Samsung UE39F5000 power supply no start no backlight

November 4, 2021
board power
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fault description:

* red LED flashes
* if I disconnect the Main Board (MB) from the source board, the backlight does not light up !!!
* all LEDs / number 91 / are OK
* I checked all the capacitors
* I checked all the diodes
* I checked all the FETs
* measured voltage when the Main Board is disconnected
* if the Main Board is connected the voltage pulsates

B13V 8.7V
B13V 8.7V
Vamp —
Vamp —
GND
B5V 5.2V
B5V 5.2V
P_DIM —
ECO_ON / OFF —
BLU_ ON / OFF 2.9V
GND
GND
A5V 5.2V
PS-ON 3.3V

but I managed to turn on the TV normally by accident and both the backlight and the image went – the error is definitely in the power supply

PS

if i want to turn on the power board on a table without a main board how can i do it? should the PS-ON pin be grounded ???


John