This amp is certainly repairable. Did you try replacing all five fuses at once (four internal 3 amp quick blow, one external 4 amp slow blow)?
Often users overlook blown fuses mounted on the fuse clips on the audio boards, and replace only one of a blown pair. This causes the fuse to blow quickly again. The amp is VERY rugged and nearly always a blown fuse is all that is wrong with it.
Regards,
Frank Van Alstine