LG 42 LK 450N won't start

[manoftheyear]

Hello

TV goes on and blue led lights on for 5 seconds, then the tv switches off and red led comes. Screen stays black all the time.

something wrong with the power supply voltages, when the TV is on for 5 seconds my DMM shows P. Dim runs between 0v and 3.1v, A. Dim somewhere between 1.7 and 3.1v, INV_ON and Error out rails go from 0v to 3.4v.

Can the inverter and the backlights be checked by jump wiring? I tried a jump wire (680 ohm resistor between) from the 3.5v rail to INV_ON and PWR_ON. Nothing happens. By doing this the main filter cap has only 330v (line is 230AC), PFC not working this way?

Service manual here https://cdn.badcaps-static.com/pdfs/...b64fd020f0.pdf

Not the exact same but many similarities and mostly same components.

[manoftheyear]

Re: LG 42 LK 450N won't start

Anyone? Where to start, which board to suspect? A little 'tick' kind of sound can be heard I believe from the PSU each time the voltages drops and raises.

[Andrew F. Ali]

Re: LG 42 LK 450N won't start

Post pics of the PSU Bd and the back of the TV.
Are you getting StdBy 5V when you plug into the AC without trying to turn it on?

[budm]

Re: LG 42 LK 450N won't start

INV ON , >2.5VDC = ON, <1VDC = OFF, the command signals are generated by the main board not by the power supply board.
How about the Voltage for running the inverter board? I cannot tell much until I can see the pictures of the boards and the whole back side of the TV.

[manoftheyear]

Re: LG 42 LK 450N won't start

Some pictures attached. IF you need more or any specific let me know...

Standby is 3.5v and working fine. While the tv is on for the five seconds, 3.5V, 12V and the 20V that is for the inverter are all ok. The ones I mentioned in the first post go up and down.

[manoftheyear]

Re: LG 42 LK 450N won't start

So so.. This LG is a no go? Any help appreciated.

[adamp91gvm]

Re: LG 42 LK 450N won't start

I have thelg 42lv450u same issue i have had it on once it clicks to come on red light blinks goes white for a second and goes off it the av board for sure

[adamp91gvm]

Re: LG 42 LK 450N won't start

Anyone ??

[manoftheyear]

Re: LG 42 LK 450N won't start

Back to my case. by putting a jumper wire from the 3.5v rail to the pwr_on and inv_on the power supply goes on, all the voltages measured from the psu connector are stable and right on the spot, even the rails that are related to the inverter circuit... but still the backlights won't come on? should I suspect the inverter board or a ccfl tube here?

[adamp91gvm]

Re: LG 42 LK 450N won't start

If i turn mine on then disconnect the av board from power my back light comes on fine ? And by on i mean press the buttom on the remote then wait for the 1st click then take av power lead out

[budm]

Re: LG 42 LK 450N won't start

Back to my case. by putting a jumper wire from the 3.5v rail to the pwr_on and inv_on the power supply goes on, all the voltages measured from the psu connector are stable and right on the spot, even the rails that are related to the inverter circuit... but still the backlights won't come on? should I suspect the inverter board or a ccfl tube here?

But do you still only get 330VDC between the two legs of the main filter cap? The inverter circuit needs that PFC boosted Voltage for it to run.

[manoftheyear]

Re: LG 42 LK 450N won't start

But do you still only get 330VDC between the two legs of the main filter cap? The inverter circuit needs that PFC boosted Voltage for it to run.

No, I don't know how I got a reading like that before. It's now 386VDC.

[budm]

Re: LG 42 LK 450N won't start

No, I don't know how I got a reading like that before. It's now 386VDC.

That is good, so the fuse (white body with 2 color stripes on it) next to the two inverter transformers is good?

[manoftheyear]

Re: LG 42 LK 450N won't start

That is good, so the fuse (white body with 2 color stripes on it) next to the two inverter transformers is good?

Well actually, it ain't... DMM shows OL. How in the hell did I miss a fuse of that size is beyond me. Probably too much going on at the moment.

Q702 that's a Mosfet 5n50 is shorted. Should I expect the diode connected to the gate to be bad too?

[budm]

Re: LG 42 LK 450N won't start

Well actually, it ain't... DMM shows OL. How in the hell did I miss a fuse of that size is beyond me. Probably too much going on at the moment.

Q702 that's a Mosfet 5n50 is shorted. Should I expect the diode connected to the gate to be bad too?

Yes, the GATE R/D network and more likely the IC that drives the Gate will be damaged also.

[manoftheyear]

Re: LG 42 LK 450N won't start

You mean the IC BD9270F? You think by changing these I might be ok?

[budm]

Re: LG 42 LK 450N won't start

You mean the IC BD9270F? You think by changing these I might be ok?

You need to check all the components connected to the MOSFET and replace the IC so you will not be taking the chance of blowing up the new MOSFET.

[manoftheyear]

Re: LG 42 LK 450N won't start

Sorry this had BD9240F that is a 28 pin IC You have datasheet for that? How about the SMD diode connected to the gate, what would be a good substitute?.

[budm]

Re: LG 42 LK 450N won't start

Bd9240 SPEC.
SMD diode connected to the gate, 1N914/4148 (the example circuit from SAMSUNG shows the diode as BAV70, so look up the spec for the electrical rating) may be fine, the diode is usually connected in parallel with 10~100 Ohms resistor so when you test the diode it will show conduction in both direction.

[manoftheyear]

Re: LG 42 LK 450N won't start

Update to this. Replaced the components mentioned in this thread. TV now works fine. Thank you.