[ms660]

In the manual and schematic I'm looking at the 100 Ohm resistor for the bias adjustment etc is connected in series with the ground end of the heads.

In that schematic that's the right hand connection to CH1 head and the right and connection to CH2 head, in other words between those connections and wiper contacts 12 and 16 respectively on the track selector switch, the common ground connection for both heads on record is via connection 22 on the preamplifier board and the record/playback switch when it's set to record, don't forget to convert from peak or peak to peak down to RMS if using a 'scope to set levels, if that's any help.

Lawrence.