Radio Noise

[N131MA]

I am hunting some radio noise in my Eagle. Radios are GNS530W and GNS430W. The noise does not break the squelch but I have problems hearing distant stations. Diagnosis so far:

1. Prominent low frequency noise in headset (like "grrrrrrrrr") when the Garmins are un-squelched. Same on both radios.
2. Tried different headsets and passenger plugs, no change -> issue does not seem to be on the audio end
3. Sound is not "whining". Switched off alternator, no change. -> no alternator problem
4. Pulled all CB, switched off all equipment (except radio), no change -> not a radiating instrument or el motor issue
5. Noise changes frequency with rmp and volume changes with radio volume -> Radiated ignition noise?
6. Mag check. No real difference between L/R Mag. -> Not a "one mag only" problem
7. Switched off both mags in flight. Noise was gone! -> For sure ignition noise.
8. Grounded shielding of the P-leads. no change.
9. Removed P-leads from mags (hot mags). No change when engine is running -> not a P-lead shielding issue
10. Mags are S6-25 series. I believe they have a decent condenser filter integrated. -> Improvement by additional mag filter unlikely
11. Both mags got the 500h overhaul @ last annual. The noise was there before and still is. -> Mag (or condenser) problem unlikely
12. Spotted the noise with a hand radio. Looks like most noise comes from the mag/harness area

Planned next steps:

• Pull the Radio antenna and check impact. If noise reduces, noise is radiated. If not, noise is conducted.
• Visual inspection of ignition harness. Looked good on first view though.
• Check grounding of ignition harness on both sides (plugs and mags).
• For a test run shield mags and ignition harness with grounded copper tape.

Any ideas or guidance is highly appreciated!

 

[ArtVandelay]

Ignition harness is tough to tell if it's good by visual inspection, microscopic cracks in the insulation, a little surface corrosion on the contacts...how old is the harness? I'm afraid to ask what a complete harness cost...

[Robert C.]

Did you recently tune a VOR and listened on that frequency? I recently left the "nav" button pressed on the audio panel and had what you described till I realized the little tell-tale light was still on. Deselecting nav made the problem go away.

[takair]

Have you checked the large engine bonding strap?  Also, frequently radios use a common ground point.  If this degrades you can start picking up noise.  

[MooneyEngineer]

• Make sure the P leads are as short as possible without straining the wires.
• Remove Comm antennas; if there's a gasket, throw it away. Area under antenna base should be bare metal, burnished clean, and treated with an electrically suitable Alodine.
• Your radios get there ground through the ground blocks (rectangular black connectors) riveted to the bottom of the cowl deck.Should be no more than .003 ohm resistance to airframe ground. 
• Same with engine bonding strap.