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

[slappala]

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

You say that the TV has to be off longer if it was running longer. That makes me think something else is getting hot, and needs to cool down; so when hot it is somehow shorted, making the power supply click on and off when trying to start up the TV(lots of times there is a startup surge current that gets drawn from the PSU). The values of components do go up and down with varying temperature.

You need to find out why there is a large current going through the fusible resistors when the TV does its on-off thing. The going on and off is from the power supply detecting an overload on one of its outputs. That is a safety feature.

-Ben

Thanks,

I saw today a 200A clamp for 28$ to use with a multimeter at a local electrics store, will go on and buy it, need it anyways in future.
They even have temperature cameras, but rent is 50e or something for a day..

The VA 60.v line is a good candidate for the root cause..Don't remember if the 15.v for IMP chip is made from this line or the VS line..

luckily I can use the new caps on my tubeamp projects.

EDIT: Got the service menu open by holding both, remote and tv menu button simultaneusly for 5s. AC detect had 13 hits for shutdown. Don't know if there is a counter that will zero this from fault memory when set is sitting off mains for some period of time and then it will let you start the TVt until gets a new ac detect error.. BUT When I can get it going I can watch the tv 8h or more easily..it just does not make sense to me why the problems comes only at start up phase.. since all the components are quite hot during running.

It just makes it really difficult to find the primary cause of the starting problem , I don#t want to toss in too much money for this old tv..can get new ones for 600$ here..alltought they last the same 4.y max porobably ..

[slappala]

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

I got the amp meter..used..was broken. waiting for new one to isolate the problem to y-sus board IC

[tom66]

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

Just for the sake of eliminating the fault, check the AC detect.

You can disable it. The only downside is if the power fails, the speakers make a "pop" sound sometimes. It does not cause EEPROM corruption as far as I can tell.

[slappala]

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

Finally watching/repairing the tv again. I still did not manage to get nice amp clamp.

I bit the bullet and bought 2 new STK chips..changed the Y-sus board STK chip and have been testing it for few hours now..no problems yet.

Cross my fingers...and thumbs..next weeks will show it I guess..

VA and VS are turned down for now..I don't have back protection on the tv if It still gives me some !§$&%/&(..

power slopes at VS and VA look consistent when powering on/off...

Less red sparks also.

PS: AC detect/protection was the fault that kept shutting it down.

Have to add fans to the STK chip cooling sinks if it works now to keep it cool in the future. I have not changed the Z-sus STK chip.

Some advice on the STK chip..I found it impossible to use my soldering station(blower and normal) to take the chip out..even if cleaning of all the excess solder from pins some of the holes are so small that the residue will be enought to keep the pins attached.

You should have some special long tips or air ducts for welding station to get it off with welders and be able to heat all same side pins at the same time...

Or do as I did..I desoldered big caps and big coils that were on the way and just used cutter to cut pins from the chip. Then took 1 pin at the time of and cleaned the pads. Worked like a charm.

[slappala]

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

after 4h looks to be a ok..on/off working normally also even when hot.
Have to use it for a while to call it done.

[slappala]

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

Ok, today the same symptoms appeared again.. :/
It's freaking wierd. maybe I'll try to bypass the AC detection..

[slappala]

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

had it off the mains for 1/2h and now it started fine again..it was sitting on teh mains for 3 days after a long period of the mains..

somehow this is connected to plugging the unit in.
If I take off the mains cord when not watching I'll bet it works. What is causing this? something on the PSU primary side??

Or some software reset thing on main cpu(control cpu:s...

Any help will be appreciated

[tom66]

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

Check the AC_DETECT pin before and after the failure. If it drops to zero it's probably your fault.

Not figured out yet what causes the AC detect on these TVs to fail but it seems to be very common.

[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..

[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

[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

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.

[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

Powered by Nokia N8

[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.

[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.

[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

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

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

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