On May 27, 11:28 pm, mikem wrote:
On May 27, 5:32 am, Steve - KDMW wrote:

I had my old Narco COM 120 replaced with a new Garmin SL40 and now
when the new radio is turned on my intercom volume drops by at least
half.

Steve, coupla questions?

Is the "HEADPHONE" OUTPUT of the SL40 wired into a COM1 or 2 INPUT on
the Audio Panel?

Is the OUTPUT of the audio panel wired to the SPA400 INPUT?

Is the SPA OUTPUT tied to the aircraft headphones?

Have you defeated the built-in intercom inside the SL40?

Did everything work right before the SL40 install? (All the installer
should have had to do was wire the SL40 headphone out to the whereever
the previous COM was wired)

It sounds like they may have tied two OUTPUTs together, which with
modern avionics is a no-no. Even though the headphone audio is
nominally at 500 Ohms impedance, the OUTPUT impedance of the headphone
amplifier inside the SL40, and the one inside the audio panel are
really a voltage source with VERY LOW output impedance, and two of
these cannot be tied together ( and there is actually no reason to tie
them together) without using series resistors, otherwise they will
fight each other, sort of like your symptoms.

Thanks for your suggestions. The COM 120 was wired to the COM 2 input
of the audio panel (KMA-134), and the audio panel output wired to the
SPA-4000. I believe the shop just removed the COM 120 and wired the
SL40 in it's place, so it should still be the same. I haven't noticed
anything different or unusual, i.e. if I select COM 2 I only hear COM
2.

I didn't check the intercom mode of the SL40. I'll do that.