Microwave, Sharp R 957 not heating?

Sharp R 957 not heating up. Initial inspection revealed that the 8amp mini fuse was blown. Replaced this and tried but it blew straight away. Replaced fuse again. This time i disconnected the bulb and one of the front (control Panel) PCB connections - these are the 2 things the fuse seems connected to. I tried again and the fuse only blew when it was connected to the front PCB. Does this mean i need a new PCB? are these available? Could it be anything else? Can i replace the fuse with a bolt (maybe not!). Please advise if you can, thanks very much.