Fuse blows. Drawing too much current ?cause.

Question

Not my machine. Was told it just quit. Found a blown fuse. Had no replacement so I used a thin copper wire. With nothing in the oven sounded like it was running. As I watched the fuse became red hot. After 10 seconds before it blew the breaker tripped on the main board. Several tries with same result. Checked the magnetron, transformer, turntable motor and fan. The first 2 gave identical ohm readings as 2 other used units I had on hand. The motors function normally. No shorts detected. I dont know how to check the capacitors