constant noise on cabin speaker

[NicoN]

in our 1980 M20K we have a constant noise on the cabin speaker.

It is nothing new, it is it least since 3 years when I joined the group.

it is not serious and alo not really disturbing, but it is noticeable, especially on ground with no headsets on.

We have the venerable KMA 23 I think and the noise is always there - no matter if some or no audio source is activated for the speaker on the KMA.

The audio quality on the speaker is poor, but  not due to noise and communication via headsets is excellent and definitive not a problem.

 

So, let me ask if this is a common or even "well-known" effect which can be eliminated?

 

Thank you

[LANCECASPER]

I've never heard of a KMA23. They made a KMA 20 and a KMA 24 in those days.

[tmo]

KMA24 most likely.  I believe mine (also a 1980 M20K-231, sn 25-0396) also has the same "feature" regardless of which outputs I route to the speaker on the KMA.  Been ignoring it till I replace the KMA with a PMA...  Will confirm next time I go see the plane.

@NicoN - do you have an intercom in the panel, or just the KMA24?

Edited February 5, 2020 by tmo

[PT20J]

Mine doesn’t do that. The KMA 24 has separate amplifiers for the speaker and headphones that should be powered by separate circuit breakers. Try pulling the one that powers the speaker and see if that resolves it. 
Skip

[LANCECASPER]

If you decide it's time for a new audio panel, these are pin compatible with the KMA24:

http://www.ps-engineering.com/PMA7000_Series.shtml

[Hank]

You can always remove the wires from the back of the speaker . . . Only one needs to be disconnected.

[Frank B.]

10 hours ago, LANCECASPER said:

If you decide it's time for a new audio panel, these are pin compatible with the KMA24:

http://www.ps-engineering.com/PMA7000_Series.shtml

That’s good to know. I never knew that the PMA would slide back in the King tray. I have used them in place of the Garmin but had no idea that they were a slide in replacement for the King.

[tmo]

12 hours ago, LANCECASPER said:

If you decide it's time for a new audio panel, these are pin compatible with the KMA24:

http://www.ps-engineering.com/PMA7000_Series.shtml

I'm afraid it's not that simple, or I'd have done it already.  While the 7000 series from PS Engineering is pin-compatible with the KMA24 as far as the audio panel part goes, it also does have an extra connector for the intercom functionality (which the KMA24 sadly/suprisingly lacks).  To quote from the page you linked: The PMA7000 series are KMA24 pin compatible, only the intercom harness has to be made, existing connector remains.

So if one wants to take full advantage of the new panel, some sort of new wiring is needed.

Can someone in the know tell us how much work is needed to add the intercom connectors (assuming no intercom installed and assuming there is already a separate intercom in the panel) versus redoing the whole audio setup for eg. a PMA450B?

[LANCECASPER]

4 hours ago, Frank B. said:

That’s good to know. I never knew that the PMA would slide back in the King tray. I have used them in place of the Garmin but had no idea that they were a slide in replacement for the King.

Only the 7000 is pin compatible with the KMA24 audio panel (The 8000 is pin compatible with the KMA28), and yes if you want the intercom portion then you would need to wire that. But if you wanted just the audio panel the way you have things wired now, the 7000 is a slide-in replacement.

[NicoN]

Thank you.

Removing the wire from the speaker is the last option. The new audio panel is also not an option at the moment.

So, I can live with it.

[LANCECASPER]

There are used KMA24 Audio Panels for sale everywhere. Usually the going price on here or Beechtalk is around $100. If someone on your field has one installed in their airplane it would take about 5 minutes to swap and see if that's your issue.

[carusoam]

Often the source for noise for something like this...

Are the ground straps.

Everything should be connected by ground straps... engine, instrument panel, fuselage....

It is possible the instrument panel has lost its grounding, and now electrons are flowing towards the speaker...

Try running a temporary wire from panel to speaker... see if anything changes...  

It is possible that the usual grounds need to be cleaned every few decades...

PP thoughts only, not an instrument guy...

Best regards,

-a-

[KB4]

I also have a very faint sound coming from OH speaker KMA24.  Today 30min after departure, the mic cut out.  Twr could hear me and I could hear them when keyed.  But I couldn't hear myself going thru my checks.  Wondering if my KMA24 is shot or a just a loose wire. Tried passenger jacks and different headset same issue. Anyone with a wiring diagram for this old thing?

[Culver LFA]

1 hour ago, KB4 said:

I also have a very faint sound coming from OH speaker KMA24.  Today 30min after departure, the mic cut out.  Twr could hear me and I could hear them when keyed.  But I couldn't hear myself going thru my checks.  Wondering if my KMA24 is shot or a just a loose wire. Tried passenger jacks and different headset same issue. Anyone with a wiring diagram for this old thing?

Here is a copy of the installation manual, and a sales document on the KMA24. 

After reading this thread I noticed today that I also have some faint static on my cabin speaker without any of  the top speaker audio buttons selected.  I would guess it is EMI from a harness somewhere or just the antique audio panel, but it isn't really bothersome on my airplane?

KMA-24Install.pdf kma24.pdf

Edited February 12, 2020 by Culver LFA

[PT20J]

2 hours ago, KB4 said:

I also have a very faint sound coming from OH speaker KMA24.  Today 30min after departure, the mic cut out.  Twr could hear me and I could hear them when keyed.  But I couldn't hear myself going thru my checks.  Wondering if my KMA24 is shot or a just a loose wire. Tried passenger jacks and different headset same issue. Anyone with a wiring diagram for this old thing?

It's a little difficult to figure this out from your description. I think you are saying that you were able to transmit and receive but you could not hear yourself in your headset. So, if you only don't hear yourself when you are transmitting, that sounds like a sidetone issue and sidetone comes from the radio. Check to see if it is specific to one comm radio. If you cannot hear yourself when the mic is NOT keyed, that sounds like an intercom issue. Neither sounds like a KMA 24 issue, but perhaps I misunderstood your description.

[KB4]

Thanks  @Culver LFA and @PT20J

Correct, I cannot hear my voice as I am talking to me when the mic is NOT keyed.  Will try Com2 and see if that makes a difference.     

[KB4]

@PT20J you were correct.  After reading your post I checked and the intercom, and the On switch had been bumped down (off position), probably when turning the heat on.  Flipped it up, and all is good.   

[PT20J]

1 hour ago, KB4 said:

@PT20J you were correct.  After reading your post I checked and the intercom, and the On switch had been bumped down (off position), probably when turning the heat on.  Flipped it up, and all is good.   

Glad it wasn't serious. And thanks for letting us know so we can all learn  

[kajo]

Does turning on isolation help, like pilot isolation, co-pilot isolation?