LG 32lb9d, primary of power supply shorted

[cashkennedy]

I got this broken ~ 1.5 yr old LG set 32lb9d and the fuse was blown, i did some basic testing and both sides of the main filter cap were shorted / 0 resistance so i figured it was shorted, but went ahead and put a small jumper in place of the fuse. After pluging in the board there was a big flash around the main tranformer and some good smelly smoke. I didnt see any actual damage to any components till i flipped the board over and found a tiny SMT transistor had blown. The transformer also looks a little imperfect so the transformer might be shorted. What should i test? Or should i just see if the short is gone now that the transistor blew.


This power supply board (made specificaly by / for LG) is soldered worse then anything ive ever seen, every joint is covered in resin ... There are 3 capacitor kits available for this board (this set has only been on the market for about a year and uses a lot of samxon and samyoung caps, but seems crazy that people would be replacing them that fast), but i dont think caps on the secondary could case a short on the primary.

Last 2 pics have the damaged transistor, and the area that the flash came from when i powered it up.

[cashkennedy]

Re: LG 32lb9d, primary of power supply shorted

The 2 major FET's before the transformer are STM p9nk60z
http://www.alldatasheet.net/datashee...P9NK60ZFD.html

They both read 0 resistance between all 3 pins, so i assume somethings still shorted. The dc outputs at the bridge dont appear shorted though / and the main filter cap is no longer shorted. The 2 fets mentioned above have 0.0v voltage drop somewhat expected from a short.

[budm]

Re: LG 32lb9d, primary of power supply shorted

I think the shorted MOSFETs also take out the IC that drive them, also the components around the Gate are damaged also.
How much is the new board?

[cheapie]

Re: LG 32lb9d, primary of power supply shorted

...and this is why bypassing fuses is a bad idea.

[cashkennedy]

Re: LG 32lb9d, primary of power supply shorted

I think the shorted MOSFETs also take out the IC that drive them, also the components around the Gate are damaged also.
How much is the new board?

Well I cant say for sure the mosfets are shorted without lifting them from the board. But likely that small SMT transistor wasnt the culprit, and replacing 2 FETs if they are bad would bring me to about 12 in repair cost.

The boards for this arent that cheap, i see one on ebay for 40+10s/h, could try to talk them down to 35 total.

Ill probably buy a replacement board if i cant 100% isolate what part was originaly bad, but just seeing if theres anything more i should test. I supposed pulling the 2 FET's would be logical if i werent so lazy

Plus im always baffled by how transformers are connected in SMPS's , not sure if i should expect a low resistance for all parts after the FET's or pratically 0 since the transformers just a few coils of wire.

[budm]

Re: LG 32lb9d, primary of power supply shorted

You should make one of this, it will put the lamp (75~100 Watts) in series with the load (your TV). It will light up brightly if the shorts are still there, otherwise it will light up bright and then dims down once the main cap is charged up.
http://s807.photobucket.com/user/bud...?sort=3&page=1
I would trace out the Gate to see which IC is connected to and replace the IC, otherwise you may blow up your new MOSFETs.