This TV attempted to turn on but there was no backlight.  It was probably flashing a diagnostic blink code (possibly 10 blinks) but the front LED was out.

A service manual with all schematics is downloadable from here.  Power supply part number is TNPA5123CA.

Checking the voltages at the power supply connector, standby voltages at TUNER6V and VSB were present and stable at 6VDC.  With the power button pressed, 6V appears at RELAY and 2.3V on TV_SUB_ON which turns on the PFC circuit supplying DTV12V and 17V and 32V.  These voltages drop when turned on indicating an error condition was causing a shutdown.

Checking the main filter capacitor, I measured 390VDC on it when the TV was briefly on.   To force the power supply to stay on, I disconnected the connector to the main board and applied 6V to the RELAY pin and used a 10K external resistor from VSB to TV_SUB_ON.  With the PS on, the 390VDC stayed on but DTV12V and 17V was still shutting down.  This lead me to suspect IC7301 (SSC9512).  I measured  R7303, R7305 and R7306 by lifting one leg up.  R7306 measured a bit high – 1.3M instead of 1M so I replaced it and the TV powered up.

I still had the problem of no front LED and this was traced to a broken connector on the cable connecting to the main board.  I had to test and wire all the lines from the IR/LED board to test points on the main board  to bypass any bad connections.