LG 42PC3RA SMPS power_ysus problems on/off/on click and fixes

[MoDu]

Re: LG 42PC3RA SMPS power_ysus problems on/off/on click and fixes

I tried turning on the sound, to see if I could hear the speaker thump you mentioned, but it was barely noticeable. It only happens when I unplug the cord, simulating a power outage.

BTW, disabling the speakers in the menu is enough to NOT have this side-effect.

[MoDu]

Re: LG 42PC3RA SMPS power_ysus problems on/off/on click and fixes

Thanks for the tips.

[tom66]

Re: LG 42PC3RA SMPS power_ysus problems on/off/on click and fixes

I doubt it will do any damage to the sustain boards. My 42" actually ran for half a second with this bridge made, you could see picture slowly fade out before it cut out completely, so it does shut down reasonably cleanly.

The speakers will make click noise even if nothing normally comes out of them, so to avoid this simply unplug the speaker connector on the AV board (board with SCART etc on it.)

[MoDu]

Re: LG 42PC3RA SMPS power_ysus problems on/off/on click and fixes

Audio is a non issue for me, I don't use the TV's audio at all.

I was able to easily fix the sparklies with some tinkering on the Var resistors, they're now invisible at a regular viewing distance.

So my only concern right now is whether this will destroy what's left of the Z board.

[tom66]

Re: LG 42PC3RA SMPS power_ysus problems on/off/on click and fixes

I had it running for a month, but sold the TV on after that due to the LG "sparklies" issue.

The only disadvantage that I can see is the audio doesn't mute on power off, so you'll get a loud crackling sound. This is a common phenomenon with audio amplifiers. Some can detect the failure of their supply voltage and mute immediately, whereas others require an external signal. AC detect provides this signal, along with the signal to the processor to tell it to keep running.

[MoDu]

Re: LG 42PC3RA SMPS power_ysus problems on/off/on click and fixes

How long have you had this "hack" force tv to not auto-shutdown?

I've had all the problems described here, even went as far as replacing all big caps and even a whole YSUS board, but alas, it remained the same.

I shorted out the suggested pins on the opto-couplers and so far so good.

Will this kill my tv in a few months? Or will it just keep going until a fuse or a IC burns?

[tom66]

Re: LG 42PC3RA SMPS power_ysus problems on/off/on click and fixes

I guess U702 monitors the AC primary's two sides with these optocouplers.

When the 3-4 pins are joined the U702 thinks the AC is on all the time by circulating the 5.v signal back to the IC. Basically thinks the opto is on state what it really is also except during the 200ms at the start where the opto ON signal would be lost sometimes.

Yep, something wrong with the optos or the AC primary waveform at the start..TV works now again without hickups so before some PSU components fail completely I will just use it like it is

I figure the same. Pins 3 and 4 make the optocoupler seem always on. It might be the PFC bus caps aging or the TL431s being damaged. (These sensed the voltage on the primary on my model.)

[slappala]

Re: LG 42PC3RA SMPS power_ysus problems on/off/on click and fixes

Same here. No idea how it fixes it but it does. Seems very common with LG plasmas.

I guess U702 monitors the AC primary's two sides with these optocouplers.

When the 3-4 pins are joined the U702 thinks the AC is on all the time by circulating the 5.v signal back to the IC. Basically thinks the opto is on state what it really is also except during the 200ms at the start where the opto ON signal would be lost sometimes.

Yep, something wrong with the optos or the AC primary waveform at the start..TV works now again without hickups so before some PSU components fail completely I will just use it like it is

[tom66]

Re: LG 42PC3RA SMPS power_ysus problems on/off/on click and fixes

Same here. No idea how it fixes it but it does. Seems very common with LG plasmas.

[slappala]

Re: LG 42PC3RA SMPS power_ysus problems on/off/on click and fixes

I checked the datasheet again and tried it out. shorting these 3-4 pins on those 2 optocouplers help and gets the tv going when it's clicking.

phew..

I just don't know what could cause this..and frankly don't care .

I measured the signals on both sides of the opto transistor and voltage's drop out for some reason when clicking..

The scope waveform to the opto led side changes when this happens (dirtyer, but peak to peak just a hint)
AC voltages don't drop out too much..

might be voltage/current spike during PSU secondary startup phase that causes the primary side waveform to change and optos to collapse.

This could be caused by a million things...old panel drawing some amps to get the PSU going etc. endless different reasons..

I'm just wondering would new optocouplers help..the enviroment is violent and this PSU's voltages are not the cleanest ones I've seen on this earth..probably not since both act the same way.

For now I just bridge the pins and start watching a movie..(and play around with some setting in the hidden menu maybe ..)

Thanks for the help everyone!

[tom66]

Re: LG 42PC3RA SMPS power_ysus problems on/off/on click and fixes

Okay, pin 4 on the optocouplers is on the phototransistor side.

They're the only signals coming FROM the primary to the secondary, normally the signals go to the primary side from the secondary.

Need a closer pic of under these devices.

You will probably need to place a solder bridge between pins 4-3 on both optocouplers. That's how I fixed mine.

[slappala]

Re: LG 42PC3RA SMPS power_ysus problems on/off/on click and fixes

sorry about that, I will change the pics to attachments. I tought it was best to sent these as links.

parts are NEC 2571 / SJ605, not included in the repairmanual BOM.

Might take 1h or so to delete the files from imageshack(Said when deleting the links.)

[tom66]

Re: LG 42PC3RA SMPS power_ysus problems on/off/on click and fixes

PC502,PC503 are probably the suspect devices - near the relays, bottom left. Post a part # for them.

Also please don't post inline - I've got crap internet service and it takes forever to load pages like this.

[Jasgriff]

Re: LG 42PC3RA SMPS power_ysus problems on/off/on click and fixes

It works much better when you just add them as attachments.

[slappala]

Re: LG 42PC3RA SMPS power_ysus problems on/off/on click and fixes

Powered by Nokia N8

[slappala]

Re: LG 42PC3RA SMPS power_ysus problems on/off/on click and fixes

http://*************/g/9/201203081821.jpg/

There they are in imageshack, let me know if you can see the pics or not.

[slappala]

Re: LG 42PC3RA SMPS power_ysus problems on/off/on click and fixes

Yep,
when the 5.v gets pulled straight from stby line it still deactivates 3.4v feed to the main board...

Maybe that's why it start's on the second try. I will post pics. My board is different. I did not see optocouplers at fast look :/.

Also I don't have schematics since it's PC3ra version and service manual does not include them.

[tom66]

Re: LG 42PC3RA SMPS power_ysus problems on/off/on click and fixes

I bypassed the AC detect and hooked it to a constant standby 5.v line. It works now, but the set starts twice quickly. Second time it stays on.
I would need a 5.v clean constant signal from some point. Where did you take it?

Also the line I used is really messy on my scope. not a clean 5.v I would say..and has a 0.5v spike that could cause the second start attempt/reset.

I think the PSU control chip might take the AC detect line down in case of the voltages don't rise enough quick. Is it coming from PSU control chip?

EDIT: I was right..the AC detect line comes from teh PSU control chip. Just measured it.

Now, where to take a clean 5.v signal or should I filter the standby signal with some caps to get a cleaner signal...I will measure the constant 5.v feed line if it is on all the time before the VA and VS lines are taken up.

Then I will call it "fixed" until it burns some fuses...like I said teh PSU is working A ok when teh set is on..only the voltage rise times are having trouble. see the pics on earlyer pages. and we are not talking seconds here

Post a picture of your board - the fix actually involves disabling the AC detect optocouplers with a bit of solder. I didn't try to pull to 5V on my board.

The signal goes through the PSU controller (a microcontroller of some kind which sequences the outputs and runs some checks.) The actual AC detect signal is lost on the primary side somehow; I'm not sure why or how it gets lost.

The disadvantage of disabling the AC detect on some TVs is you will hear a loud popping sound from the speakers if power is lost while it is running. However, it does not damage anything. Mine lasts 1 second with no power because of the lack of AC detect. And it doesn't happen when switched to standby.

[slappala]

Re: LG 42PC3RA SMPS power_ysus problems on/off/on click and fixes

I bypassed the AC detect and hooked it to a constant standby 5.v line. It works now, but the set starts twice quickly. Second time it stays on.
I would need a 5.v clean constant signal from some point. Where did you take it?

Also the line I used is really messy on my scope. not a clean 5.v I would say..and has a 0.5v spike that could cause the second start attempt/reset.

I think the PSU control chip might take the AC detect line down in case of the voltages don't rise enough quick. Is it coming from PSU control chip?

EDIT: I was right..the AC detect line comes from teh PSU control chip. Just measured it.

Now, where to take a clean 5.v signal or should I filter the standby signal with some caps to get a cleaner signal...I will measure the constant 5.v feed line if it is on all the time before the VA and VS lines are taken up.

Then I will call it "fixed" until it burns some fuses...like I said teh PSU is working A ok when teh set is on..only the voltage rise times are having trouble. see the pics on earlyer pages. and we are not talking seconds here

[slappala]

Re: LG 42PC3RA SMPS power_ysus problems on/off/on click and fixes

Hi, yep.
I think the reason is the powersupply can't get the voltage ramped up upon start enought quick--> mainboard shuts down the powersupply board.

Maybe this is the reason some tv:s have had software update..to wait some time for ramping up the voltages in older PSU:s..

AC detect is I think 5.v signal that comes when the secondary side is switched on. I think that works for my set as I have measured it before. Off course goes to zero after some MC forces the PSU down because it will not get to the normal voltage level enough quick.

Now is this small delay caused from some other board sucking too much current in start or PSU not giving enough juice as fast as nececcary by software protection is not clear for me :/..

I'm blown away the fact that there are billion emails about this problem with different brands and no one has "solution" for it. Or it's a big "trade" secret..thinking that TV companies sell the whole boards for repair maybe not so many tv tech's have not been tackling these problems.

I guess it can be a bad cap somewhere in any board sucking up the juice or then some other hidden problem in other parts.

TV functions super when I can get it going..maybe I should by remote controlled maincord plug and see when the component that is failing is dead for sure..would ease up things a bit.

Anyone have a thermocam?

PS: I'm very tempted to try some trickery here, but since I can't fool the PSU control chip that is sniffing the VA and VS voltages wihtout getting into trouble I have not done that yet. We don't know how the PSU control chip works and what pin is for what purpose excactly.

I thought the IPS chip was leaking to ground when starting the unit, but not..we'll I got rid of those red sparkels.. I hope this will be solved some day..