LG 60LB5200 No power , completely dead

[Anooc7]

Hi,
I have this lg that is giving me trouble for the past 2 days with a faulty power supply unit (I assume because this is an always on power supply) .
Initial findings:
Got it dead. So I opened and checked for any badcaps’ or mosfets and interestingly everything was fine, except a R33 resistor that got infinity reading This particular resistor is connected to one of the secondary winding of a transformer to the HV pin of IC tea1755t . The big filter caps reading was 168v . Both 24v and 12v outputs were absent (0v) . I have seen an older post for this same tv and found that r33 was a 10k resistor but unfortunately I didn’t have one in stock. So I paralleled two resistors and managed to make around 9k. But still the caps showing the same voltage and the cold side is dead. I checked the pin 1 (vcc) to pin 2 (ground) voltage and it reads few milli volts only. After 2-3 minutes one of the resistor which I paralleled (which was a 1/4 watt one , pls see the pics) started to produce smoke. So I stopped working on the board now and realized that my experience is not enough to move forward in this situation. Any thoughts?
The following is the link to the schematic of the power supply.



Thanks
Anoop

[R_J]

Re: LG 60LB5200 No power , completely dead

It is very likely the ic is bad, it is about the only part that would cause r33 to open.

[Anooc7]

Re: LG 60LB5200 No power , completely dead

It is very likely the ic is bad, it is about the only part that would cause r33 to open.

Okay, Sounds good. Thanks

[Anooc7]

Re: LG 60LB5200 No power , completely dead

It is very likely the ic is bad, it is about the only part that would cause r33 to open.

Hi, I replaced the IC but still showing less voltage on the 24 v and 12 v connection, as well as 169 volts at the 400v places.
Any inputs at this point?
Thanks

[Davi.p]

Re: LG 60LB5200 No power , completely dead

This is the first time i see a main ic controller with pfc controller, so check big mosfet Q8 and near R48, plus D12..

[Anooc7]

Re: LG 60LB5200 No power , completely dead

This is the first time i see a main ic controller with pfc controller, so check big mosfet Q8 and near R48, plus D12..

Q8 - 1 and 3 pin shows few milli volts, 2- where d12 meets - 170v the other side of diode - 169.5

[Davi.p]

Re: LG 60LB5200 No power , completely dead

No these measurings are wrong, i was meaning test the components if are good, so without current in, test mosfet and diode for short in diode test mode of dmm, and resistor for open in ohm mode.

[Anooc7]

Re: LG 60LB5200 No power , completely dead

No these measurings are wrong, i was meaning test the components if are good, so without current in, test mosfet and diode for short in diode test mode of dmm, and resistor for open in ohm mode.

I checked before measuring the volts, they are fine.

[Davi.p]

Re: LG 60LB5200 No power , completely dead

if those and the resistors between the ic and Q8 are ok then the only answer is bad new ic or their solderings. How much volts on the 12 and 24 v outputs?

[Anooc7]

Re: LG 60LB5200 No power , completely dead

if those and the resistors between the ic and Q8 are ok then the only answer is bad new ic or their solderings. How much volts on the 12 and 24 v outputs?

8 and 16 volts respectively. The IC is carefully soldered, and it is not burning that r33 resistor like before.

[Davi.p]

Re: LG 60LB5200 No power , completely dead

i have not seen the schematic but there's no secondary command to switch on pfc.. so i see no other components involved in pfc start..

[R_J]

Re: LG 60LB5200 No power , completely dead

Is this power supply connected to the main board? The voltages would indicate it is in standby, It has a pfc circuit but it is not operating, What is the VCC voltage across C25? Also what's the voltage across C51 (or check the voltage on pin5 of the ic (carefully))

[Davi.p]

Re: LG 60LB5200 No power , completely dead

this is an always on card because it is well visible one only optocoupler.. until pfc is off i guess output tensions are low due to transformer ratio.

how do you are testing diode, mosfet and resistor and which are readings?

[R_J]

Re: LG 60LB5200 No power , completely dead

Check if R48 is ok and not open circuit. I know this is an ALWAYS ON power supply, BUT it likely operates in a standby condition when the tv is turned off and not drawing high current.

[Davi.p]

Re: LG 60LB5200 No power , completely dead

Yes, may i remember of a Samsung power board that the table was saying 8v standby 13v on

[Anooc7]

Re: LG 60LB5200 No power , completely dead

Is this power supply connected to the main board? The voltages would indicate it is in standby, It has a pfc circuit but it is not operating, What is the VCC voltage across C25? Also what's the voltage across C51 (or check the voltage on pin5 of the ic (carefully))

No, it is not connected to the main board at this time. The voltage across c25 is 15v and across c51 is 20v . I am going to connect the psb to main board soon.

Thanks

[Anooc7]

Re: LG 60LB5200 No power , completely dead

this is an always on card because it is well visible one only optocoupler.. until pfc is off i guess output tensions are low due to transformer ratio.

how do you are testing diode, mosfet and resistor and which are readings?

I checked them in diode mode and found that the N-channel mosfets are conducting properly so as the diodes and resistors.
I will update if this setup works with attached main board.
Thanks

[Anooc7]

Re: LG 60LB5200 No power , completely dead

Update: So I hooked the psb to main board and the voltages (24 and 12 outputs) dropped to 4.2v and 2.2v (where it was 16 and 8 before). I checked the r33 resistor (10k)that I replaced and it is showing Mega Ohms.
I am including a practical circuit which I found from the data sheet of Tea1755t ic , there is no resistance in between the hv sens pin and the transformer, so can I short that with a jumper? (Instead of r33)

[Davi.p]

Re: LG 60LB5200 No power , completely dead

Have you checked the value of r33 in the schematics? It is very strange there is no resistor in this schematic, i don't have seen datasheet, if hv pin is the startup pin the current must be highly reduced, but something excapes from me, why a central take in the primary winding and not the end wire if resistors are integrated in chip? i must read the datasheet.. never seen a primary side with more than 4 terminals. Very odd.

[Anooc7]

Re: LG 60LB5200 No power , completely dead

Have you checked the value of r33 in the schematics?

From the schematic it is labeled as 103 , I found from a previous forum posting that its a half watt 10k resistor.