LG 42PC5DC plasma tv clicking on off

[jose6326]

I picked up this tv today

The tv powers on and the red led illuminates in front

When I push the power button the red led turns off then a green led comes on then you hear clicking on the ps board and the red led comes back on

When I push the power button there is no illumination of the screen because it clicks on and off quickly

There are 2 fuses on the ps board which I found 1 blown 4A fuse on the ps board (see pic with arrow)
There are no visible blown/bulging caps

I did replace the blown fuse with a 3.15A fuse that I took from a spare ps board
Hoping this would fix the problem it did not it only blew the fuse when I pressed the power button
This fuse that keeps blowing is located in between the heat sink and the 2 big caps to the right of the center of the ps board (see pic with arrow). The other fuse that did not blow is located by the connector for line voltage

I ordered 10 fuses from digikey so I can try to figure out what is the problem which im guessing is a short somewhere.

Im guessing I have to disconnect the cables going to the other boards to determine if the short is coming from tbe ps board?

[capkid]

Re: LG 42PC5DC plasma tv clicking on off

You most likely have a failed component on the primary side of the PSU. I'd pull the PSU and look for obviously blown or burned components with a flashlight first.

[ben7]

Re: LG 42PC5DC plasma tv clicking on off

Checked the MOSFETs on the heatsink next to the fuse that keeps blowing. See if any of them are shorted.

[jose6326]

Re: LG 42PC5DC plasma tv clicking on off

thanks i will check for burned components later on today

how do you check a mosfet?

[jose6326]

Re: LG 42PC5DC plasma tv clicking on off

is ok to use glass fuses instead of ceramic as long as the amperage is the same?

[tom66]

Re: LG 42PC5DC plasma tv clicking on off

In general not unless you like picking out embedded glass fragments from your skin when the fuse blows. Most glass fuses can't safely interrupt high fault currents such as those available from your average plug, which will result in them exploding in many cases. I have made this mistake before, luckily I was in front of the set when fuse went pop. Ceramic fuses will retain their integrity when they fuse open.

[jose6326]

Re: LG 42PC5DC plasma tv clicking on off

yea i figured the glass would go everywhere if it blows.

right now i found 3.15A fuses so i will use those for now until the ones from digikey get here

[jose6326]

Re: LG 42PC5DC plasma tv clicking on off

Ok so I put in a good fuse, disconnected the cables going to the other boards

I plugged in the power cord and turned on the power to the surge protector then the relay on the psu started clicking and you can hear one of the transformers powering on and of in sequence with the relay and about 5-8 seconds the ceramic fuse blew

So im guessing the problem is isolated to the psu?

I checked the board and I couldnt find any visible burns or defective components

Now I need help checking the mosfets on the heat sinks

How do I do this?

What kind of readings am I looking for

[capkid]

Re: LG 42PC5DC plasma tv clicking on off

You probably have N-channel mosfets: https://www.badcaps.net/forum/showpo...0&postcount=40

Edit: You sometimes have to test them out of circuit if your readings are questionable. You can usually just unsolder one of the mosfet's leads.

[jose6326]

Re: LG 42PC5DC plasma tv clicking on off

I think I figured it out

I put the dial on diode mode and I tested the first 2 mosfets closest to the fuse that keeps blowing

These are the readings

1-2 : 0.002
2-3: 0.000
1-3: 0.002

1-2: 0.000
2-3: 0.000
1-3: 0.000

Did I test these correct?
Please see pics
The 2 mosfets in the yellow box are the closest to the fuse that keeps blowing

The number on the mosfets I tested is W20NK50Z
Not sure who the maker is but the logo is St

[capkid]

Re: LG 42PC5DC plasma tv clicking on off

It looks like you have shorted mosfet(s). I'd try testing them off the board.

http://www.mouser.com/ProductDetail/...2fw3iiJbnjc%3d

[jose6326]

Re: LG 42PC5DC plasma tv clicking on off

i will test the other 2 mosfets on the same heatsink furthest form the fuse blowing.

i didnt get a chance to last night

[tom66]

Re: LG 42PC5DC plasma tv clicking on off

Depending on the design you -may- also need to replace the L6598 chip too.

[jose6326]

Re: LG 42PC5DC plasma tv clicking on off

i wish i would have seen your post earlier tom

i could of purchased this chip together with the mosfets to avoid paying multiple shipping charges

oh well

i will replace the mosfets first and if it does not fix the issue i will replace the chip


thanks capkid and tom

edit: i was able to add the chip to my order just in time

[jose6326]

Re: LG 42PC5DC plasma tv clicking on off

How do I pull the mosfet off of the heat sink?

[capkid]

Re: LG 42PC5DC plasma tv clicking on off

You should be able to unsolder the mosfet from the board then unscrew it from the heat sink. Make sure you save the insulators that sit between the heat sink and mosfets.

[jose6326]

Re: LG 42PC5DC plasma tv clicking on off

I desoldered the legs from the board and removed the screws

But the mosfet is like glued to the heat sink
I cant get it off
I even tried pulling it off with flat pliers and nothing

[capkid]

Re: LG 42PC5DC plasma tv clicking on off

Those can be a pain. I have some bent needle nose pliers that I use to get a grip on the mosfets when they're stuck. Make sure you've removed all the solder then you'll have to grip the mosfet and wiggle it until it's free.

Edit: You might want to try heating the back side of the sink with a hairdryer if there are no components close by.

[jose6326]

Re: LG 42PC5DC plasma tv clicking on off

I got them off I just needed to use a little bit of muscle

Capkid can you check my other thread for the samsung tv
I need help going further on testing

Thanks

[jose6326]

Re: LG 42PC5DC plasma tv clicking on off

Should I replace the caps on the psu even if they are not bulged?

I really need to order an esr meter