KMA 24 extremely low audio through headset, speaker works fine.

[Jml624]

I have a Mooney M20j, just flew it a little over 10 hours and the radios and intercom worked great. Went to take my son up in the plane this evening and could not hear audio in headset, pilot or co-pilot. Upon shutting down the engine and trying to do a little troubleshooting I could barely hear the ATIS in the head set with all volumes maxed out. Then I switched to aircraft speaker on the KMA 24 audio panel and it was loud and clear. I did a radio check through the headset and got a loud and clear response through the airplanes speaker. I did not have a side tone though. Both king radio are transmitting and recieving fine, ICS works fine between pilot and co-pilot just can't hear the radios through the headset on the pilot and co-pilot.  Any help would be awesome. Thank you in advance.

[Minivation]

Assuming you have a separate intercom panel and given the fact that both pilot and copilot stations are having difficulty hearing both radios, my prime suspect is the headphone amplifier circuitry is at fault inside the KMA24. Reference the basic block diagram of the KMA24 circuitry below:

In order to "mix" various audio sources (i.e. marker beacon, comm 1, nav 1, comm 2, nav 2, etc.) into one headset output, these audio sources are passively mixed through a network of resistors, which has the side-effect of attenuating each respective signal. In order to bring these signals back up to original amplitude, the mixed audio is then run through an amplifier before being sent out to the headphone jacks (or perhaps, in your case, the "radio" input of the intercom unit). The cabin speaker and headphones use separate amplifier circuits, which may explain why your cabin speaker still works OK.

As for resolving this issue, my suggestion is to take it to an avionics shop that is knowledgeable with old-school avionics (i.e. won't throw new boxes at it right away). There exists an adjustable potentiometer (R201) which controls the amplification level of the Headphone Amp, so perhaps an attempt could be made first to see if that makes any difference.

If adjusting the R201 potentiometer doesn't make any appreciable difference, there is a good chance the voltage regulator that supplies power to the amplifier itself is at fault. (If this is the case, then this might explain why you're still able to *faintly* hear the radios - the amplifier is inop and is operating at 1:1)

 

Reference: King KMA24 Maintenance Manual 006-05180-0001 Revision 1

[Jml624]

Thank you

[carusoam]

Welcome aboard Jml...

Trouble shooting all the variations of wiring for headsets.... keeps a lot of avionics shops busy...

How well was the side-tone working?

Are the headphones new to you as well?

PP thoughts only, getting everything set up properly is a blessing...

Best regards,

-a-

[Jml624]

Headphones are not that new to me. I have had them for about a year. They were working fine Monday and so was the radio audio. 

[Gary0747]

It appears the KMA24 may have mechanical switches?  My KA134 has mechanical push button switches and the contacts on mine need to be exercised and cleaned with contact cleaner occasionally or I can get weak or no signal from some of the audio inputs. I wish mine had the audio isolation amps that it sounds like yours has. 

[Jml624]

Got it fixed the other day when I had some tools and time to troubleshoot. I found a loose wire on a bus bar. I am guessing that the voltage drop was just enough to not power the audio control box. Tighten and everything came alive again.

[carusoam]

Great follow-up jml!

Thanks for the closure...

Best regards,

-a-

[rbridges]

8 hours ago, Jml624 said:

Got it fixed the other day when I had some tools and time to troubleshoot. I found a loose wire on a bus bar. I am guessing that the voltage drop was just enough to not power the audio control box. Tighten and everything came alive again.

good deal.  I was going to say it was probably a bad capacitor.  I had the same symptoms, and that's what it turned out to be in my case.

[Minivation]

Sweet! Best case troubleshooting scenario. Hope the KMA24 serves you well for the foreseeable future.

[ZuluZulu]

I still have a used KMA24 on the shelf from a 28V airplane if anyone wants to pick up a spare.... #shamelessplug