Radio Noise

[Scott Aviation]

So I've put 20 hours on the 201 since restoration including new engine/radios etc. Since day 1 there has been noise in the radio when recieving a radio call, or whenever there was traffic on the radio, Ive got 2 kx-155's and an audio panel. The description of the noise almost sounds like whenever I recieve a call a microphone in the cabin is switched on and all you can hear is the sound of the engine buzzing sound underneath the radio chatter. Its not there during startup and taxi, I find its only present when the plane is at full power in cruise or on takeoff etc. There is also no noise when talking over intercom All crystal clear.

So far I have tested

-Turning off the intercom-still there

-Switching between com1 and com2 -noise still there

-the p-leads and ensured they were grounded and shielded properly.

-I have cycled between left and right mag in flight with no change.

-I had the magneto rebuilt with new capacitors I had the ignition harness tested and had new plugs installed.

-Radios are new as is all the wiring for them.

I have yet to test the spark plug resistance <5000ohms and the ignition harness shielding is not damaged.

Im just wondering before I rip cowlings back off and start pulling all the plugs and plug screws off if anyone has experienced this issue before

[N601RX]

Try pulling the field and output circuit breaker on the alternator. My P leads looked new out front, but I discovered they had been spliced twice behind the panel.One of the splices didn't even splice the shield, just the center conductor. Also verify the new radio wiring and its routing to the radio. Verify the grounds and shielding are terminated correctly.

[Txbyker]

I had very similar situation. Radio noise on climb out and in cruise. Distant radios very faint. No problems in the cockpit but static and noise with external communications at the higher rpms. Had the radios and intercom bench tested, antennas checked. Got new plugs, still no help. Turned out to be the capacitors not seated in the mag correctly and they were actually floating around in their bases. I realize you say the mag was rebuilt and capicitors replaced so that may not be of help.

[Scott Aviation]

Try pulling the field and output circuit breaker on the alternator. My P leads looked new out front, but I discovered they had been spliced twice behind the panel.One of the splices didn't even splice the shield, just the center conductor. Also verify the new radio wiring and its routing to the radio. Verify the grounds and shielding are terminated correctly.

Tried this already too Should have mentioned that. No change. Regarding the P-leads I think I do recall 1 of them being spliced under the panel, I guess I should have a closer look, does the P-lead shield being grounded make that much difference in radio noise? Any thoughts?

[Scott Aviation]

I had very similar situation. Radio noise on climb out and in cruise. Distant radios very faint. No problems in the cockpit but static and noise with external communications at the higher rpms. Had the radios and intercom bench tested, antennas checked. Got new plugs, still no help. Turned out to be the capacitors not seated in the mag correctly and they were actually floating around in their bases. I realize you say the mag was rebuilt and capicitors replaced so that may not be of help.

What did you do to resolve the cap floating around and not seating correctly? How should they be secured? I didnt do the work myself to the cap's and mag's I had an engine shop test and overhaul them.

[PilotDerek]

Maybe a stupid question, but have you tried a different headset?

[Scott Aviation]

Maybe a stupid question, but have you tried a different headset?

Yup, tried a Zulu 2, Zulu and DC 10.3

no change

[N601RX]

Tried this already too Should have mentioned that. No change. Regarding the P-leads I think I do recall 1 of them being spliced under the panel, I guess I should have a closer look, does the P-lead shield being grounded make that much difference in radio noise? Any thoughts?

The shield serves 2 purposes. When the engine is running in flight the voltage on the P-leades is pulsating at a high voltage. The shield helps prevent it from radiating to other electronics. The second purpose is the shield is the return path from the switch that grounds the mags out when the switch is turned off. I would not splice one, I would replace it.

Also make sure the ground strap is attached between the engine and firewall.

[BigTex]

I had something very similar and it turned out to be an isolation wire missing from the pilot's jack and the left backseat's jack. It didn't take the avionics guy 30 minutes to find the problem. It's amazing the difference that made in the quality of my transmissions.