No power, no standby light - standby voltage is present though.

I have a 65x900f that will not power on using either the power button or the remote. I have confirmed that standby power is present on the power supply board where the cable leads to the main board. I have also confirmed that the standby power makes its way to the main board connector. One thing I have noticed that is strangem, is that the “power_on” pin on the power supply board constantly reads 3.5V. This doesn’t change when the switch is being pressed. There doesn’t seem to be power going to the TCON board, there is a 12V signal going to the LD board , but the LED_VCC are (obviously) not getting the 12V that they require.

When the main board is connected to the power supply board, 12V is available to the LED driver board, but when the main board is unplugged no 12V is present at the connector from the power supply to the driver board.

I have scoured the internet for any documents relating to this model but have found nothing. Does anyone have experience working on a 900f? I am willing to assume that swapping the main board might fix the issue - but am unfamiliar with the function of some of these boards (ie. is the main board switching anything on/off on the power supply board, or am I to assume that the absence of no 12Vcc on the LD board means the power supply is faulty too?)

Any help is appreciated!

Thanks