LG 50PQ6000 unstable 5v standby

[Baileybum1234]

Hi I picked this TV up at work because it was going to be thrown out. I have been looking around the net and asking questions for weeks and learning in the process.

I have a decent understanding of electronics but not TV related experience and so loving the challenge. I must be mad but im sure some of you can relate.

It seams I have the dreaded relay clicking issue that could be linked to many different possible faults in the TV. Its how the TV protects itsself when there is an issue some where so I understand.

The only voltages I can find are the standby 5v lines x 2. These are 5v until I plug in the main board then its drops to 3.5v and few components get a little warm on the PSU board. One transistor and one diode. I have tested the diode and replaced the 2 capasitors here. As for the transistor im not sure (IC 301 a STR W6252 power IC)

I have currently decided something must be wrong on the 5vstandby lines of the main board but cant fine any scematics or training manuals with the PD92A EAX57566204 main board.

There is no stable AC detect just appears to pulse very quickly so I cannot read it with my DVM.

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

Turns out there is an AC detect done some testing today any ideas folks?

PIN 23 3.5V 0.15-0.18A

PIN 11 3.5V 0.12-1.14A

With no obvious pattern with the relays clicking.

I thought I would do another PIN voltage test while I was there:

5v stb 3.5V

M5v on 230mV

RLon 3.2V

5vDet 6.9mV

AC Det 3.4V

VS on 130mV

[ben7]

Re: LG 50PQ6000 unstable 5v standby

Check the all the small electrolytic capacitors in the PSU. Also check the resistance of the 5v standby rail that is on the mainboard.

[tom66]

Re: LG 50PQ6000 unstable 5v standby

It would seem like your 5V is being loaded down by the main board but you can check if it is the power supply by connecting a 10 ohm 5W resistor across the output. This will provide approx 0.5A load (2.5W.) If the 5V drops out, then the problem is on the PSU.

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

Interesting find:

Reading P47 of the 50PS60 training manual

TV off:

PIN 24 (Key On) 4.3v

PIN 18 AC detect: millivolts.

TV on:

PIN 24 (Key On) 0v

PIN 18 AC detect 3.4v

Hi ben7:

All the small caps pass a visual check. Any suggestions on exact caps to pull out and test?

infinate ohm 5vstandby pin 23

infinate ohm 5v standby pin 11

[ben7]

Re: LG 50PQ6000 unstable 5v standby

Interesting find:

Reading P47 of the 50PS60 training manual

TV off:

PIN 24 (Key On) 4.3v

PIN 18 AC detect: millivolts.

TV on:

PIN 24 (Key On) 0v

PIN 18 AC detect 3.4v

Hi ben7:

All the small caps pass a visual check. Any suggestions on exact caps to pull out and test?

infinate ohm 5vstandby pin 23

infinate ohm 5v standby pin 11

Try doing what Tom sugguested.

Seems like a dried capacitor (maybe more) on the PSU, with no blatently obvious short circuits.

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

It would seem like your 5V is being loaded down by the main board but you can check if it is the power supply by connecting a 10 ohm 5W resistor across the output. This will provide approx 0.5A load (2.5W.) If the 5V drops out, then the problem is on the PSU.

Hi tom,

I have some 100ohm (would they be too high?) resistors to hand. What output and where do I connect?

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

I get you now load the standby 5v lines. I will pick up some 10ohm 5w resistors tomorrow.

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

Shouldnt I have an 5VDC AC DETECT voltage in TV off state?

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

AC Detect is generated on the SMPS, by rectifying a small sample of the
A/C Line at D102 and associated circuitry and routed to the Sub Micon (IC701) where it outputs at pin 15 and sent to P814
pin 18 to the Main Board where it is sensed and monitored by the Main Microprocessor (IC1). The AC Det in this set works
differently than most. If AC Det is missing the Microprocessor will turn off the television in about 10 seconds after turn on.
This will happen each time turn on is attempted.

you never know! Guess its my next move unless you can think of anything else? Start checking these things tomorrow.

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

Ok I fitted a 2 x 10 ohm resistor across the 2 x 5v standby lines and the voltage dropped to 3.4v

[tom66]

Re: LG 50PQ6000 unstable 5v standby

OK, then the problem is clearly with that part. I would replace the capacitor on the output of the 5V standby. Can we have a picture?

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby



Hope this is a good enough image. Its a picture I found on the web and exactly the same board.

The things I have ringed are what I think is the IC that supplies the 5v, I have tested the zener I ringed and already replaced the capasitors I have ringed but there obviously not the right ones

I also notice that zener and IC getting very hot (burn mark under the zener diode eventhough tested fine) when connected to the main board.

[tom66]

Re: LG 50PQ6000 unstable 5v standby

It's not really good enough, plus we need pictures from underneath...

I have heard of some people replacing that IC on the heatsink, though cannot find the video I watched on it right now...

[ben7]

Re: LG 50PQ6000 unstable 5v standby

Hm, replace all the secondary filter capacitors in the bottom right of the PSU.

Also, is there a snubber network on the primary side? (usually is a power resistor, and a film capacitor, and a diode. Sometimes also a zener)

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

Do you think the voltage is supposed to drop to 3.3V under load??

I looked on P138 of the training lg50pq30 manual and IC 301 that delivers the 5vstandby voltage is labeled up:

5VST to 3.3V VST REG and some test voltages are there. I have no idea what this means.

[tom66]

Re: LG 50PQ6000 unstable 5v standby

No, I doubt the 5V should drop under load.

[ben7]

Re: LG 50PQ6000 unstable 5v standby

I looked on P138 of the training lg50pq30 manual and IC 301 that delivers the 5vstandby voltage is labeled up:

5VST to 3.3V VST REG and some test voltages are there. I have no idea what this means.

It clearly says, 5v standby to 3.3v standby regulator. It's a regulator that regulates some of the power from the 5v standby bus to 3.3 standby power.

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

Ok looking at the training manual for a simular board the 5v standby should be 5v in standby and run.

[ben7]

Re: LG 50PQ6000 unstable 5v standby

Ok looking at the training manual for a simular board the 5v standby should be 5v in standby and run.

I think either there is oscillation (bad caps?), or a bad snubber network (too high voltage, SMPS IC mosfet is breaking down) that is causing the low voltage and hot SMPS IC issue. It could be a partially failed rectifier diode on the low voltage side heatsink.