Radio transmissions clear for some and garbled for others.

[slowflyin]

I have an unusual challenge with a Decathlon I recently purchased.   It has the original KX170B and Sigtronics Intercom.  When transmitting some listeners hear 5x5 and to others it's completely unintelligible.  

For example, my son was doing some pattern work and the local Tower Chief calls my on my cell to advise my son's transmissions are unreadable.   We had flow the AC home as a flight of two and didn't have any com issues so I was surprised.  I turn on my Icom handheld and can hear the aircraft just fine.  Not perfect, typical noisy cockpit,  but no issues understanding.    I walk over to the base of the tower (it's a small 50' tower) and I can hear what they hear via the loud speaker.  They were not exaggerating, completely garbled.   I turn up the Icom handheld so they can hear what I hear and were all scratching our heads.   Simultaneous reception, one 4x5 and one useless.    On a subsequent flight I picked up a Yaesu handheld to listen in and was surprised to hear terrible coms.  Turned on the Icom and things sounded fine. 

It's almost like the Icom handheld might be receiving a wider bandwidth.   Both the Tower and the Yaesu were reminiscent of a transceiver that was tuned one digit off freq.  

Reception in the AC from all transmitters is fine.

 

Thoughts?

 

 

[EricJ]

2 hours ago, slowflyin said:

It's almost like the Icom handheld might be receiving a wider bandwidth.

Maybe a narrower bandwidth if there's a local interferer, or a transmit spur in your KX170B, that the other radios get and the iCom doesn't.    It's hard to say without a deep dive with a spectrum analyzer or other tools.

2 hours ago, slowflyin said:

Both the Tower and the Yaesu were reminiscent of a transceiver that was tuned one digit off freq.  

It could be that the oscillator in the KX170B is old and drifted a bit.   This could also cause the issues you're describing, and would explain why it receives itself fine.    Or, as you suggested earlier, the iCom is less sensitive to drift in the receiver.    For AM transmission there's a lot of insensitivity to drift unless it gets bad, but it's possible that's what's happening.

 

2 hours ago, slowflyin said:

Reception in the AC from all transmitters is fine.

Is there just one transceiver in the airplane or multiple?    If there are multiple, is it a problem with all of them?

You might see if somebody local has a TKM MX170  that you can borrow.   It has more modern electronics and will slide-in the KX170 cage.   That'll tell you whether it's the KX170 or something else.    An experienced radio person with a spectrum analyzer might also be able to help sort out if it's a signal issue or a spur or something else.

[slowflyin]

@EricJ

I'm not an RF guy but you comments match my thoughts suspecting drift.  I'll see if I can find a spectrum analyzer to take a look.

 

Only, one transceiver.   

 

Yes, we are actively looking for a MX170 or even a proven KX170B to help run this to ground.

 

Thanks for you comments!