this is for the coil? or to see if the rectifier is bad?

 

If your getting AC, it's because the diodes in the voltage regulator (AKA..rectifier) has gone bad.

They only allow one way current flow which take AC from the stator and changes it into DC coming out of regulator. (Regulator reads battery condition and adjusts voltage from ballpark 13.5 DC to 15DC)

Usually diodes burn out from heat. They can short across. AC can take out a DC aftermarket electrical ignition controls.

Wasn't sure about coil, and obviously it needs to be running and you need to do all the static checks first mentioned above.

Make sure even if no AC that you also don’t have over 15 volts DC at a high rpm.

 

Well. IMO the OP doesn't have a meter that can measure the 1/2 ohm that the primary winding should be so there is likely nothing wrong with the coil.

 

He can also try and arc the coil to test it, ground one side of the primary to negative 12V source, and touch and release rapidly, the other primary coil terminal to 12v positive and see if it sparks while the plug is grounded to the negative 12v power supply.