need help diagnosing radio static

[RobertE]

I've got radio noise that, I'm sure, is like many have had before (found lots of examples searching the archives) but my symptoms, along with what I've learned so far by troubleshooting, are sufficiently unique that I need help from what is the very big collective brain of Mooneyspace.  Here goes.

Symptoms - When receiving any signal but the very strongest (in other words, pretty much everything except a tower controller 10 miles out or a approach or center controller when near an antenna) I get  staccato-type, high frequency static in receive mode that is loud enough to make it very hard to hear a transmission.  This is a crackling sound, not a whine.  This happens in both radios (a Garmin 430 and KX155).  The static registers only when receiving a signal.  When not receiving there is no noise although, occasionally my 430 blinks that it is receiving something and opens the receive circuit when there is no one broadcasting.  I'm guessing that whatever is going on is producing enough noise that the radio thinks there is a signal to receive.

Troubleshooting so far -   Turning off the field to the alternator doesn't stop the static, nor does turning off power to every circuit in the panel.  The static persists when running on both mags, the left alone or the right alone.  The static does change with RPM.  At my normal cruise setting of 2,500 it is loudest, the pitch grows when I go to 2,700 and, oddly, it disappears entirely when I slow to 2,300 or below (maybe the squelch filter is strong enough to block the noise at that frequency?)

Tomorrow I'll run the RPM test with the squelch disabled and will also turn off both mags (at altitude) to see if that stops the static.  Other than those two tests I don't know what to do and neither does my A&P or Avionics guy.  Does anyone have a guess as to root cause or what other troubleshooting steps I ought to take?

Oh.  One final oddity.  I suffered with this problem during about 8 hours of a 10 hour return from Oshkosh.  In the last two minutes of those 8 hours when in the traffic pattern at home the radio continually had an open receive circuit with lots of static and, briefly, a sound that sounded a bit like an ELT signal.  Tomorrow I'll pull the battery out of that ELT to see if that solves it.  But if the ELT is the problem why would the static change with RPM?  Odd.

Again, any comments would be appreciated.

 

[BKlott]

Since it seems to be related to rpm, you would likely suspect either an alternator or magneto related issue. 

[wishboneash]

Check ground connection. Usually that is cause of most noise/static problems.

[Yetti]

Rerack radio several times.  Dirty connectors in the back. Also someone may have put a screw through your antenna coax.  An SWR meter could be helpful.  Do you have new or old coax?

[RobertE]

12 hours ago, Yetti said:

Rerack radio several times.  Dirty connectors in the back. Also someone may have put a screw through your antenna coax.  An SWR meter could be helpful.  Do you have new or old coax?

I’m sure I have old coax.  I can, of course, repack the radios but this is a problem that presented itself simultaneously to both radios so I should think any problem would have to have a single, common source.  Headed out soon to do more troubleshooting.

[Yetti]

The ground suggestion is also a good place to start.

[Jblanton]

I fought a static issue, and it ended up being the USB power adapter plugged into power outlet.  long phone charging cord also made static when plugged in as well.

[RobertE]

Well, I’ve found the source of the problem - a defective ELT (121.5). The radios are back to normal - no static, no odd sounds - when the ELT battery is removed and become defective again when reinstalled.  Now there remain some mysteries, such as if that ELT was activated (and I’m told the transmission from that antenna can bleed into the com radio antennas) why didn’t someone report hearing a signal over 1k miles of flight and how come the static varied with RPM?  But I’ll take this victory and call it a day.  Thought I should close the loop on this.