3-9
3.12
MAIN AND GUARD SQUELCH ADJUSTMENT
The squelch on both the main and guard receivers is factory set to open at approximately
1.0 microvolt. This adjustment can be made or altered to suit local conditions as follows:
1.
Set the main receiver of the transceiver to 156.000 MHz. Connect a signal
generator to the antenna input of the transceiver.
2.
Set the signal generator to produce a ±3 kHz deviation with a 1 kHz tone on
156.000 MHz. Set the signal generator RF level to 1.0 uV . The squelch indicator
LED is on and signal is heard. Set the generator to 0.6uV. The squelch indicator
should go off and no signal heard.
3.
If not, re-adjust main receiver squelch potentiometer, R3 through the access hole
located on the bottom of the transceiver chassis (see Figure 3-4).
4.
Repeat the above procedure to adjust the guard receiver squelch setting using guard
receiver squelch adjustment potentiometer, R4 (see Figure 3-4).
3.13
TRANSMITTER DEVIATION ADJUSTMENT
1.
Remove and retain the eight (8) No. 4-40 screws securing the top cover of the
transceiver to its chassis. You should now have access to the Main Rx/Tx Module.
2.
Set the transceiver operating frequency to 156.000 MHz and connect an appropriate
test receiver to the RF output connector. Ensure that the output of the transceiver is
terminated into a proper dummy load.
3.
Key the transmitter and input a +10 dBm (2.5 VRMS), 1 kHz audio signal into the
microphone input.
4.
Adjust the wideband deviation limit potentimeter, R30 on the main Rx/Tx module
(see Figure 3-5) to produce a ±4.45 kHz deviation.
Adjust the narrowband deviation
limit potentimeter, R76 on the main Rx/Tx module to produce a ±2.2 kHz deviation.
5.
Verify that the deviation does not exceed ±5 kHz for wideband and ±2.5 kHz for
narrowband on the following frequencies: 138.000 MHz, 162.000 MHz and 174.000
MHz. Re-adjust R30 or R76 as required, if the deviation exceeds ±5 kHz or ±2.5
kHz, respectively.
6.
Place top cover on transceiver chassis and secure all eight (8) screws.
3.14
GUARD RECEIVER AUDIO LIMIT FEATURE
Upon special request, 1 mW of guard receiver audio bleed with the guard volume control in
the fully CCW (OFF) position, can be provided. This feature can be disconnected as follows:
1.
Remove and retain the seven (7) No. 4-40 screws securing the bottom cover of the
transceiver to its chassis.
2.
Find the jumper located at the bottom of the front panel is connected between the
CCW position of the guard audio potentiometer (R2) and ground. Remove this
jumper.
3.
Replace the bottom cover of the transceiver and secure with the seven (7) screws
removed in step 1.
Summary of Contents for TFM-138
Page 13: ...2 2 FIGURE 2 1 Operator s Switches and Controls TFM 138 F10 B software...
Page 24: ...3 2 FIGURE 3 1 Outline Drawing for Model TFM 138 Transceiver...
Page 26: ...3 4 Figure 3 2 Wiring Connections for TFM 138 Transceiver...
Page 30: ...3 8 FIGURE 3 4 Microphone and Sidetone Level Main and Guard Squelch Adjustment Access Holes...
Page 42: ...STEP SYSTEM PASS FAIL NOTES A 11...
