MFJ-1270C/1274C MANUAL RADIO INTERFACING
Page 27
If you built the external interface box as in Fig.3-7, then follow this procedure to
adjust R(s).
1. Install JMP J on the MFJ TNC PC board.
2. Temporarily solder a variable resistor in place of R(s) Fig. 3-7. The
maximum value of this resistor must be determined by experiment, but 500K
should handle most cases. Connect your MFJ TNC to the radio. Connect the
microphone to the radio, or to the interface box if one is being used. Connect
the radio to a dummy load and listen to the transmission with another nearby
radio. Adjust R(s) for proper modulation as follow:
Transmit Audio Level Adjustment for Interface Method II
1. Turn on the MFJ TNC and computer and start your terminal program.
Enter the modem calibration procedure by typing
CALIBRA
Press the K key to key the transmitter, then tap the space bar until the higher of
the two tones is heard. Pressing the K key again will unkey the transmitter.
After the transmitter has been keyed for a few seconds, it will be shut off
automatically by the transmit watch- dog circuit. As you perform the
adjustments below, you will have to periodically unkey then re-key the
transmitter by typing the K key. If you wish to defeat the watch-dog timer, place
a shorting jumper at JMP4.
2. With the MFJ TNC keying the transmitter and transmitting the higher of
the two tones, adjust the transmit audio level as follows.
3. Adjust R(s) the variable resistor installed for proper modulation level
(typically between 3.0 and 4.5 kHz deviation. for Amateur FM). If FM test
equipment is not available, adjust R(S) until the audio signal on the monitoring
receiver is reduced by half of the maximum level. This can be estimated by ear
or accurately determined by measuring the output voltage across the speaker
with an oscilloscope or A.C. voltmeter. If there is not adequate audio level from
the MFJ TNC to make adjustment of R(s), then you can increase the output
level of the MFJ TNC by the transmit audio control on the left side of the TNC.
The transmitting audio output of the radio port is factory set at 250 mV p-p.
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