I was able to find the schematics for this mainboard. apperently Sylvania uses the same main board.
anyways here is the link to the service manual pdf
http://forum-files2.fobby.net/0004/5937/SYLVANIA%206420FE.pdf
I had the same problem as you, and was able to find a bad diode in the bridge rectifier.
alterintel
November 2008