LG Plasma 50" TV repair (Model: 50PS6000) - Feb 2016

Nb/ This repair was first posted by me (bodeplot) on www.austech.info forum on 20 Mar 2016. The contents of this post has been replicated here.

I repaired an LG Plasma 50" TV (Model: 50PS6000) unit for a coworker in February 2016. This isn't my line of work now days, but like to keep the skills sharpened in case I return to the service industry. It's my first plasma TV repair. A picture of the unit is below:

This unit would power up for 2 seconds, then power down. Relays could be heard energising (clicking) and then de-energising. No life on the screen. Without pulling the covers off, it looked like a short out.

Covers off, I isolated the power supply board and powered on. Fault had not changed, so investigated the power supply board. This is a very sophisticated power supply that has a power factor correction feature.

Chopper MOSFETS W20NK50Z (Q802 and Q801) in the switch mode section of the P/S were short. A slow blow fuse T4A H250V that powered the MOSFETS (F801) was also blown. See pic of PCB below. Nb/ the components are removed, and yellow squares indicate component locations:

The faulty bits:

Flipping over the board, the high voltage resonant controller (switch mode IC) L6999 (IC801) had also been taken out by the chopper transistors.

Replaced all four components from RS Components for under $40:

- 2 x RS Stock No. 486-3114 STMicroelectronics STW20NK50Z N-channel MOSFET Transistor, 17 A, 500 V, 3-Pin TO-247

- Packet RS Stock No. 541-3798 Cartridge Fuse, T, 4A, 250 Vac

- Packet RS Stock No. 167-6018 Fuseholder 5x20mm OGN 10A

- 1 x RS Stock No. 686-5011 STMicroelectronics L6599D, Resonant Controller, 16-Pin, SOIC

I also reflowed the area indicated by the magenta box in the above pic. There were lots of dry joints. This appliance not so lead free now :)

I think the source of the problem is lack of support for the power supply board. Other boards in this unit have support in the middle of the board. For some strange reason, the power supply board doesn't. It's a very heavy board which is prone to flex, and with the crappy solder used, and all the surface mounted electronics on the back of the board, it's a recipe for an early failure. The owner said this unit was faulty for a long time. He hadn't got around getting it fixed, so it faulted within a few years of ownership. It had also been transported a lot since ownership, face down. So, I reckon all that flexing of the board during transport damaged the soldering on this board, causing the eventual failure.

I recommended to the owner to never to transport TV face down, or even better, don't transport it at all !