If it is one of the models with IPMs, they often fail but I've successfully replaced just the IPM instead of the whole board. A blown IPM will generally fail shorted so will blow a fuse, usually a 4A ceramic 5x20mm, so check fuses first on the Y-Sus and Z-Sus boards. If one has blown, check for a short on that board, typically Vs to ground is what shorts due to a blown FET/IGBT. If this model uses IPMs, the blown FETs will likely be inside the hybrid module, requiring replacement (though some people have bodged on new external FETs
). It's a straightforward task with some solder wick and a decent iron, though a desoldering station is quicker.
If the fuses are all good (look for surface mount ones, too), move on to standard plasma troubleshooting... Check your high voltages... The Vs and Va testpoints are usually on the power supply board and the other voltages on the LGs are usually called things like -Vy on the Y-Sus and and Vzb on the Z-Sus board in LG parlance.
The proper voltages will be on the panel sticker and, IIRC, the second line on the LG stickers below where it calls out the Va (the address voltage for the X boards at the bottom) and Vs (the sustain voltage that does most of the power, comes out the Y-Sus board through the Y-Drives) goes in this order: Set-Up / -Vy / Vsc / Ve / Vzb
Some of these will probably be missing and say N.A. on the sticker as most LG sets do not have all of these voltages but they have maintained the sticker format. Just look for the test points for each and see what voltages you actually have at the testpoints, then I can point you in the right direction for troubleshooting that section.
Vs and Va may only come up for a few seconds at power-on if it senses a fault and goes into protection but you should have time to measure them. A MAX function if your multimeter has one can be helpful.
The service manual is avilable at:
http://www.toms-service-manuals.com/...&mfg=LG-Zenith