I tried all standard basic diagnosing (change plug, change outlet, change cable, change everything)

I opened the TV found the following:

Power supply Board:

• model: BN44-00876D

Main Board:

• type: BN94-12395C
• model: BN41-02568
• code: BN97-12813A

I tried disconnecting the main board from the power supply and I can observe all backlights coming on though the holes in the back

I took out the power button and tested for connectivity.

I replaced the following parts:

1. Power supply with identical model (though it was the power supply)
   1. Checked all voltages on the connector that goes to the mainboard and are all in line with what is expected based on other articles here
   2. Checked voltages going to the backlight and they are in line with what is on the board (and the backlight turns on so that should be ok)
2. Main Board with a SIMILAR model (BN94-11914C) which according to the Samsung parts website, is compatible with my model (Samsung BN94-11914C Main Pcb Assembly). In fact the label on the new main board lists my tv model: UN55MU7600FXZC

After replacing the main board I am getting different symptoms:

When plugged in, the backlights turn on or a second, they go off for a second, they turn back on at half brightness for a second, they turn off, and after another second the red standby light comes on. When the red standby light comes on, you can hear a buzz from the power supply (as if it tries to power on) and then the buzz goes away, the standby light goes off. Wait 5 seconds, the cycle starts again.

Things I tried (with both power boards)

• Press the power button - no change, cycle continues uninterrupted
• Press the power on the remote control - no change. (Although you can see the IR sensor on the tv blinking rapidly when the remote control button is pressed.
• Disconnected the speakers from mainboard - no change
• Disconnected the wifi module from main board - no change
• Disconnected the mainboard from power supply - backlights come on, solid, stay on.
• Disconnected the backlight connector from power supply - nothing turns on.

Someone on the internet suggested this could be a faulty screen... but I find it hard to believe. could it be that the main board is a different code than my TV that is causing this? Even if the Samsung website recommends them as compatible?

Thank you!

EDIT:

Note: I temporarily disconnected the speakers and the WiFi module since they seemed to have no impact connected vs disconnected

Video with behavior when plugged in:

(documents.cdn.ifixit.com/igo/video/ifixi...)

⟐ Commented November 14, 2023 at 5:57 MST