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Section IV Operation (Continued)
pitched whistle or a high-pitched whistle) you will hear the increase in the output
tone of the receiver. If the incorrect mode is selected, an increase in tone of a
whistle applied to the transmitter will cause a decrease in the resultant tone from
the receiver.
Thus when a voice is used in place of a whistle or tone, in the proper listening
mode the voice will be received correctly whereas in the incorrect mode, the
voice will be translated backwards and cannot be made intelligible by the voice
lock control. When listening to an AM transmission, a correct sideband is heard
in either mode since both upper and lower sideband are received.
Once the desired SSB mode has been selected, frequency adjustment may be
necessary in order to make the incoming signal intelligible, the VOICE LOCK
control allows the operator to vary frequency above and below the exact-center
frequency of the received signal. If the sound of the incoming signal is high or
low pitched, adjust the operation of the VOICE LOCK. Consider it as performing
the same function as a phonograph speed control. When the speed is set too
high, voices will be high-pitched and if set too low, voices will be low-pitched.
Also, there is only one correct speed that will make a particular record produce
the same sound that was recorded. If the record is played on a turntable that
rotates in the wrong direction (opposite sideband) no amount of speed control
(VOICE LOCK) will produce an intelligible sound.
An AM signal received while listening in one of the SSB modes will produce a
steady tone (carrier) in addition to the intelligence, unless the SSB receiver is
tuned to exactly the same frequency by the VOICE LOCK control. For simplicity it
is recommended that the AM modes be used to listen to AM signals.
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Section IV Operation (Continued)
ALTERNATE MICROPHONES AND INSTALLATION
For best results, the user should select a low-impedance dynamic type microphone
or a transistorized microphone. Transistorized type microphones have a low output
impedance characteristic. The microphones must be provided with a 4-lead cable.
The audio conductor and its shielded lead comprise two of the leads. The third
lead is for receive control, the forth is for transmit control.
The microphone should provide the functions shown in schematic below:
4 WIRE MIC CABLE
Pin Number Mic Cable Lead
1 Grounding
2Audio Lead
3Transmit Control
4 Receive Control
Fig. 1. Cobra 148GTL microphone schematic.
If the microphone to be used is provided with pre-cut leads, they must be revised
as follows:
1. Cut leads so that they extend 7/16” (11 mm) beyond the plastic insulating
jacket of the microphone cable (see Fig. 2.)
2. All leads should be cut to the same length. Strip the ends of each wire 1/8”
(2 mm) and tin the exposed wire.
Before beginning the actual wiring, read carefully, the circuit and wiring
information provided with the microphone you select. Use the minimum heat
required in soldering the connections. Keep the exposed wire lengths to a
minimum to avoid shorting when the microphone plug is reassembled.
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