LG 50PQ6000 unstable 5v standby

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

If possible. I suspect the 5V MAIN is derived off here...

If the 12V is also low by about the same amount (30%), then the suspect would be the primary side, controller IC, or TL431.

I say TL431 because I now recall budm had a Samsung monitor with low rails, and it turned out to be the TL431 (voltage reference IC; for the feedback loop.)

There is no 12VDC at all on P813 in fact its of negative voltage slightly with the board connected and dissconnected.

The only voltages I have found are the 5vstandby that drops when connected.

[tom66]

Re: LG 50PQ6000 unstable 5v standby

OK, looking at the layout, it seems that the 12V is switched on through a MOSFET. It is always present, but only presented to the main board as required.

There is a jumper link, it looks like J22. Can you get to this from the top side?

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

Yes I can get a probe on J22

[tom66]

Re: LG 50PQ6000 unstable 5v standby

OK, it should measure:

12V +/- 0.5V, if there is a fault on 5V side.
Not 12V (possibly around 9V) if there is a fault on the primary or TL431 fault.

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

0vDC to GND spikes to -0.1mV when the relay clicks

[tom66]

Re: LG 50PQ6000 unstable 5v standby

Hmm OK I will need to rethink this!

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

Can you see the components ok? Would you like me to shout out any numbers?

[tom66]

Re: LG 50PQ6000 unstable 5v standby

Can you get a little wider shot of the back (green side) -- include the transformer pins.

[ben7]

Re: LG 50PQ6000 unstable 5v standby

Just a side note, not going to try and distract you from tom66's expertise! xD
IC201, is a 5v regulator. I bet that it regulates the 5v standby. It looks to be a KIA278R000PI. It's max input voltage is 10v. Could it be failed from 12v input?

You thinking that the rectifying diode is shorted out?

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

Any better?

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

Just a side note, not going to try and distract you from tom66's expertise! xD
IC201, is a 5v regulator. I bet that it regulates the 5v standby. It looks to be a KIA278R000PI. It's max input voltage is 10v. Could it be failed from 12v input?

You thinking that the rectifying diode is shorted out?

IC 201 is a KIA278R000PI

D201?? 0.52v one way and 0v the other (in circuit) meter on diode test.

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

Sorry just read the post again you ment D351

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

Can you test these like normal diodes?

[ben7]

Re: LG 50PQ6000 unstable 5v standby

Sorry just read the post again you ment D351

Hm yes, D351.

I say that you should replace it. How did you measure it? did you have your negative probe on the center pin, and your positive probe on the outer pins (one at a time)? These are dual diode packages, and I am guessing the common pin is the cathode, based on the PCB layout (filter capacitor after the dual diode).

If one of them reads short, try reversing the meter leads. If it still reads short, take the diode off the board, and measure. If it still is shorted - it's bad, causing the voltages to be wonky, and overloading the SMPS IC.

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

0.22v one way and 0v the other on both diodes in the component. (Diode removed from board and tested)

[ben7]

Re: LG 50PQ6000 unstable 5v standby

0.22v one way and 0v the other on both diodes in the component.

Hm, that's fishy.

The diode seems good.

Something else(?) must be shorted.

Does the manual have a schematic of the PSU? if so, could you upload it?

[tom66]

Re: LG 50PQ6000 unstable 5v standby

Thanks for the big photo but I meant just a little bit expanded around the same area, not the whole board. I am trying to figure out where the 12V comes from.

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

This is all I have from another TV. The PSU is very simular but the rest of the TV is different from what I have found out so far.

I did have to remove C351 to get the D351 out and do have a funny reading but my capasitence meter isnt amazing I have to admit. ITs just a setting on my DVM.

[ben7]

Re: LG 50PQ6000 unstable 5v standby

This is all I have from another TV. The PSU is very simular but the rest of the TV is different from what I have found out so far.

I did have to remove C351 to get the D351 out and do have a funny reading but my capasitence meter isnt amazing I have to admit. ITs just a setting on my DVM.

Most DVM's [that I know of] don't measure capacitance above 10uF.

What is the reading of the diodes, when you took D351 out?

P.S. We are both on different paths (me and tom66), feels awkward, doesn't it?

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

Its hard to avoid flash reflection but hope this helps.