Backlight out but can still see Apple logo

Hello, I have a customer’s iMac in which he stated he heard a “pop”, and the screen went dark. I originally assumed this was the backlight fuse, which I was told is inside the display, and I have done the following and still no results.

PRAM and SMC

New Display (confirmed working on other unit)

New LED DisplayPort Cable (tested on both displays)

The Apple logo can be seen, but only under extreme light in a very very dark environment.

External monitor is only functional when the LED is not connected to the unit.

All 4 LED diagnostic lights are lit up when the unit boots.

I’m leaning towards logic board or power supply, but I don’t know where to go from here. Any help or advice would be appreciated. Thank you!