Emerson LF391EM4 No Power No Standby

This TV was dead with no standby light and no power.  A service manual is available for download here.

The power supply board has jumpers as test points.  I measured 20VDC P-ON+21V and 0V on AMP+13V.  AMP+13V provides power to the main board.  There is a FET which could short and blow a fuse but the 20VDC reading indicates the fuse is ok.  Checking the resistance of AMP+13V to GND with the board alone, I measured a short.

In the service manual, there is a flowchart for this line.  AMP+13V is not output.  No-> Check D653, D655, C654, C655, C668 and their periphery circuit. Repair it if defective.

Pulled off C654 and C655 and they both tested good.  I lifted one leg of both of the diodes and D653 was shorted.  The part number is SB3A0BH which is a 100V, 3A Schottky diode.  Equivalent substitute at Digikey is SB3100 (641-1418-1-ND).

I replaced both diodes and TV powered up.  Luckily, there was no damage to other parts when the diode shorted.


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