Telex ANR TM 500 - Manual (Page 5)

Microphone Gain Adjustment
The microphone gain has been factory-adjusted to the nominal level required
for normal radio operation and should normally not require adjustment. If
needed, adjustment by a qualified avionics technician is recommended. To
access the microphone gain, insert a small phillips head screwdriver through
the access hole in the headset (refer to Figure 4). Clockwise rotation
increases the microphone gain.
NOTE: The microphone gain adjustment does not have a stop; at one point the
gain will go abruptly from maximum to minimum and will remain at
minimum gain for approximately 30
° of rotation.
MICROPHONE BIAS VOLTAGE REQUIREMENTS
The boom microphone requires a bias voltage of 8-16 VDC through 470
Ω DC
resistance, 150
Ω AC resistance (per RTCA DO-170). If you are uncertain
whether your avionics equipment meets this requirement, consult the avionics
equipment manufacturer.
4.
Microphone
Gain Adjustment
Figure 4: Microphone Gain Adjustment