MFJ-1270C/1274C MANUAL RADIO INTERFACING
Page 24
The transmit audio level for the radio port is factory preset at 250 mV p-p to be
compatible with the mic input of most radios. However, if the transmit audio
is too low or distorted, adjustment may be needed. Use the following procedure
to calibrate:
Transmit Audio Level Adjustment for Method I Interface
1. Connect your MFJ TNC and radio as shown in Fig. 3-5. Turn on the MFJ
TNC and computer and start your terminal program. Connect the radio to a
dummy load and listen to the transmission with another nearby radio.
2. Enter the modem calibration procedure by typing
CALIBRA
and a carriage return. Press the K key on your keyboard 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, install a shorting jumper at JMP4.
3. With the MFJ TNC keying the transmitter and transmitting the higher of
the two tones, adjust the transmit audio level as follows. With a small
screwdriver, adjust trimpot located on the left side of them MFJ TNC while you
listen to the monitoring receiver. Turn the adjustment on the trimpot clockwise
(CW) until no increase in output level is heard at the monitoring receiver.
4. Rotate the adjustment on the trimpot counter-clockwise until the audio
signal on the monitoring receiver is reduced by half of the maximum level. This
can be estimated by ear or accurately by measuring the output voltage at the
transmitting audio pin of the radio port with an oscilloscope or A.C. voltmeter.
5. Press the K key to return to receive mode and type Q to exit the calibration
routine. Be sure to remove JMP4 if you placed it to defeat the watch-dog timer.
You have now set your transmitter deviation to approximately the correct level.
If you notice a significant hum level in the monitored audio in Step 3, take
measures to remove it. This may require shielded wire (recommended in any
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