[Collins] 51S-1 Problem.

[Freeberg, Scott (STP)]

Interesting.  I don't have a schematic of the 51S1 so I can only assume a
few things.  If the fuse is designed to be on the hot, before the switch,
(which is good design technique),  and the radio is blowing fuses with the
switch in the OFF position, then this is what comes to my mind:
1.  worst case, the on/off switch is shorted ON and something high current
went on past the switch to cause it to short, and what ever that is, its
still doing it.
2.  More likely case, shorted ac plug, shorted or damaged ac cable, or
something bad in the switch if theres a ground connection potential.
3.  The usual stuff that you've already looked at, like the fuse holder
connections contacting the chassis, or a wire splinter nearly invisible
touching a ground connection.  

I assume you measure 0 ohms or close to it when the power switch is OFF and
you measure between hot and neutral or hot and ground.   You might just
unsolder the hot side of the AC cord and see if the short disappears from
the circuit.

Its fun pondering and trying to come up with possible solutions. 

73,  Scott WA9WFA