Bendix/King
KMA 28B Audio Selector Panel and Intercom System
Installation and Operator’s Manual
D201106000060
Page 2-4
June 2011
2.4.4.1
Speaker Load
The KMA 28B contains a speaker amplifier. Some units with internal speaker amplifiers, such as the
King Radio KX170-series, require a resistive load to prevent damage if their speaker amplifier is not used.
Connect the speaker output from the unit to the load input on the KMA 28B (J1, pins 19 and L, 16 and M.
The speaker load is 16
, 3W.
2.4.4.2
Fail Safe Connection
The KMA 28B connects the pilot microphone and headphone directly to COM 1 input when power is re-
moved. The headphone failsafe is the RIGHT channel, connected to the TIP of the headphone jack, to be
compatible with monaural headphones. Unswitched input #1 (Bottom Connector pin T) is also provided to
the pilot headset, right channel, in fail-safe.
NOTE: The pilot headphone MUST be wired in accordance with Appendix C, Bottom connector Pin 3 to
Pilot headphone tip (right) to ensure proper failsafe operation. Verify failsafe operation in accordance with
Section 2.11.2, step 3.
2.4.5
Com 3 Duplex (TEL) Function for Cell Phones
As installed in the standard configuration, the KMA 28B Com 3 function operates conventionally. Push-
ing the Com 3 XMT button places the receive audio from Com 3 in the headset and applies the pilot or
copilot microphone to the Com 3 when the appropriate PTT is activated.
If J2, Pin J is connected to aircraft ground, the KMA 28B is forced into Com 3-Duplex mode. Copilot and
passenger audio as selected by hook switches are provided to the COM 3 output, the PTT for Com 3 is
inactive, and audio from Com 3 is presented to the headset. This allows a telephone-like audio interface.
The COM 3 input and output are compatible with aviation radios.
The COM 3 input and output are compatible with general aviation radios. However, if J2, Pin J is con-
nected to aircraft ground (either directly or through a switch), the KMA 28B is forced into Com 3-Duplex
mode. In this mode, the COM 3 input and output is compatible with many cellular telephones utilizing the
hands-free headset interface. A 1/8" or 3/32" jack can be installed on the aircraft panel, which is inter-
faced with the KMA 28B as shown below. To connect the cellular telephone to the jack wired in to COM
3, a patch cord is required. This patch cord is available from PS Engineering under P/N 425-006-7026
(3/32" to 3/32"). A 1/8 to 3/32 adapter is P/N 425-006-2535.
3/32" Cellular Jack
COM 3 Mic Input
Com 3 Audio
Audio Lo
J
K
1
KMA 28B
Bottom Connector
Cellular Plug (typical)
Tip= Microphone out
Ring= Speaker audio
Base=Ground
For Duplex (cell phone) operation
Pin J of top connector MUST BE GROUNDED
This is a typical interconnect
PS Engineering does not guarantee
compatability in all cases.
Cellular Phone
Interconnect
Figure 2-2 Cellular telephone interface
