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Magnetos are a huge source of interference, think of them as little noise generators. Magnetos generate a large amount of hash into the P-lead harness. The unshielded P-lead acts like an antenna for all of the radiated RF interference. The best practice is to have a P-lead filter on the magneto. The wiring from the switch to the P-lead terminal needs to be a shielded wire. The shield is only grounded at the magneto. Ideally there should be no more than an inch or so of unshielded wire at the magneto.
Make up your mag leads as one continuous shielded assembly. The shields ground at each mag and stay insulated at the switch. You just have the core wires attached at the switch.
It sounds like your install was done on the Q&D (quick and dirty). This may be a significant part of your static problem.
There is another thing to consider. Each of your radios has filtering on the audio output stage for just the symptoms you describe. That filtering is simply foils capacitors. The caps dry out over time and fail. The caps are there for the very problems that you are chasing. Proper mag shielding may not solve all of your problems if the output caps are bad.
While we are talking about filter caps, do you have a capacitor on the alternator (B+) output terminal?
Let me share a project I just finished. Cessna 172 with noise in headsets from: beacon popping, alternator, flaps, PWM dimmer (Max-Dim), turn coordinator, and generally poor sounding audio. I rewired the alternator output to the bus. Replaced the field circuit as it had >450mv. line drop. After replacing the field breaker, every wire, and master switch the voltage drop was 165mv. I central point grounded the beacon and the flap switch. I also added an output cap to the alternator. Further inspection found sever breaks in the shielding on the intercom leads to the back seats. I semi-carefully measured all the intercom runs and had PS Engineering build a new harness for me. The end result was an absolutely quiet, both audio and electrically, system.
Getting the PWM dimmer quiet was the toughest. It turned out that the speaker was still local grounded under the screw. This uses the airframe as the ground path, a no-no. The problem ended up being the output audio filtering in the audio panel. Once repaired 95% of the noise in the speaker went away. There is no noise in the headset. It is dead silent.
Joe @ Max-Dim (Seaton) is wonderful to work with.
Please report your findings with the list of items mentioned in a previous post.
Dave
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