Radios in my 1974 Cessna 172

Bob, liked your article on Legacy Avionics. I have a couple questions about my radios in my 1974 Cessna 172. My #1 nav/com is a KX-155. It works well except occasionally I will have a lot of static on a channel - seems like more in the lower range - say in the 118-119 frequency range, and have trouble hearing ATC, but then when I get handed off to another controller on a different frequency the noise goes away and I can hear fine. Any idea what might cause this? On my #2 nav/com, it is a TKM MX-12. Until I read your article, I thought this was an old wore out original radio, but instead I guess it is a TKM replacement. I haven’t looked in the logbooks to see how old it is, I bought the aircraft in 2004. The problems with it are the volume knob is sticky, in other words it will be real low or real high, or fluctuate and not have a consistent range of adjustment. Is there a fix for this, or does TKM service these? Also my glide slope indication is weak to none on this radio but course deviation works good. Thanks for your help. Scott Needham


  • Scott ShererScott Sherer COO Forum Moderator
    Hi Scott, thanks for writing. Bob logs in every few days and I'm sure he'll see this and respond. In the meantime, TKM is a very good company and you can send your MX-12 in for service. First, visit the website and get an RMA. Their website is Regarding the KX-155. Service is expensive and this may be a throw away item at this point. I'm interested in Bob's opinion on this. Also, your wiring is circa 1974. Buying new radios and/or servicing old radios may not totally eliminate your issues. You may have to have a shop evaluate your wiring. It'll cost a couple of thousand for new wiring, in addition to buying new and/or servicing old radios. Good luck. Bob, are you there?

    Scott Sherer
    Wright Brothers Master Pilot, FAA Commercial Pilot
    Aviation Director, Cessna Owners Organization Forum Moderator and Cessna Owners Author.

    Need help? Let me know!

  • Sorry Scott(s) for the delay. I'm buying a Condo and it's been a major "distraction" Please be aware that I am not an avionics technician, however there are a few things that you can do that might alleviate the KX-155 issues. Remove the radio and get some spray contact cleaner. Clean the connector in the back of the tray and on the back of the radio. Disconnect the antenna connections on the back of the radio and also clean with contact cleaner. Make sure to connect them back in the same spot! One is comm, one is nav and a third is glideslope (if you have that on the KX-155). If you can get to the other end of the antenna connections, clean them also. You likely have a little box under the panel where the antennas connect and split the signal. Clean those connections. Make sure the radio is seated tight in the tray. If that doesn't improve things, you need to get it bench checked. The TKM MX-12 is fully supported by TKM. Contact Vic Casebolt at (480) 991-5351. You would have originally had a Narco MK12 or MK12B in the aircraft and the MX-12 was the replacement. The issue with these radios is the other Narco peripherals that are part of the system. You likely have a VOA-5 or VOA-9 CDI in the aircraft and a UGR-(1A, 2 or 2A) glideslope receiver, The issue is that both of these units are not aging gracefully. Since the KX-155 and the MX-12 likely share the same antenna, that is not suspect but the antenna cable and connections could be. Check and clean them. If you have old RG-8 black antenna cable, that breaks down and should be replaced (next annual?). If you suspect that the UGR glideslope receiver is bad, contact Troy Ellison at Ellison Avionics in TX. (254) 640-9024. He can help! Thanks! Bob Hart
  • Thanks Scott and Bob for the advice. I’ll give these things a try and also contact TKM on the MX-12.
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