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Hi John and welcome to the site, I’m Louie. Sorry you had to wait awhile but I’ll do my best to help.
Initially, bear with us; re-confirm and post back with the complete model number of the Samsung.
John, might it perhaps be an S50FH-YB03 panel? The model number would be something like PL51D530A3FXZA.
First order is to determine if the power supply is working; i.e. are all the secondary voltages present...consistently.
Then determine if the panel is priming; i.e. upon powering on, the panels turn from all black to soft gray.
To serve as common reference, please post back with clear pictures of the rear of the TV showing the lboard layout. Click HERE.
The "...brown circuit board in the middle..." is the power supply board.
The expected voltages would normally be stenciled/printed right across or very near the connectors.
Two (2) very important voltages for the panel would be the Va and Vs.
Please post back with a very clear picture of the power supply board. Clear enough to be able to identify the board label.
Appreciate the pictures.
And yes, these are the expected voltages when measuring the pins of the connectors.
As posted above, the Va and Vs are critical voltages.
Please check/measure the different connector pins and match reading with printed values.
Somewhere on the panel would be a sticker where the expected Va and Vs voltage would be printed. Please post back these values.
Still, since the voltages are not consistent would make this "...brown circuit board in the middle..." power supply board highly suspect.