I had the same symptom with a BM6500. Of course, that doesn't mean it was the same problem . For me, it was the vertical silver can-type capacitor near the fuse. (On page 10 of the service manual you'll find this to the right on the rectifier D3). Page 19, lists it as a 4700uF -10 + 50% 16V Cap. However, as it's an older receiver and that is a large electrolytic capacitor, you may want to check it first.
If you have the service manual (available to Gold Members) you should be able to isolate the issue fairly quickly. Since you know the fuse is blowing instantly, start from there and check every potential path to ground.
Good Luck!
-Rob