I don't have an operations manual for the 238, but I have the service manual and from looking at the schematics it looks like the FILTER switch is the dbx defeat for channel 8. When you slide the TAPE SYNC switch IN (the one next to the little level trimmer) then you have the option to leave the dbx on (FILTER OUT) or to turn the dbx off (FILTER IN).
Not 100% sure about that...anybody else with a 238 know the answer for certain?