Emerson LF391EM4 No Power

[neilc6]

Lots of repair kits for this model including FET and replacement fuse on the PS board, EEPROM, chip caps for the main board and repair service where you need to send both boards in.

On my board there is no front LED. Fuse is not blown and I get 20V on P-ON+21V line. However, AMP+13V which powers main board measures 0V and is shorted to GND on PS board.

Service manual with schematic is available and replacement PS board around $20USD on ebay.

Will dig a bit further since I have a schematic before giving in and buying the PS board.

[budm]

Re: Emerson LF391EM4 No Power

"AMP+13V which powers main board measures 0V and is shorted to GND on PS board." Is that resistance test done with the cable still connected between power supply and the main board?
Look to see if the main board has cap designator C3214 or not.

[neilc6]

Re: Emerson LF391EM4 No Power

I am measuring the short on the power supply board directly with no other boards attached.

[budm]

Re: Emerson LF391EM4 No Power

1) If that 13V line shorted, it should have cause the power supply to squeal like crazy.
Can we see the pictures of the whole top and bottom side of the board and also show the test points you use.
2) The P-ON 21V line will not be at 20V unless it get the PS-ON from the main board, when in standby, the +20 will be about 12V, the 13 will be about 8V, they will go up to full Voltage when PS-ON is present, so there is something not right about the measurement some where.

[neilc6]

Re: Emerson LF391EM4 No Power

Haven't had time to investigate too much but here are the pics requested. Also verified short across C654 which goes from AMP+13V to GND.

[neilc6]

Re: Emerson LF391EM4 No Power

Pulled up C654 and C655 and they tested ok. Lifted one leg of both D655 and D653 and D653 is shorted. Schottky diode SB3A0BH. 100V 3A. Digikey substitute SB3100.

[budm]

Re: Emerson LF391EM4 No Power

Good job locating the bad part.

[CapLeaker]

Re: Emerson LF391EM4 No Power

those 100V 3A Shottky diodes are very often the culprit, but usually there blows more than just a diode. I've replaced these before too, but I put something better in it than a SB3100. I can't remember what it was.
For reference here is a schematic.

[neilc6]

Re: Emerson LF391EM4 No Power

Part arrived today and tv is working. The troubleshooting flowchart in the service manual is correct.

AMP+13V is not output. No-> Check D653, D655, C654, C655, C668 and
their periphery circuit. Repair it if defective.

Most service manuals would say replace the power supply board. I replaced both D653 and D655 since it's the same part.

[CapLeaker]

Re: Emerson LF391EM4 No Power

sweet!