Garmin AT
Rev --
7-26
GX Service Manual
© 2004 Garmin AT
Modulation Capability:
Input a standard modulator test signal (100 mV RMS at 1 kHz) into the microphone audio. Using
a linear detector, measure the AM modulation on the transmitter’s carrier signal.
118.00MHz _______ NLT 85% 126.5MHz ______ NLT 85% 135.97MHz ______ NLT 85%
Sidetone Audio Level Adjustment:
1. Connect a 100 ohm resistor and AC power meter (or voltmeter) across the headphone
output.
2. Press and hold NAV button (about 3 seconds) until the SW VER X.X is displayed.
3. Rotate the large knob until SIDTONE LVL is displayed. Record reading in customer defaults
on the service data sheets.
4. Transmit with a standard modulator test signal injected into the microphone audio.
5. Turn up the sidetone level value (small knob right side) to make sure the output power can be
greater than 280 milliwatts (5.29Vrms across 100 ohms).
6. Adjust the sidetone level (small knob right side) to 28 milliwatts (1.67 Vrms across 100
ohms).
Sidetone drive capable of 280 mW: _____ OK Sidetone reading for 28 mW _____
Microphone Bias
Measure the voltage across the microphone input with a 1 k ohm load resistor connected in
parallel.
6.3 volts +/- 0.4 V: ______ OK
Modulation Audio Frequency Response
This is an optional test not required for returning a unit to service
1. Inject a 20 mV 1 kHz audio signal into the MIC1 input
2. Key the transmitter and adjust the audio generator’s amplitude to obtain 85% carrier output.
3. Measure and record the audio frequency depth at 1 kHz.
4. Change the audio frequency input (MIC1) to 350 Hz.
5. Measure and record the AF Depth difference with respect to 1 kHz.
6. Change the audio frequency input (MIC1) to 2.5 kHz.
7. Measure and record the AF Depth difference with respect to 1 kHz.
350 Hz Depth: __________ LT 4dB
2.5 kHz Depth: __________ LT 4 dB
Frequency Tolerance
1. Key the transmitter on a clear channel between 118.000 MHz and 135.95 MHz.
2. Using a frequency counter or spectrum analyzer, measure the output frequency of the
transmitter.
Frequency tolerance: ______ NMT ± 15 PPM from -20
°
C to +70
°
C.
Summary of Contents for APOLLO GX SERIES
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