This Toshiba TV would turn on but there would be no picture or no backlight. Probing the power supply outputs, I detected no 30V, no 20V and no 24V (powering the inverters for the backlights) but the rest of the voltages were present. I was able to find a schematic online and it pointed to a regulator STR-Z4579 which provides these voltages. I ordered the part from ebay and when I replaced it, I still got the same results as before.
I checked every diode, resistor and capacitor in the circuit but didn’t find any bad components. I then suspected the regulator was shutting down due to a fault. I unplugged the 24V connectors going to the inverter boards and the 30V and 24V were now present. The 20V appeared to be unused from the schematic.
I measured the resistance of the 24V input on the master and slave inverter boards and found a short on the master board. Checking the four transistors, I found shorts on two of them. I ordered the replacement FDD8447L transistors on Digikey but when I searched on ebay afterwards, I found repair kits consisting of four transistors and the IC BD9897FS. If the transistors don’t fix the problem, I will need to order the IC as well. Searching the board part number 6632L-0450A (PPW-EE42VT-M) on ebay also turned up a repair kit. Looks like this failure also appears on Toshiba model 42HL67.
Update: Replacing the two shorted transistors and putting the board back in caused them to short again. I have ordered the repair kit with the controller IC.
Update 2: Replacing the controller IC didn’t help though the 24V was stable and there is an LED on the inverter board that flashed when powered up. I suspected bad soldering on my part.
Searching replacement boards, I found an Aliexpress listing indicating the following boards were all compatible.
6632L-0450A / 6632L-0451A
6632L-0371 / 6632L-0372
6632L-0448A / 6632L-0449A
6632L-0474A / 6632L-0475A
6632L-0470A / 6632L-0471A
6632L-0465B / 6632L-0466B
My TV used a 450A/451A pair but the boards were $50 USD on ebay. Found a 474A/475A pair for $13 USD so I ordered it.
When they arrived, I first checked for shorts and then plugged them in. The TV backlights finally powered up.