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MFJ-1278B MULTI-MODE RADIO INTERFACING
Transmit Audio Level Adjustment for Interface Method II
1. Turn on the MFJ-1278B 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 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-1278B 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 decreased 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 AC voltmeter. If there is
not adequate audio level from the MFJ-1278B to make adjustment of R(s), then you can
increase the output level of the MFJ-1278B by adjusting R157 for radio port 1 or R158
for radio port 2. The transmitting audio output of both radio ports is factory set at 250
mV p-p.
4. Press the K key to return to receive mode and type Q to exit the calibration routine.
You have now set your transmitter deviation to approximately the correct level.
Remove JMP4 if you placed it in Step 4.
5. Carefully remove the variable resistor and measure its value. This is the proper value of
R(S) for your particular radio.
6. Select the nearest standard value fixed resistor (1/4 watt is fine) and permanently install
this resistor as R(S) in the interface circuit.
7. If you have access to FM test equipment, check to see that the modulation level is still
within the limits of 3 to 4.5 kHz deviation. If the modulation level is not within 3 to
4.5kHz limits, then make a final adjustment with the MFJ-1278B transmit audio level
controls. These controls are R157 for Radio 1 and R158 for Radio 2.