Computer interfacing, covered in the previous chapter, is only half the interfacing task. The
other half is connecting your MFJ-1278B to your radios.
MFJ-1278B Radio Ports
Interfacing the MFJ-1278B to your radios involves connecting the following signals at Radio
Port 1 and Radio Port 2. The pin outs of Radio Port 1 and Radio Port 2 are shown in Fig. 3-1.
Pin1 Microphone audio, from the MFJ-1278B to your transmitter.
Pin 2 Ground, audio and PTT common.
Pin3 Push-to-talk, to allow the MFJ-1278B to key your transmitter.
Pin 4 Receive audio, from your receiver to the MFJ-1278B.
Pin 5 Squelch input (optional) to allow the MFJ-1278B to detect activity on a
shared-mode channel.
Fig.3-1 Radio Port 1 and Radio Port 2 Connector
This chapter describes how to connect the MFJ-1278B to your radio and how to adjust the
receive and transmit audio levels appropriately. The interconnection needs to be well thought
out, to minimize pickup of stray audio and RF noise by the lines. If possible, you should set
up your station with a monitor speaker and be able to operate on voice without disconnecting
the MFJ-1278B.
MFJ-1278B MULTI-MODE RADIO INTERFACING
RADIO PORTS CONNECTION
The MFJ-1278B gives the user two (2) radio ports. This allows for both FM and HF
operation from either radio port. Since the radio ports are independent of each other, the user
is not restricted to FM or HF operations. The radio ports on the MFJ-1278B allow an FM
radio and an HF radio to be connected at all times. In fact you can have any combination of
HF radios or VHF radios connected to the radio ports at any time. The pin designations for
both radio ports are exactly the same. See Fig. 3-1 for the radio port pin designations. The
radio ports on the MFJ-1278B are switched by using the RADio command. We will discuss
radio port switching in the next chapter.
Once the user obtains the proper microphone connector (not provided), use Fig. 3-2 to wire
the 5 pin DIN cable (provided) to the microphone connector. If two radios are to be used
with the MFJ-1278B, you will need to wire two radio cables.
You may obtain the specific pin designations for your radio's microphone connector from
your radio's manual. Appendix A at the end of this Instruction Manual lists pin assignments
for some of the most popular radios. The accuracy of this information is not guaranteed.
You should verify this information with your radio manual.
Fig. 3-2 Radio Connector Pin Diagram
Two 5-pin male DIN cables are provided with the MFJ-1278B. These cables have a 5-pin
DIN connector on one end, with the other end of each cable being unterminated. The
unterminated end of each cable is for wiring the appropriate microphone connector which
matches your radio.
CHECK THIS CABLE WITH AN OHM METER TO IDENTIFY EACH
WIRE BEFORE WIRING IT TO THE MIC CONNECTOR THAT FITS
YOUR RADIO.
MFJ-1278B MULTI-MODE RADIO INTERFACING
HANDHELD RADIO CONNECTION
Some HTs key the transmitter by drawing a small amount of current from the microphone
input pin (see Fig. 3-3 below). Radios with this type of special keying circuit are ICOM-2AT
(tm) and Yaesu FT-x09, FT-x3, FT-727 (tm) and others.
Appendix A at the end of this instruction manual provided pin designation for some of the
radios. Also consult the instruction manual of your radio.
If your HT has this type of microphone circuit, you can wire the microphone like the one
shown in Fig. 3-3 or you can remove the cover of the MFJ-1278B and install a shorting
jumper at JMP L for Radio Port 1 or JMP K for Radio Port 2. Installing JMP K or JMP L
will eliminating the need of soldering "Cx" and "Rx" to the microphone cable. "Cx" and
"Rx" are installed on the MFJ-1278B mother board. Fig. 3-4 shows the location of JMP L
and JMP K connectors. On the MFJ-1278B mother board, the "Rx" resistor for Radio Port 1
is R140 and the "Rx" resistor for Radio Port 2 is R107. If your radio still will not key
properly after installing JMP L or JMP K, you may need to change R140 or R107 to a
smaller value. Be sure to remove JMP K or JMP L when connecting the MFJ-1278B to
another type of radio.
FIG. 3-3 HT Special Keying Circuit
MFJ-1278B MULTI-MODE RADIO INTERFACING
RADIO INTERFACING METHODS
The MFJ-1278B allows radio connection without any modifications to the radio or any signal
balancing devices in the cables. There are two types interfacing methods presented in this
chapter.
Method 1: Direct Connection to Microphone and Speaker
For Method 1, shown in Fig. 3-5, the MFJ-1278B's audio is fed directly into the microphone
connector or similarly connected auxiliary jack. The output of the MFJ-1278B will be
adjusted to give a proper modulation level. The receiver audio will be taken from an
earphone plug or speaker jack and fed directly to the MFJ-1278B. The user can connect a
monitor speaker to the SPEAKER jack of the MFJ-1278B. This allows you to monitor the
transmit and receive audio on the channel.
Fig. 3-5 Method One Interconnect.
The transmit audio levels for both radio ports are 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, then adjust the appropriate output level control as stated in the section. Use the
following procedure to calibrate:
Transmit Audio Level Adjustment for Method I Interface
1. Connect your MFJ-1278B and radio as shown in Fig. 3-5. Turn on the MFJ-1278B 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
MFJ-1278B MULTI-MODE RADIO INTERFACING
3. With the MFJ-1278B keying the transmitter and transmitting the higher of the two
tones, adjust the transmit audio level as follows. With a small flat-tipped screwdriver,
adjust trimpot located on the left side of them MFJ-1278B (R157 for radio 1 or R158
for radio 2) while you listen to the monitoring receiver. Adjusting the trimpot CW
increases the output, while CCW decreases the output. 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 decreases by half of the maximum level. This can be estimated by
ear or accurately by measuring the output voltage at the transmit audio pin of the radio
port with an oscilloscope or AC 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 event) in your microphone
audio circuit. The use of shielded cable is always necessary, in projects such as this. If your
transmitter has an adjustable microphone gain control, try reducing the sensitivity of the
transmitter microphone circuit and increasing the signal level from the MFJ-1278B to
minimize hum or other noise problems.
Setting the Receiver Audio Input Level
The modem in your MFJ-1278B implements an advanced phase coherence type data carrier
detection (DCD). There is a threshold control and a sensitive tuning indicator. Together they
set the correct receive audio level for the modem, also to optimize the DCD characteristics
for the various methods of operation.
With your radio in the receive mode, open the squelch control so that a steady hiss is present
on a speaker. Set the volume control to the minimum volume position. The tuning indicator
on the MFJ-1278B 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 AC voltmeter. This will be near the correct amount of audio for NBFM operation. Levels
higher than this will not degrade the modem performance as long as the receiver audio
amplifier is capable of producing the chosen output level without distortion (clipping).
MFJ-1278B MULTI-MODE RADIO INTERFACING
The bandwidth of the receiver audio will have an effect on modem sensitivity to false DCD.
Some receivers produce wideband audio that will NOT produce any false DCD activity
regardless of the threshold control setting. In this case, set the DCD threshold control to its
maximum clockwise rotation. This will not effect modem performance.
Method 2: Accessory Jack or Interface Box Connection
If your radio has an accessory jack with PTT, transmit audio, and receive audio signals, the
interconnection can be done through this jack (shown in Fig. 3-6).
Fig. 3-6 Accessory Jack Interface.
If your radio does not have an accessory jack and you don't wish to add a connector to your
radio, you may construct a separate external interface box. This box will permit simultaneous
connection of your MFJ-1278B and a microphone. Figure 3-7 shows a schematic of an
external interface box.
A microphone interface box similar to the one shown in Fig. 3-7 is available from MFJ
Enterprises, Inc. or from any dealer of MFJ products. The Model No. of the microphone
interface box is MFJ-1272B.
Regardless of whether you use an accessory jack or an external interface box, you should use
shielded wire for all signal carrying leads. The connectors and fittings on your radio will be
the deciding factor, as to what hardware you will need. The user may also want to refer to
MFJ-1278B MULTI-MODE RADIO INTERFACING
Fig. 3-7 External Interface Box
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-1278B PC board.
2. Temporarily solder a variable resistor in place of R(s) Fig. 3-7. The maximum value of
this resistor can be determined by experiment. However, a 500K resistor should be
adequate most cases. Connect your MFJ-1278B 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 the next sections describe.
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