If you feel comfortable to work under power then unplug the microwave and open the control panel. Remove the wires from the HV transformer primary winding and attach voltmeter probes to these wires. Plug the microwave back. Start the microwave as usual (the door is closed) and measure the voltage. Post the result.
Here are the breakdown diagrams and Parts for Whirlpool MH1170XSB3 Microwave - AppliancePartsPros.com
P.S. How the microwave is installed into the cabinet is described in the Installation instructions