LG 42PC3D - UD Turns on and off till power is unplugged

[shizzel]

Hello all,
I've been scouring the forum looking for a solution to my problem. Please forgive me if this question has already been answered. Here's the back story on the unit. The power on/off happened intermittently for about 6 months till the set popped and the picture went out. I traced it to a bad y-sus board. I purchased a new board and the tv worked fine for about a month.

Turned tv on a pop. This time the z-sus board went. I had the z-sus board repaired by a reputable business mentioned n this site. About a month after the z-sus board was replaced, pop. Y-sus board went bad. I sent the ysus board in for repair. Just got it back last Wednesday and now the tv turns itself on and off until I unplug the power chord from the tv. Before I turn it back on and hear another pop does anyone have any ideas as to what could be causing the ysus to fail?

I read that the power supply could be bad. I opened up the tv and there are no signs of bulged caps. Any help would be greatly appreciated.

[tom66]

Re: LG 42PC3D - UD Turns on and off till power is unplugged

This could be the common AC detect issue; which is fixable.

The Y-sus and Z-sus should not fail so often. Your Vs voltage may be too high.

[thelmores]

Re: LG 42PC3D - UD Turns on and off till power is unplugged

Lots of TV companies suggest replacing the y-sus and z-sus at the same time..... looks like you learned that lesson the hard way. Some even suggest changing the y-buffers when you change the y-sus.....

Seems like something could be loading down your power supply, like a bad y-sus or z-sus...... or possible bad y-buffers.....

I would disconnect the y-sus and z-sus one at a time, and see if the power supply stays on. You may even have some bad y-buffers........ If you disconnect the y-sus from the power supply, and the power supply stays on, You may even have some bad y-buffers....... you would have to disconnect the buffers from the y-sus to isolate the fault between the two if thats what it comes down to.....

obviously, after the boards have been installed, and tv is running, you want to verify and tweak panel voltages, labeled on the sticker on the panel.

[shizzel]

Re: LG 42PC3D - UD Turns on and off till power is unplugged

Thanks for the quick replies. Unplugged ysus. Power stays on. Vs voltage 188.6 and 188.5. Both in and out of the power supply. I metered power coming out of the power supply to the ysus at 5.21 volts. Metered the power in to the ysus at 5.19 volts. Plugged ysus back in and popped the fuse....... UGH!!! heading to get some fuses. If that works I'll unplug the y buffer....

[tom66]

Re: LG 42PC3D - UD Turns on and off till power is unplugged

These TVs are notorious for blowing their Y- and Z-sus due to the extreme amount of heat produced.

[shizzel]

Re: LG 42PC3D - UD Turns on and off till power is unplugged

Good news sort of, i replaced the fuse and plugged ysus back in. Picture works. Metered the ac det. Stand at 4.89 volts when the tv is on or off. while I had the meter hooked up the tv cycled off and the volts dropped down to .38 I had to manually turn the tv back on and the voltage went back up to 4.89.

Tom66, I found an ac detect post of your in another thread. Could it be that the ac detect is sending a surge to the ysus when the power cycles n and off causing the ysus to fail? I noticed that the fix involves soldering pins 3 and 4 together optocouplers. Should the ac detect read anything when switching the tv on? I'm thinking of doing your bypass procedure.

[tom66]

Re: LG 42PC3D - UD Turns on and off till power is unplugged

AC detect is unrelated to Y sus failures. Face it, LG designed these badly. The early revisions kill the Y sus and the replacement IPMs don't seem to last too well either.

The bypass procedure comes only at the risk that the sound will click or pop when turned off. Also I've only tried it on the 42PC1D, so can't confirm compatibility with 42PC3D.

[shizzel]

Re: LG 42PC3D - UD Turns on and off till power is unplugged

Thanks Tom, was hoping for a magic bullet answer. My power supply board is the exact same as the board you posted in the other thread.

[shizzel]

Re: LG 42PC3D - UD Turns on and off till power is unplugged

Tom66, bypassed optos. No on and off... Looking good so far. Thanks for the suggestion. I want to put fans on the ysus and zsus. Should i pick up power from any particular place? Thanks again for the opto bypass!

[shizzel]

Re: LG 42PC3D - UD Turns on and off till power is unplugged

Got it handled. scabbed 2 5v fans out of an old laptop fan. Connected power to the molex into the ysus and zsus. Tv has been on for 3hrs. Hasn't shut down once and heat sinks on the ysus and zsus are slightly warm to the touch. Thanks to everyone for their advice.

[electronicsurplus]

Re: LG 42PC3D - UD Turns on and off till power is unplugged

Great work tom

Just used the same method (opto bypass) on the same television and so far still working. I never thought of that. excellent

Thanks
Jon

[robk627]

Re: LG 42PC3D - UD Turns on and off till power is unplugged

This may be a dumb question, but are the solder points for the opto bypass the same on the 42PC3D as they were on the 42PC1D?

I am having the same problem shizzle had.

Havent pulled open my 42PC3D since changing the 10 caps in the Power Supply.

[tom66]

Re: LG 42PC3D - UD Turns on and off till power is unplugged

Post a picture of the topside and underside of PSU and I will highlight them.

[tblock]

Re: LG 42PC3D - UD Turns on and off till power is unplugged

Can someone please post a link to this 'opto bypass' thing? I picked up a 42PC3D for $10 because it had a problem and the guy just wanted to get rid of it.

Basically, I plug it in and it just keeps clicking on and off. The display briefly lights up and turns off with the sounds of the click.

Looking at the power supply, I can't see any bulging in any of the caps. I unplugged the z-sus board and tested the 5V pins with a multimeter, each one wouldn't give steady readings.. the numbers jumped around.