Magneto grounding and radio interference

[nichzimmerman]

Ok, so this isn't quite Bearhawk related (except that the airplane in question is temporarily sharing my hangar with some Bearhawk parts) but I would be grateful for some advice here.

My brother just bought a Cherokee Six and is keeping it in my hangar until the engine gets pulled for a Bob Barrows overhaul this fall. As we flew his new-to-him bird home, we noticed that there was a point when COM 1 (GNS-430) had so much noise that the squelch wouldn't quiet it down and it has remained essentially unusable. COM 2 also has a lot of noise. but is still usable. Next morning during an engine run-up mag check, we noticed that turning the key to right mag resulted in no RPM loss. Turning the key all the way off did not kill the engine.

Facts:
Both radios have a LOT of noise (problem manifested in flight)
Right mag is not grounded (problem noticed on first run-up after radio problem)
The airplane just came out of annual, during which both mags were shipped off for some work that didn't quite constitute an overhaul (previous owner did this, not sure what was done)

Are these probably related? Is there anything I can check before taking it to a mechanic? I would like to see if there is an obvious disconnect in a P-lead but not exactly sure what I'm looking for as I haven't done my Bearhawk's FWF yet. How would you troubleshoot this?

This happed while in IMC. It reminded me that there is a multitude of possible issues that can result in a complete com failure outside of independent failure of both radios.

Thanks in advance,
-Nic (BH 1217)

[stinger]

Checking In! Hot magneto, There are a couple magneto switches that have service bulletins, old switches with poor internal contacts. The P leads should be grounded at one location not both ends, such as mag end only. Perform continuity test on P leads. Some old magnetos require filters, example Bendix . Good Luck Stinger

[JimParker256]

It sounds to me like two problems that have different direct causes. (Although "sloppy maintenance" might be the root cause of both...)

The engine not having any RPM drop when running on the Right Mag only and not shutting down when the switch is turned to OFF indicates that either the P-lead for the Left Mag is not properly grounded or the switch itself has failed. Since the mags were removed and replaced for maintenance, it is more likely a wiring issue. The wire itself may be damaged (bad crimp, broken inside the insulation, etc.) or you may have a loose connection. As Stinger said, continuity testing should tell if the wire is good, but I would check all the connections as well.

Noise (static) is often ground related, as well. There are LOTS of circuits in an airplane that can cause static on the radios if their grounds are flaky. And although it's statistically far less likely to be your issue, I had a problem with my push-to-talk switch that resulted in LOTS of static in the headset.

Good luck, and hope you find it quickly!

[Battson]

Sounds like a broken P-lead and broken P-lead shield to earth connection.
You could also add in a P-lead filter to help resolve it.

[nichzimmerman]

Thanks for the info folks. I learned something new out of each of your responses.

I took the cowl off yesterday, expexting I would have trouble finding a bad ground. In fact, the left mag had a wire coming out of it (p-lead) with a mag filter on the end and it was dangling freely. The mag filter bracket had broken so it was hanging by the two wires until the terminal on one end broke. So after a new bracket and new terminal, it's all working as advertised.

It makes me look forward to experimental aircraft maintenance a little bit more.