MFJ-1270C/1274C MANUAL RADIO INTERFACING
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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 it is not, make a final
adjustment with the MFJ TNC transmit audio level control.
Receiver Audio Setting
The modem in your MFJ TNC is equipped with an advanced phase coherence
type data carrier detector (DCD) with a sensitive tuning indicator. These can be
used in combination to set the correct receive audio level for the modem and to
optimize the DCD characteristics for HF operation.
With your radio in the receive mode, open the squelch control so that a steady
hiss is heard on a speaker. Set the volume control to the minimum volume
position. The tuning indicator on the MFJ TNC should drift off to one side of
the display and become stationary. It may drift enough to disappear off the end
of the display. Slowly advance the audio output level with the volume control
until the tuning indicator "springs to life" and dances around a point near the
middle of the display. This is the absolute minimum audio level for marginal
copy. Continue to advance the volume control until there is approximately twice
as much audio present at the receiver output. This can be estimated by ear or
measured with an oscilloscope or A.C. voltmeter. This will be near the correct
amount of audio for NBFM operation. Levels higher than this will not degrade
the modem performance solong as the receiver audio amplifier is capable of
producing the chosen output level without distortion (clipping).
IMPORTANT NOTE: Harmonics Interference
If you experience interference from one of the harmonics of the crystal oscillator
in the MFJ TNC , trimmer capacitor, C47 (near the crystal) can be adjusted to
move the frequency of the harmonic.
Monitor Speaker Connection
