The coils in this machine are DC and you'd expect to see ~40VDC present at the coil at all times when measured to ground (the ground strap in the cabinet is a good place to test too). The SDB completes a path to ground for the coils when they fire.
The AC voltage at this coil is concerning. Check the voltage at the rectifier board. With your DMM on VAC put the black lead on E8. This is one of the connection points for the transformer. Put the red lead on TP4. You should have ~7.3VAC. If this is right then test the other TPs with black on ground. If they are fine then test the diode on the coil in question with the machine off. If it is OK then maybe the 20VAC on the coil was a misreading due to the coil firing.
Here is a link to the manual if you don't have one https://www.ipdb.org/files/2406/Bally_1978_Strikes_and_Spares_Manual_w_Schematics.pdf