Progressive headset noise during run up

[NC Bill]

I search the database but haven't found an answer to this issue.

On return flight both headsets developed interference loud enough that I couldn't understand approach nor tower. Fortunately the copilot had better hearing and was just able to make out the instructions. The noise stopped after landing and during taxi.

Today I was ready for a test flight but the issue reoccurred at run up as the RPM exceeded 3500 and progressively increased beyond that.

Later I returned to the airport and after engine start I increased to 4000RPM and proceeded to turn on the avionics switches 1 at a time.

I found that the INTERCOM switch toggled the noise ON and OFF. Nothing else started or stopped the noise. Once again the noise did stop below 3500RPM even with INTERCOM ON

Suggestions appreciated.

 

[Tom Baker]

You want hear anything in your headsets unless the intercom is on. What you need to do is shut off individual items with the circuit breakers one at a time to isolate the problem. It could be anything from a bad connection on the intercom to a ground loop issue from another piece of equipment. I suppose it could possibly be the capacitor for the electrical system too. 

[BravoFoxtrot]

The PM3000 has a fail-safe mode:

"The PM3000 has an automatic fail-safe interconnect to the aircraft radios. If power is disrupted to the intercom for any reason, an internal relay will connect the pilot's headset to the aircraft radio allowing continued radio communications."

I would expect that leaving the intercom off you would still hear the radio. You could test this by tuning to a weather broadcast station.

[NC Bill]

I was tuned to ATIS and the signal came thru CLEAR to the headset with INTERCOM OFF. Interference returned with INTERCOM ON.

[BravoFoxtrot]

I think that puts you back to Tom's troubleshooting suggestions above. On the bright side, you should be able to fly the plane in the meantime with the intercom off.

[Anticept]

Do a voltage drop test with all equipment running from the ground bus to the battery.

It shouldn't be more than a hundred millivolts or so. Upgrading the capacitor to a 100k millifarad goes a long way too, but wont help if you have a dirty ground.

[johnnyjr]

I’d start with checking your grounds behind the panel on the fire wall as well as the ones under the cowl. There are jumpers from the firewall to the engine mount as well as mount to the engine block. I found a missing ground that was left off after overhaul that had be chasing gremlins for months. And then one day I had the cowl off and slid under to check things out and saw the mount to block cable just hanging. Reattached and bam all the issues were gone. 
 

When checking the grounds behind the panel it’s not a bad idea to loosen the nuts almost till they are at the end of the stub. Spray some electrical contact clearer, wiggle the wires to get them all wetted and re tighten the nuts. Be generous and spray a couple times to make sure any crud get cleaned out. 

[cdarza]

After you've gone through the above mentioned recommendations (ground etc etc) and probably last is the intercom itself.  I went  through months of trouble shooting on a whine that would typically come on only after takeoff.     Eventually found what i think was a burnt connection in the intercom.  Im no expert and cant explain the details but after fixing the burnt connection its been perfect thus far. 

[cdarza]

On 5/17/2021 at 7:21 AM, BravoFoxtrot said:

The PM3000 has a fail-safe mode:

"The PM3000 has an automatic fail-safe interconnect to the aircraft radios. If power is disrupted to the intercom for any reason, an internal relay will connect the pilot's headset to the aircraft radio allowing continued radio communications."

I would expect that leaving the intercom off you would still hear the radio. You could test this by tuning to a weather broadcast station.

This is correct but in my case with Bose headset i had the headset switch on stereo instead of mono and i couldn't hear anything.   YEARS later - whilst trying to figure out if the intercom wasnt wired correctly,  i flipped the switch on the headset and now i can hear without intercom but only mono.