According to the Boeing Maintenance Manual (D&0) 24-00-00....
"The SOURCE OFF light does not indicate that the AC transfer bus is de-energized. In flight, the left SOURCE OFF light comes on when GCG 1 trips. However, the bus transfer function lets IDG 2 power AC transfer bus 1. These are the examples:
-IDG1 fails
-The bus transfer closes the BTBs
-Both AC transfer buses get power from IDG2"
This appears to contradict the earlier responses.
Rgds.
NSEU.