Radio / Intercom Issues

[fishdoc]

Since my last annual my digital com radio has refused to do anything but make a horrible buzzing sound when I push the PTT button and the numbers flash and change like they are possessed.
The mechanic swears they did nothing that would have affected the radios at all. They did pull the radio out and send it back to the maker since it was not serviceable in the field.
The manufacturer sent it back saying there was nothing wrong with it.
I took the plane to another shop where they spent a fair amount of time tracing down wiring and even repairing a broken wire (on something) but no change with the problem.
I also have a King 170B radio that works fine although it is now getting more and more static / background noise that interferes with passenger communications. It transmits and receives fine.
3 years ago at the time of purchase the previous owner replaced the intercom due to sudden problems being able to transmit.
The plane is equipped with a Flightcom 403 MC.
I know this is a loaded question and could be a can of worms but... could the intercom be the root of tall this?
Thanks for any input you may offer.

[BeeMan]

You didn't change anything else did you? I had similar problems transmitting soon after I bought new headsets. Worked fine initially but gradually developed a loud squeal when the PTT was keyed. The problem was eventually tracked to a mis-match between the mic and com gain settings.

Hope that helps, Bill

[c170b53]

Radio problems are hard to pin down especially when its an intermittent problem. So the devil is in the details and you'll need to provide more info. What radio are we talking about? Is the problem there as soon as you turn it on or does it appear after a few minutes ? Got a friend with the same radio, swap it out and isolate the problem. When transmitting the PTT is simply providing a ground to signal the transmitter section to transmit, if its causing the tuning section to scramble the frequencies that ground is getting to other inputs within the radio.

[minton]

I have heard of "Audio feedback" happening when the speaker is either removed and the system does'nt have a suitable "dummy load" installed to replace the speaker or the speaker side somehow malfunctions This could apply to headsets

[DWood]

I assume you have tried all the normal things like turning off the intercom and asking for a radio check. It sounds like you are getting a ground out on more than the Mic when you key up. Are you only getting feedback in the headsets with static and is the radio actually transmitting correctly? The suggestion of swapping out similar headsets, PTT, etc makes sense.
What else do you have hooked up to the radio? Is the radio or the intercom the flight com (I assume the intercom) what is the radio type?
Did they yellow tag the radio when they sent it back so the gain is correct?
Good luck
Dan

[Bill Hart]

Was the Interior removed during the annual? Try checking the mic jacks for grounding on the airframe.

[fishdoc]

Lots of great info and suggestions - thanks!
One radio works fine for transmit and receive and the other is unusable to transmit 100% of the time.
The buzzing when the PPT in depressed will come from either the headsets or the overhead speaker - whichever is selected - every time the button is pushed. That same radio will receive but is weak and noisy. It used to be crystal clear before all this started.
It sure seems to make sense that something is going to ground that didn't use to but only off the #2 radio position.
I will take a closer look at things that could have been affected by removal of the front inside panels.
Thanks again.
I'll post what ever I discover so maybe to save someone else some headaches.

[GAHorn]

If they both have BNC connectors, try trading their antenna leads. It's a simple matter to do w/BNC connectors and if you have a shorted antenna it might cause these symptoms. (Better yet, use a VOM first to determine the suspect antenna is not shorted so you don't blow the output of the transmitter.)

[minton]

What I'm hearing is that only one radio has issues. This leads me to think that the weak one has internal issues OR the weak one has issues within the interface with the intercom. I would do my best to unplug everything I could and as I reintroduced the various units keep testing until the system malfunctions OR take it to someone who deals with electrons all the time.

[JHKeeton171]

Had similar problem that turned out to be racking issue of radio unit not smoothly fitting into rack and all pins in the connectors making good contact. Check all pins in the plugs in the back of the rack and on the radio unit itself and make sure none are loose, bent, broken or corroded and that radio unit is fully and completely seated in rack properly. Hope it is something that simple for you.

[170C]

Having read your posts I would add one suggestion that you have likely already checked, but if not it might be worth the effort. Check that all items are properly grounded. Even if you know they are/or were prior to the annual. Sometimes what was a good ground can suddenly become a poor one due to its being slightly moved, vibrated, etc.. Been there & done that.

[Green Bean]

Would also agree, check the grounds.. especially because of the feed back squelch.

[GAHorn]

Grounds can be a problem...especially TOO MANY GROUNDS! ...in the form of ground-loop feedback. If an intercom has shielded cables (and most do) and if they are grounded at BOTH ends...they will create a ground-loop feedback fault. (loud buzzing, humming, squealing.)
They should ONLY be grounded at ONE end.

[minton]

Understand that the radios feed through the intercomm, thus may be the root of the problem.

As far as the intercomm issue, many require that the entire system "float" thus having one common ground. This is to eliminate "Noise" from alternators and the loop feedback issues, etc.