Kensandyeggo wrote:
...
I took her in for my annual, and the A&P said
that the old Bendix mags do not have shielded wiring, and that's the
cause of the noise...
Am I correct in assuming that the hash gets
into the radio from the pos. input, or can the ground contribute?

The unshielded ignition is radiating radio frequency interference at
the COM frequencies (118+MHz) all the way from the plug wires to the
COM antenna. You have a whole battery of spark gap transmitters running
at the same time. The interference is NOT getting into your radio via
its power wiring.

Would wiring the radio power directly to the pos anode of the battery
with a circuit breaker inline, and running a separate ground for the
radio help, or am I just whistling Dixie?

Dixie! The only way to cure it is to shield the ignition system. Unless
you know for a fact that the "towers" on your existing mags are
internally shielded, you will have to replace the mags too. Don't
forget to install shielded P-leads, too.