Though I was avoiding plasma TV’s due to their weight and lower resale value, I did get a hold of this one which was a bit newer. Research indicated the 50PV400 is a Costco version of the 50PV450 and pretty much identical.
Plasmas contain lethal high voltages so you need to be extra careful working with one which is currently or recently plugged in. I was able to find a training manual for the 50PV450 here.
This TV would not turn on and you could hear the relay click when the power button was pressed. Measuring the voltages on the power supply connector to the main board indicated the voltages were present but the power supply was performing a self-shutdown due to a fault somewhere.
Measuring the resistance of the high power output voltages going to the display (VS, VA and M5V) found a short on the VS line. The short was traced to the ZSUS board and close inspection of the board found a entire row of components under a heat sink with scorch marks at their bases. These parts were FGP4633 IGBT transistors (Q106, Q109, Q107 & Q110) and RD2003 diodes (D114 & D118). Checking the diodes and transistors in-circuit with a multimeter indicated they were all shorted.
Replacement FGP4633 transistors can be found on Ebay or Mouser but not Digikey. I wasn’t able to find an equivalent for the diodes. I ended up ordering a replacement ZSUS board since they were under $20 USD on Ebay.
Update: I got the replacement board and the TV powered on but the picture was splotchy. Not sure if there was a problem with the zsus board or if the problem was elsewhere.