Garmin GNC 420 Maintenance Manual page 73

5.7.2.17 Audio Frequency Response
1. With compression disabled, apply a 30% modulated, 1 mV signal to the unit.
2. Verify the audio output does not change more than 6 dB from 350 Hz to 2.5 kHz.
3. Verify the audio-frequency response above 2500 Hz decreases, and at all frequencies above
4000 Hz, it is at least 18 dB below the output obtained at 1000 Hz.
4. Measure the audio output in the 25 kHz channel mode.
350 Hz ______ dB
1000 Hz ______ dB
1500 Hz ______ dB
2500 Hz ______ dB
5. Measure the audio output in the 8.33 kHz channel mode.
350 Hz ______ dB
1000 Hz ______ dB
1500 Hz ______ dB
2500 Hz ______ dB
5.7.2.18 Receiver Audio Compressor Test
1. Enable compressor.
µV
2. Apply a 100 1 kHz, 20% modulated audio signal.
3. Adjust the power/volume control for 7.07 Vrms com audio output.
4. Set the com audio output reference level to 0 dB.
5. Adjust the modulation depth to 85% and verify the com audio output level does not vary more than
3 dB.
25% to 85% audio variation __________ (dB)
Page 5-22
Rev. C
400 SERIES MAINTENANCE MANUAL
P/N 190-00140-05