDC (should this not be higher?)'
Is that what you are getting measured between the two legs of the main filter cap?
We need the dcv readings of each pin of the connector with the name of each pin so we can see what your are missing.

BudM,
I have posted the voltage readings. When you get time, could you please let me know what I could check next. Thank you.

[budm]

Re: LG, Model 47LG50, not coming ON

OK, the power supply is getting the correct command signals and the PFC Voltage booster is working because you are getting the 384VDC on the filter cap but the switched 12V/24V power supply section is not coming on, that will be the section to check next.
One signal Voltage that I ma not sure is the 'ACD 0' AC detect, I do not know if 0V is normal or not.
http://www.shopjimmy.com/lg-eay40505...upply-unit.htm

[tictac]

Re: LG, Model 47LG50, not coming ON

There are a number of dual diodes with their cathodes connected to large heat sinks. If I measure the anodes, with ground as a reference, should I be getting AC or pulsating DC?

[budm]

Re: LG, Model 47LG50, not coming ON

At this point we need the good clear pictures of the whole bottom side of the power supply board.

[tictac]

Re: LG, Model 47LG50, not coming ON

At this point we need the good clear pictures of the whole bottom side of the power supply board.

Here they are. I may be able to get better closeups tomorrow in the daylight if you need.

[budm]

Re: LG, Model 47LG50, not coming ON

Thanks, they are a little too much glares from the flash, I may also need you to take the closed up pictures in section so I can stitch them together to able to see the traces in details.

[budm]

Re: LG, Model 47LG50, not coming ON

https://www.badcaps.net/forum/attach...9&d=1453766077

What are the P/N as printed on those two ICs with 16 pins (or may be 20 pins)?

[tictac]

Re: LG, Model 47LG50, not coming ON

https://www.badcaps.net/forum/attach...9&d=1453766077

What are the P/N as printed on those two ICs with 16 pins (or may be 20 pins)?

I will get good closeup pictures in the daylight tomorrow morning.
The IC part numbers are on the attached picture.

[budm]

Re: LG, Model 47LG50, not coming ON

NPC1396 is the SMPS IC for the 12V/24V power supply, we need to find out if it is getting the Voltage between VCC pin and GND pin to run.

[tictac]

Re: LG, Model 47LG50, not coming ON

NPC1396 is the SMPS IC for the 12V/24V power supply, we need to find out if it is getting the Voltage between VCC pin and GND pin to run.

With the TV plugged in and turned OFF, VCC-pin12 is at 2.69V. With the TV turned ON, VCC-pin12 is at 14.32V. These are measured with pin 10 as the reference.

A new observation: Just after measuring these voltages, the back lights came on for about 15-20 seconds and then went off again. I had no time to measure any other voltages while the lights were ON. I tapped the power supply everywhere, thinking there may be a bad connection some place, but I couldn't get the back lights to come on again.

Attached are 6 new pictures of the board.

[budm]

Re: LG, Model 47LG50, not coming ON

With the TV plugged in and turned OFF, VCC-pin12 is at 2.69V. With the TV turned ON, VCC-pin12 is at 14.32V. These are measured with pin 10 as the reference.

A new observation: Just after measuring these voltages, the back lights came on for about 15-20 seconds and then went off again. I had no time to measure any other voltages while the lights were ON. I tapped the power supply everywhere, thinking there may be a bad connection some place, but I couldn't get the back lights to come on again.

Attached are 6 new pictures of the board.

So the IC gets enough DCV to run (VCC ON Threshold level min is 12.3V, Typ is 13.4V, Max 14.3V).
I would resolder the legs of the power transformers T101, T102, MOSFET Q110, Q111, Caps C114, C125.

[tictac]

Re: LG, Model 47LG50, not coming ON

I re-soldered the components you suggested and the TV turned on as normal. Both, picture and sound, are good. I will leave it on to test run for 4-5 hours just to be sure that was the problem. Thank you for your advice and time.

[budm]

Re: LG, Model 47LG50, not coming ON

Please also make sure all the boards mounting screws are tight since the GNDs to the chassis are done through the board mounting screws and the GND pads on the circuit boards..

[tictac]

Re: LG, Model 47LG50, not coming ON

Please also make sure all the boards mounting screws are tight since the GNDs to the chassis are done through the board mounting screws and the GND pads on the circuit boards..

OK. It's been working fine for a number of hours now, so I believe we fixed it.
Thanks again. I'll leave a donation.

[theyankeemoto]

Re: LG, Model 47LG50, not coming ON

Hi guys, I have the exact same problem with my 47lg50 tv.

I replace the power supply board with one off eBay that I thought would most likely be the problem, but didn't fix it. Could it be the backlights inverters ?

[budm]

Re: LG, Model 47LG50, not coming ON

Read post 3 and later and then tell us what Voltages you have like what the OP had provided..