The International Cessna^® 170 Association

Well, it has been a long winter of rewiring behind the panel of my 170A. I replaced fuses with circuit breakers and added a switch on an empty piano key for new LED anti-collision lights. I didn’t make any changes to my intercom or radio other than crimping on new terminals to the power and ground wires and I re-routed the wires for my pilot headset jacks. No circuit changes. Oh, I did replace the coax for my radio antenna.

However... now I am getting an annoying and loud tone in the headset when I hit the push to talk button. I didn’t have any other headsets plugged in at the time. I switched the intercom to isolate and still get the tone when transmitting. Any thoughts on what I should look for? Is this indicative of a ground loop?

For reference I have a KX170B nav/com and an Intervox four place intercom.

Looking for troubleshooting advice. Thanks in advance. You guys are great.

Doug

Maybe you missed some isolation washers on your jacks ?

Yes, it might be a “ground loop” which commonly makes a “tone”.

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Thanks! I’ll double check and see if they are missing.

Well, I installed insulated washers on all of the pilot, copilot and handheld microphone jacks with no improvement. The rear headset jacks are on plastic so they are already insulated from the airframe.

So I’m not sure where to go next. The only other thing I touched as part of the updates is I replaced the com antenna coax cable because the old cable was rubbing and worn through to the shielding in one spot. Could a mistake on installing the new cable cause this tone?

Is the tone received by anyone to whom you are broadcasting?

gahorn wrote:Is the tone received by anyone to whom you are broadcasting?

Good question... I’ll check that today. I’m at a small grass airport that hasn’t been busy. When I have tested it.

Check that the new coax braid is connected at both ends. You could be getting RF into the mic audio circuit.

Mike Gourd wrote:Check that the new coax braid is connected at both ends. You could be getting RF into the mic audio circuit.

Mike, when you say braid, you mean check the outer shielding? How would you test it?

The braided shield should be grounded to the end fitting when crimped. Checking for continuity between the out side of one crimped fitting and the other would prove that both fittings contact the shield.

You should be able to pull the cable off one end and test continuity between the outside of the crimp and ground since the other side of the cable should be eventually grounded at the end of the run either at the radio or antenna.

If you don't get continuity at one end testing above. Take the test one step further and disconnect both ends of the cable. Connect your multi meter with long leads between each crimp. Don't have long enough leads. Use a jumper an one end or the other to jump the crimp to ground and then test the other side for continuity to ground.

I agree with Bruce.

Trying to add to what's already been said ...

In addition to a ground loop, I'd look for supply voltage disturbances or instability. Transmit draws a lot of current, so high resistance in the supply breaker or wire connections could do this, as could high ground resistance. The KX170B is an old radio with only one ground wire, so make sure all the new terminals you crimped are solid. Is the intercom ground point the same physical location as the radio? Did you change it?

Do you hear the noise in the NAV audio?

Is this occurring with the engine not running? If so, what's the battery voltage? What's the voltage at the output of the radio breaker? Can you change the frequency or volume of the noise by changing other electrical loads?

Is the new headset wiring shielded? Is it bundled with the antenna coax? What kind of cable is the antenna coax, RG-400? RG-58? Did you fabricate the cable and crimp the terminals yourself? Did you replace the right-angle chassis connector too?



Dave

If the side tone just started it probably is not the jacks but it could be. I would check to see that you are grounding well. No more then 3 ground connections on any lug in avionics and also make sure that the battery ground to frame is good. no corrosion. grounding is the leading cause to avionics trouble. Also if it started after you touched something go back to that something and redo it. Good Luck.

Thanks, guys. I’ll dig deeper into the grounds and the shield on the new coax. Hopefully have some news by Wednesday if I can get free to head to the hangar.