The switches are good then. If the heating elements are no longer turning on by themselves, the next step is going to be testing the transformer of the magnetron to identify if it may have failed. With the microwave sitting unplugged for 10 minutes, the capacitor will need to be discharged to make sure there isn’t a charge in it. This can be done using a screwdriver with an insulated handle. With the red wire disconnected, the magnetron transformer would be tested for resistance from the red-wired terminal to the white-wired terminal (on the primary windings) and it should measure 0.5 Ohms. From the red/black wired terminals (on the secondary winding) to the chassis should measure 87 Ohms. The harness going to the magnetron should measure 0.2 Ohms when tested from wire to wire. If the transformer checks good, the magnetron would need to be removed and inspected for any indications of arcing or shorting on the antenna end. What findings do you get?
Ok, I think I followed. I discharged the capacitor and unplugged the red and black wire from it. Tested that to the chassis and got 29.5ohms. Then on the other side of the transformer were 2 terminals, one with 2 red and one with 2 white wires connected to them, testing between those terminals gave me .8ohms. Did I test the right things here?
It seems like you did. However, the resistance reading from the red/black to chassis is definitely low. A reduced resistance can allow for excess current, resulting in a blown fuse. Since this only occurs when the microwave function is started, we can rule out wires as a possibility. The door switches tested good, ruling that out as well. Since the relay control was replaced, it leaves the high voltage transformer as the likely cause for the fuse issue, especially since the resistance was low when testing to the chassis. Here is a link to the information page for the transformer. WB17X10030 Microwave High-Voltage Transformer | Genuine Ge OEM In Stock