Rewiring KX155 Nav/Comm to KMA24 audio panel

Sorting some wiring on an avionics stack that I suspect came out of another aircraft and was never really properly wired in the first place..... It consists of a KMA24 audio panel , two KX155 radios and a KLN89 GPS unit. Com 2 has the GS option and is connected to a KI204 ILS indicator. The indicator has a converter to resolve a composite VOR/LOC signal.

Its probably a generic answer, but I don't understand how the Comm and Nav audio and microphone connections should go together and the diagrams aren't helping me.

On the radio I have Audio HI and Audio LO. I presume that audio HI goes to audio input on the audio panel, but what about audio LO - does that go to earth? Same earth or different earth to the shields? Is there a reason why the Nav and Com audios cannot be wired using one piece of 4 conductor shielded cable rather than two bits of two conductor cable?

The mic and mic key connection between radio and audio panel seems clear enough but where (and this is probably really dim of me) is the connection to the microphone jack?

I'm guessing that the headphone output goes to the headphone jack, and there is a return to the (specific) speaker earth on the audio panel?

Be really grateful for some help on this.....

Many thanks

Chris

Yes, Low is ground or earth.
Standard practice, to minimize the chance of unwanted noise, is to ground at one point, the audio panel. So run low on the radio to low on the audio panel. If there is no low, use ground. Same for jacks for headphones and mike: Run high and low thru thru shielded cable to the panel, connect low to low or ground at the audio panel. Connect all shields to ground only at the audio panel. Use fiber washers with a shoulder on one to insulate jacks from the airframe.
You can probably run nav and com audio, hi and lo, thru one shielded cable, since these are both high level signals and unlikely to have issues with cross talk. But I wouldn't!

Okay, I downloaded a KMA24 manual off the internet.
The headphone goes to pin 3; the PTT (tip on mike jack) to pin Y; and the mike output (ring terminal) goes to pin 8; all on J241.
The manual says to ground the headphones (and speaker, if used) to pin 22. Use pin 1 for all other equipment grounds.
If you have more than one mike, you must isolate them as shown using relays controlled by that person's PTT switch, as shown on the schematic.
This is a very old design, and if you are not using a speaker, then the speaker outputs must be connected to an internal load resistor. Read the manual carefully. The same may be true for the KX155 speaker outputs but you'll have to read its manual - I don't know.

Bob, thanks, sitting staring at the manuals.....
Com1 mic and mic key, pins P and R are inputs to the audio panel from the radio; pins Y and 8 are the inputs from the mic and PTT to the audio panel ( just trying to be clear with the terminology). Pins 3 and 22 are the outputs to the headphones?? Ok I think that's clear. In fact I think it is clearer on page 2-15thanks for your help - got it now. Been a bit of a derrrr-brain as my children might say!

Chris

terminology

chris mitchell said:

Bob, thanks, sitting staring at the manuals.....
Com1 mic and mic key, pins P and R are inputs to the audio panel from the radio; pins Y and 8 are the inputs from the mic and PTT to the audio panel ( just trying to be clear with the terminology). Pins 3 and 22 are the outputs to the headphones?? Ok I think that's clear. In fact I think it is clearer on page 2-15thanks for your help - got it now. Been a bit of a derrrr-brain as my children might say!

Chris

Click to expand...

I think you've got it. I would say pins P and R are OUTputs from the audio panel to the radio, since that's the direction the sigals flow, not from com to panel. And pin Y is the input from PTT ( and pin 8 the input from the mike to the panel, again the direction of the signal flow). Make sure pin 22 is used as the ground connection to the headphones.