LG 42LH30 Power Supply Help

[jrh215]

Hi,

I have recently fixed a Power supply from an LG tv that has been the victim of a power surge. I replaced two MOSFETs that had shorted out, along with other traces that were leading up to them. The problem is that when connected back to the rest of the tv, it works like it should until the picture starts to display, then something on the board makes a faint clicking sound about three times in a row then shuts off (red light displays again). The ticking also takes the inverters power away, making the screen flash. Im assuming that it has something to do with the inverter section of the board, but cant seem to figure out what is making the ticking noise.

Otherwise, the power going to the tuner board never loses any power when the ticking occurs.

Any help would be greatly Appriciated

 

[jrh215]

The ticking seems to be coming from one or both of the inverter transformers, but im still stuck on why they would make any ticking sound or short out.


Anybody know of this problem??

 

[bountyhunter]

I am almost dead certain that when you hear a power supply "ticking" it means some kind of overload protection is clicking in. In most cases, it waits a specific time interval and tries again. If the overload is continuous, it will "lock out" after a specific number of faults.

You may have repaired the power supply, however: when that supply got hit by a high voltage surge and things blew, it likely passed high voltage through to other parts of the unit which are now blown and present an overload when the power supply tries to feed them.

It's also possible that not all of the power supply outputs are working as designed.

 

[jrh215]

Could the ticking be caused by the inverter transformers turning on/off??

The two out of four MOSFETs I replaced give power to the transformers when the rest of the power supply is told to turn on.

These can still fail even if they never got to the extent of shorting out can't they??

 

[bountyhunter]

I am just saying that:

makes a faint clicking sound about three times in a row then shuts off (red light displays again).

this sounds exactly like a protection circuit cycling and then locking off. Something is seriously wrong, either in one or more of the P/S outputs or the circuits that are driven by the power supply.

The switch FETs can fail from a variety of reasons, but a voltage surge would apply a voltage to them that exceeds their limit and they would break down with high current flowing and a large voltage across them which blows them up quickly.

 

[jrh215]

Well......whatever was wrong in the first place just blew the FETS I replaced, so im just going to give up on this piece of crap and try to get a new board.

 

[RhapsodyOfFire]

I have the feeling one of the capacitors is bulked/blown - the big on the upper right of the second picture. It could be optical illusion because the image is not good .

 

[jrh215]

They all look fine. Its just the picture.