Hi, looking for advice/ideas on the following.
Replacing MX170 nav/com(slide in replacement for KX170) with GTR255 8.33 com and binning NAV side.
A GMA340 audio panel has been retrofitted some time ago, connectors appear to have a birds nest attached to them :-), …and wires going nowhere!
Have taken relevant wires from KX170 connector and connected to GTR255, all works, including intercom, except on tx get a squeal/ noise in headset and on transmitted freq.
Disconnecting mic audio/com mic 1 audio in hi gives carrier as expected.
Above wire runs in shielded cable along with Mic Key
So I presume the problem is between GMA340 and GTR255 connection above?
KX170 connector also had an ICS wire to it, presume not required ?
Thanks
Pete
If you’re getting a squeal on TX, it sounds like RF is getting back into the system and causing feedback (basically as soon as the RF gets into the audio stages, anything that ends up rectifying it will act as an envelope detector – similar to how a GSM phone near an audio amplifier or audio cables will give you those “bip b b bip b b bip” sounds). The upshot of that is that you’ll have feedback if RF is getting into the audio (either in the audio panel or the radio).
Have you removed all the wires going “nowhere”? Are all the audio cables shielded and the shield properly connected as recommended by the GTR225 installation manual? Have you tried putting some ferrites on the audio cables leading into the intercom and radio? (snap-on ferrites are available so you don’t have to disconnect any wiring to add them) Does your antenna require a balun (e.g. dipoles – if you have a dipole antenna without a balun, the coax shield will end up radiating and you’ll have RF in the cockpit). Do you still get the squeal on the transmitted frequency when the headsets are disconnected? Have you tried transmitting into a dummy load instead of an antenna, and if so do you still get feedback?
Also check for audio feedback. When you transmit, the radio will produce side tone so you can hear yourself while transmitting. If something’s configured or wired wrong, such that the audio out is making it back into the audio in, in this case you’ll end up with feedback. Try disabling the side tone on the GTR225 and see if the feedback goes away (or changes in character – I don’t remember if it’s possible to entirely disable the side tone on the GTR). Also make sure the GTR’s internal intercom is disabled.
Hi, thanks, slept on issue last night…it always helps!…quite right, problem was rf feedback….due poor ground…wooden aircraft!
All sorted and fully fuctioning!
Great :-) Enjoy the radio – I fitted a GTR225 a couple of years back, and I’ve been very satisfied with it. (Then again I was replacing a Narco radio, so the new one didn’t really have such a high bar to clear!)
Alioth
I think you are being a bit hard on NARCO the 12D was quite a good radio for its time and their Starnav multi sensor navigation system was so far ahead of the game GA failed to understand what an advanced system it was ( and failed to buy it ).
On the other hand their ADF & DME offerings simply could not compete with much better offerings from King.
This was a later Narco radio that I had, with the high voltage 7 segment display. They were notorious for failing digits – ours failed 3 times in 7 years.
