Main Fuse blowing after nil use for a year.

My amp has been unused and sat in a cold workshop for about 12 months. When I powered it up, I hear a click and nothing. I check the main fust - and it is open circuit. I replace it with a slow blow 500mA as parts list, and it blows again. I have identified the load problem is on the transformer secondary circuit that feeds the diode rectifier section. I am a qualified electronics designer technician, but have no experience of this amp. Any ideas?