Won't heat any foods. - Whirlpool MH7140XF Microwave Oven

You have a problem with your magnetron tube, and when the magnetron tube goes bad, the diode almost always goes out with it. you should be able to order a replacement tube from your manufacturers website, if you cant let me know and ill try to help you more.

here are the steps to replace the magnetron tube, if you want to do it yourself that is. if you have a professional do it, it can be pretty costly.

1. Unplug Microwave at least few hours before
2. Remove Microwave from above oven range by somebody holding Microwave from ground unscrew the screws from cabinet
3. Remove top cover of Microwave
4. Make sure to keep screws correctly in a place you remember what goes where
   
5. Make sure capacitor is discharged, GE manual inside Microwave says capacitor will discharge within 30 seconds once it is off. As a precaution check the voltage in capacitor to make sure there is no electricity
   
6. Remove other panels around magnetron part
7. You need to remove lot of connecting wires, should be careful to note down what goes where
   
8. If possible take digital photos to refer back when you put everything back together
   
9. You may need to remove transformer to easily remove and replace Magnetron. If you can't remove Magnetron easily then remove bottom panel of the Microwave and remove transformer
   
10. Now it should be easy to remove magnetron
    
11. Replace new magnetron and screw it. We had to put one outside part from old magnetron to new one because new one was missing that. Check to make sure both old and new one are exactly the same
    
12. Make sure put all wires properly back
    
13. Now put transformer back and screw it
    
14. Put capacitor panel in place and screw it
    
15. This might be a good time to test the Microwave to make sure it works fine now
    
16. Now cover the top panels and start to screw everything
17. Once everything assembled put the Microwave back above range put screws from top