Maintenance Manual

MAINTENANCE
MM101886V1 R1A 11
4.3
CRYSTAL FILTER TUNING
4.3.1 Tuning FL1 and FL2
1. Connect test equipment as shown in Fig. 10.1.
2. Connect the high impedance probe to either side of resistor R152.
3. Set the span of the network analyzer to 20 kHz, and the center frequency to 21.4
MHz.
4. Set the analyzer RF output level to obtain –75 dBm at the input of the module.
5. Adjust C9, C12, and C17 for maximum response at 21.4 MHz.
4.3.2 Tuning FL3 and FL4
1. Adjust C24, C29, and C36 for maximum response at 21.4 MHz.
2. The 6 dB bandwidth displayed on the network analyzer shall be between fc +/- 6.1
and fc +/- 7.3 kHz. The peak-to-valley ripple shall be less than 1 dB.
3. If the requirements of step 2 are not met, adjust C9, C12, C17, C24, C29, and C36 as
needed until the requirements are met.
NOTE
The bandwidth is controlled primarily by C12 and C29.
4. Set the squelch pot on the test fixture to maximum (maximum means to set the wiper
for maximum signal at the Sq-Arm terminal)
4.4
AUDIO AND DATA OUTPUT ADJUSTMENTS
1. In the 25 kHz mode, apply a standard input signal (-75 dBm, 21.4000 MHz signal
modulated with a 1kHz Audio Frequency (AF), 3kHz peak deviation) to the module
IF input. Turn the FM modulation off. Monitor TP5 with a high impedance probe
connected to the frequency counter. Adjust L102 for a reading of 455 kHz ±100 Hz.
2. Turn the RF generator FM modulation on. Set R203 for 1.0 Vrms ±3% AF output at
module output (Pin 31C on 96 pin connector, Pin 52).
3. Remove the FM modulation from the test signal. Monitor the 455kHz+ jack on the
test box with an oscilloscope using a high-impedance probe. Adjust R100 for a
2.2±0.1 volts peak-to-peak level. The 455 kHz-jack shall have a level of 2.2 ± 0.2 V
peak-to-peak.
4.5
TROUBLESHOOTING
When troubleshooting the module, it is most convenient if the standard test fixture is
used. The following conditions are with the module in the 25 kHz mode. This can be set
up using a PC with the necessary software connected to the test box. Alternatively, a wire
link can be soldered between pads H1 and H2 on the PC board.