Samsung UN40F5500 Doesn’t Turn On No Backlight

Model UN40F5500 UN40F5500AF UN40F5500AFXZC

When the power button is pushed, the front LED blinks but the TV doesn’t turn on. Unplugging the main board from the power supply, the backlight flickers. Checking the strips with an LED tester, they all test good.

The voltage across the leads on the big main capacitor go the expected 390VDC. Checking other voltages, B5V was at 5.2V but B13V was unstable drifting around 11V +/- 1VDC. Since B13V supplies the LED driver IC, this unstable voltage was causing the flickering.

The power supply board was BN44-00645D and I found a schematic for
BN44-00645A. The suffix is usually a different revision but compatible part number.

Checking the components around the unstable part of the circuit, the capacitors all tested good.

Since I couldn’t find the fault, I ended buying a replacement board on Aliexpress. With the new board installed, the TV powered up.

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