LG 50PQ6000 unstable 5v standby

[tom66]

Re: LG 50PQ6000 unstable 5v standby

Also possible, the sense resistor may have increased resistance due to failure, leading to a low over-current limit.

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

cheers guys for the pointers, I was chasing my tail, I just need to google what all this means.

[ben7]

Re: LG 50PQ6000 unstable 5v standby

Also possible, the sense resistor may have increased resistance due to failure, leading to a low over-current limit.

Yell at me if I'm wrong, but wouldn't it just shut off before the rated load is reached? I don't think it would make the SMPS IC get very hot with a drop in output voltage. (I'm only familiar with them going 'tick...tick...tick' when in protection mode)

Of course, there's always the possibility!

Bailey, is there any way you could takes pictures of your board? (sorry if I asked that already!)

[tom66]

Re: LG 50PQ6000 unstable 5v standby

Well, depends on the controller IC... some go into lock-out/pulse-mode when the average current exceeds a set threshold... whereas others will just throttle back the PWM and the output will drop.

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

Here you go they came out better then I thought they would:

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

When you say pulse mode I can hear the 2 relays before the rectifier clicking on/off on/off ect

[ben7]

Re: LG 50PQ6000 unstable 5v standby

Here you go they came out better then I thought they would:

Check the resistance of the two power resistors, R607 and its sibling (can't see part number).

Is the clicking happening when the PSU is plugged into the TV?

How many volts are you getting on the big brown filter capacitors, C610 and C611? In standby? When trying to turn the PSU on (when it's in the TV)?

[tom66]

Re: LG 50PQ6000 unstable 5v standby

Test this resistor. If it is good, it should test less than 0.5 ohm; subtract the reading your meter shows when the leads are shorted. It is a 0.39 ohm resistor, I think.

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

Ben7

R607 = 0 ohms
R613 = 0 ohms

The clicking happens when the psu powered and not connected to the TV.

The clicking also happens when the psu is powered and connected to the TV providing the on/off switch is closed. I think its the Auto ground pin on P813 that stops it clicking when the on/off switch is open but I might be wrong.

Please could you talk me throught testing capasitors C610 and C611 I havent dont that before.

Tom66

R306 = 0.6 ohms (out of circuit)

[tom66]

Re: LG 50PQ6000 unstable 5v standby

What reading do you get when probes are shorted?

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

What reading do you get when probes are shorted?

0.0 - 0.2 ohm just seams to drift over time. When I tested earlier I didnt leave them togeather so long so just got 0.000.

[tom66]

Re: LG 50PQ6000 unstable 5v standby

It may be OK, it is just very hard to tell! If you are going to replace the capacitors, I would advise you replace the resistor at the same time.

[ben7]

Re: LG 50PQ6000 unstable 5v standby

Please could you talk me throught testing capasitors C610 and C611 I havent dont that before.

Check the voltage (DC) across each capacitors legs (on the underside of the PSU PCB). Check them with the PSU not connected to the TV, and with it connected. Please report back the readings! (Note: be careful, don't let the probes slip, and don't touch and metal on the PCB. Also, lay it on a non-conductive surface (like a [dry] towel), as some heatsinks are at different voltages, and they could possibly short out)

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

So R306 is the sense resistor? and sets the shut off protection current?

[ben7]

Re: LG 50PQ6000 unstable 5v standby

So R306 is the sense resistor? and sets the shut off protection current?

Yep.

When the current through the resistor increases, the voltage across it increases too. The SMPS IC monitors this voltage, and if it goes above a certain voltage, the SMPS IC thinks there is too much current flowing, and it shuts down.

[tom66]

Re: LG 50PQ6000 unstable 5v standby

Yes, it feeds back by measuring the current through the primary. If the resistance increases the voltage across it increases so the controller thinks the current is higher than it is, and shuts off.

Edit: Darn, the American beat me to it...

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

With the board dissconnected from the TV:

The capasitors measure 390VDC then starts to drop untill the next relay click then its 390VDC again.

With the board connected to the TV:

Its all over the place. Its hard to even get a reading.

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

and these 2 capasitors are for smoothing out the AC ripple after full wave rectification??

[ben7]

Re: LG 50PQ6000 unstable 5v standby

With the board dissconnected from the TV:

The capasitors measure 390VDC then starts to drop untill the next relay click then its 390VDC again.

With the board connected to the TV:

Its all over the place. Its hard to even get a reading.

How much does the voltage drop before the next relay click?

It almost seems like the PFC isn't functioning right.

Edit: Yes, they are sort of, the input AC power gets rectified, then stepped up by a boost circuit, and these capacitors filter the output of that boost circuit. The boost circuit is called a 'PFC' circuit. PFC = Power Factor Correction. google it

Edit2: Do you have a capacitance meter? If you do, check teh capacitance of the large red capacitor (near one of the heatsinks, it has some silicone glue on it too. take it out to measure it's capacitance)

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

drops down to 308 -311VDC

Google initiated.