Originally Posted by jliltd
There is a "sidetone" adjustment potentiometer that you can access from the top of the unit. Turn it up a little with a screwdriver and see if that helps. See the King KY 96A/97A installation manual available with a Google search. There is a picture of the accessible adjustment pot locations. There might be a similar pot for radio reception level inside.
We just removed an overhead speaker in a Super Cub with a KY97A transceiver and my memory of the manual is still fairly fresh on that unit. So....
You might want to make sure there isn't a 4 ohm resistor across pins E and 5 (speaker output designated 4 ohm audio Hi and Lo, respectively). Older King radio models required a dummy load in absense of a speaker. That could affect the headphone out circuit due to internal cross linking. The KY97A doesn't typically need that dummy load resistor but I might remember some nuance based on the exact dash number on the KY97A. Again refer to manual. Some unwitting tech in the past may have integrated that obsolete requirement for a resistor. Since you swapped the unit into a different airplane that probably rules out said resistor externally but maybe something internal or the second airframe might have that resistor while yours doesn't. Did you try putting the other aircraft's radio into your aiframe?
In the end maybe a bench check would be less trouble for initial troubleshooting than all that fiddle. Used KY97A's are readily available on the open market.
If you decide to update to a neelw transceiver you might consider Garmin GTR 200, Trig brand TY91 / TY96A, or the Icom A220 with optional King slide-in adapter.
There's nothing in the install manual about a resistor, but if it is serial number specific, then there may be in a different version. The shop estimated $85 for the bench test. We'll see what they say, hopefully by Monday. I was looking at that GTR 200. Looks like my best choice. I like the MGL comm, but the form factor doesnt really work with my panel. I have a Skyview, and considered the Dynon comm, but I dont want to ne stuck with an incompatable comm if I ever replace the EFIS.