Cobra 148gtl Cb Tranciever Manual

							How To  Use Your40-CHANNEL, 
CITIZENS BAND 
SSB/AM 2-WAY MOBILE RADIOModel 148 GTLContents Page
The CB Story ....................................................................................
1
Section I: Introduction ......................................................................2
Section II: Specifications ..............................................................3, 4
Section III: Installation ........................................................5, 6, 7, 8
Section IV: Operation ................................................................9 - 17
Controls and Indicators ....................................................................9
A.Control Functions ..............................................................9, 10, 11
B.Indicator Functions ......................................................................12
Operating Procedure to Receive ......................................................12
Operating Procedure to Transmit ....................................................13
Receiving SSB Signals ..............................................................13, 14
Alternate Microphones and Installation ..............................15, 16, 17
Section V: Maintenance and Adjustment ..................................18 -21
FCC Warning ..................................................................................18 
Section VI: Appendix ..........................................................19, 20, 21
Ten Code ........................................................................................19
A few Rules That Should Be Obeyed ..............................................20
How Your CB Can Serve You ..........................................................20
Use Channel 9 for Emergency Messages Only ................................21
Warranty Service Instructions ..........................................................22
Limited Two Year Warranty ..............................................................23
Cobra CB Radio Accessories..................................Inside Back Cover
OPERATING INSTRUCTIONS FOR YOUR
40 CHANNEL
CITIZENS BAND
SSB/AM 2-WAY 
MOBILE RADIO
Model 148 GTL 
						

							Section I Introduction
FREQUENCY RANGE
The COBRA 148GTL transceiver represents one of the most advanced SSB/AM two-
way radios ever designed for use as a Class D station in the Citizens Radio Service.
This unit features advanced Phase Lock Loop (PLL) circuitry, which is used in the AM
mode and in the upper and lower single sideband modes, providing complete cover
age of all 40 channels shown below.
ChannelChannel Frequency 
in MHz
126.965
226.975
326.985
427.005
527.015
627.025
727.035
827.005
927.065
1027.075
1127.085
1227.105
1327.115
1427.125
1527.135
1627.155
1727.165
1827.175
1927.185
ChannelChannel Frequency 
in MHz
2127.215
2227.225
2327.255
2427.235
2527.245
2627.265
2727.275
2827.285
2927.295
3027.305
3127.315
3227.325
3327.335
3427.345
3527.355
3627.365
3727.375
3827.385
3927.395
4027.405
2027.205
The COBRA 148GTL has a vastly superior receiver which includes an RF gain con-
trol and noise blanker circuitry effective in both AM and SSB modes., and an auto-
matic noise limiter effective in the AM mode. The receiver also features increased
protection against cross modulation and strong adjacent channel signals.
To obtain maximum performance please read carefully the descriptions and oper-
ating instructions in this manual.
2
The Citizens Band lies between the shortwave broadcast and 10-meter Amateur
radio bands, and was established by law in 1949. The Class D two-way communi-
cations service was opened in 1959. (CB also includes a Class A citizens band and
Class C remote control frequencies.)
FCC regulations permit only “transmissions” (one party to another) rather than
“broadcasts” (to a wide audience). Thus, advertising is not allowed on CB Channels
because that is “broadcasting.”
NOTICE:
Replacement or substitution of transistors, regular diodes or
other parts of a unique nature, with parts other than those
recommended by Cobra, may cause violation of the techni-
cal regulations of Part 95 of the FCC Rules, or violation of
Type Acceptance requirements of Part 2 of the Rules.
Citizens Band (CB) Radio operators are no longer required to
obtain an FCC license to operate their CB equipment  or
provide station identification.
An operator of a CB radio station is required to comply 
with the communications act and with the rules of CB 
Radio Operation.
Serial No.
Date of Purchase
Dealer Name
Keep this manual for detailed information about your Cobra CB radio.
SAVE YOUR SALES RECEIPT, THE CARTON AND “PACKING”
MATERIALS FOR POSSIBLE FUTURE USE.
The CB Story
1 
						

							RECEIVER
SensitivitySSB: 0.25 
µV for 10dB (S+N)/N at greater
than 1/2-watt of audio output. 
AM: 0.5 
µV for 10 dB (S+N)/ at greater 
than 1/2-watt of audio output.
SelectivityAM: 6dB @ 3 KHz, 50 dB @ 9 KHz. 
SSB: 6 dB @ 1.1 KHz, 60 dB @ 2.3 KHz.
Image RejectionMore than 65 dB.
IF FrequencyAM: 7.8 MHz 1st IF, 455 KHz 2nd IF. 
SSB: 7.8 MHz.
Adjacent-Channel Rejection60 dB AM & 70 dB SSB.
AM and SSB RF Gain Control40 dB adjustable for optimum signal 
reception.
Automatic Gain Control (AGC)Less than 10 dB change in audio 
output for inputs from 10 to 100,000
microvolts.
SquelchAdjustable; threshold less than 0.25 µV.
ANLSwitchable.
Noise BlankerRF type, effective on AM and SSB.
Voice Lock Range±2.5 KHz.
Audio Output Power4 watts into 8 ohms.
Frequency Response300 to 2500 Hz.
Built-in Speaker4 ohms, round.
External Speaker (Not Supplied)8 ohms; disables internal speaker 
when connected.
PA SYSTEM
Power Output4 watts into external speaker.
External Speaker for PA (Not Supplied)8 ohms.
(SPECIFICATIONS SUBJECT TO CHANGE WITHOUT NOTICE)
Section II Specifications (Cont.)
4
Section II Specifications
GENERAL
Channels40 AM, 40LSB, 40 USB.
Frequency Range26.965 to 27.405 MHz.
Frequency ControlPhase Lock Loop (PLL) synthesizer.
Frequency Tolerance0.005%
Frequency Stability0.001%
Operating Temperature Range-30°C to + 50°C
MicrophonePlug-in dynamic; with push-to-talk
switch and coiled cord.
Input Voltage13.8V DC nominal, 15.9V max., 11.7V 
min. (positive or negative ground).
Current Drain
Transmit
: AM full mod., 2.2A. SSB 12
watts PEP output, 2A. 
Receive
: Squelched, 0.25A 
Maximum audio output, 0.6A.
Size2 3
/
8”(H) x 7 7
/
8”(W) x 9 1
/
4”(D).
Weight5 lbs.
Antenna ConductorUHF, S0239.
Semiconductors3 field effect transistors, 45 transistors,
63 diodes, 6 integrated circuits, 1 two
color light emitting diode.
Meter (3-in-1)Illuminated; indicates relative output 
power, received signal strength, 
and SWR.
TRANSMITTER
Power OutputAM, 4 watts. 
SSB, 12 watts, PEP.
ModulationHigh-and low-level Class B, Amplitude 
Modulation.
Intermodulation DistortionSSB: 3rd order, more than -25 dB. 
5th order, more than -35 dB.
SSB Carrier Suppression55 dB.
Unwanted Sideband50 dB.
Frequency ResponseAM and SSB; 300 to 2500 Hz.
Output Impedance50 ohms, unbalanced
Output IndicatorsMeter shows relative RF output power
and SWR. Transmit LED glows red when
transmitter is in operation.
3 
						

							Section III Installation  (Continued)
IGNITION NOISE INTERFERENCE
Use of a mobile receiver at low signal levels is normally limited by the presence
of electrical noise. The primary source of noise in automobile installations is from
the generator and ignition system in the vehicle. Under most operating conditions,
when signal level is adequate, the background noise does not present a serious
problem. Also, when extremely low level signals are being received, the transceiv-
er may be operated with vehicle engine turned off. The unit requires very little
current and therefore will not significantly discharge the vehicle battery.
Even though the COBRA 148GTL has ANL and NB controls, in some installations
ignition interference may be high enough to make good communications impossi-
ble. The electrical noise may come from several sources. Many possibilities exist
and variations between vehicles require different solutions to reduce the noise.
Consult your COBRA dealer or a 2-way radio technician for help in locating and
correcting the source of severe noise.
ANTENNA
Since the maximum allowable power output of the transmitter is limited by the
FCC, the antenna is one important factor  affecting transmission distance. Only a
properly matched antenna system will allow maximum power transfer from the 50
ohm transmission line to the radiating element. In mobile installations (cars,
trucks, boats, etc.), an antenna system that is non-directional should be used.
A vertically polarized, quarter-wavelength whip antenna provides the most reliable
operation and greatest range. Shorter, loaded-type whip antennas are more attrac-
tive, compact and adequate for applications where the maximum possible dis-
tance is not required. Also, the loaded whips do not present the problems of
height imposed by a full quarter-wavelength whip.
Mobile whip antennas utilize the metal body of the vehicle as a ground plane.
When mounted at a corner of the vehicle they are slightly directional, in the
direction of the body of the vehicle. For all practical purposes, however, the radia-
tion pattern is nondirectional. The slight directional characteristic will be observed
only at extreme distances. A standard antenna connector (type SO239) is provided
on the transceiver for easy connection to a standard PL 259 cable termination.
If the transceiver is not mounted on a metal surface, it is necessary to run a sepa-
rate ground wire from the unit to a good metal electrical ground in the vehicle.
When installed in a boat, the transceiver will not operate at maximum efficiency
without a ground plate, unless the vessel has a steel hull.
Before installing the transceiver in a boat, consult your dealer for information re-
garding an adequate grounding system and prevention of electrolysis between fit-
tings in the hull and water.
6
Section III Installation
LOCATION
Plan the location of the transceiver and microphone bracket before starting the in-
stallation. Select a location that is convenient for operation and does not interfere
with the driver or passengers in the vehicle. In automobiles, the transceiver is usu-
ally mounted below the dash panel, with the microphone bracket beside it.
MOUNTING THE CONNECTION
The COBRA 148GTL is supplied with a universal mounting bracket. When mount-
ing the bracket and radio to your car, make sure it is mechanically strong. Also
provide a good electrical connection to the chassis of the vehicle. Proceed as fol-
lows to mount the transceiver:
1.After you have determined the most convenient location in your vehicle, hold
the COBRA 148GTL with mounting bracket in the exact location desired. If
nothing will interfere with mounting it in the desired position, remove the
mounting bolts. Before drilling the holes, make sure nothing will interfere with
the installation of the mounting bolts.
2.Connect the antenna cable plug to the standard receptacle on the rear panel.
Most CB antennas are terminated with a  type PL-259 plug and mate with the
receptacle.
3.Connect the red DC power input wire (with the fuse) to +13.8V DC. This wire
extends from the rear panel. In automobile installation, +13.8V DC is usually
obtained from the accessory contact on the ignition switch. This prevents the
set being left on accidentally when the driver leaves the car and also permits
operating the unit without the engine running. Locate the accessory contact on
most ignition switches by tracing the power wire from the AM broadcast receiv-
er in the car.
4.Connect the black lead to -13.8V DC. This is usually the chassis of the car. Any
convenient location with good electrical contact (remove paint) may be used.
5.Mount the microphone bracket on either side of the transceiver, using the two
screws supplied. When mounting in an automobile, place the bracket under
the dash so the microphone is readily accessible.
5 
						

							Section III Installation (Continued)
D.Try a different location on your car (keeping in mind the radiation pattern 
you wish).
E.Is the antenna perfectly vertical?
F.Try a different location in your neighborhood. Stay away from large metal
objects when adjusting (metal telephone or light posts, fences, etc.).
NOTE
The COBRA 148GTL will operate into an SWR of 2 to 1 indefinitely
and sustain an SWR of 20:1 for a maximum of 5 minutes at rated 
operating conditions.
EXTERNAL SPEAKER
The external speaker jack (EXT. SPK.) on the rear panel is used for remote receiver
monitoring. The external speaker should have 8 ohms impedance and be able to
handle at least 4 watts. When the external speaker is plugged in, the internal
speaker is disconnected.
PUBLIC ADDRESS
To use the transceiver as a public address system connect an external 8 ohm
speaker (4 watts minimum) to the PA SPK. jack located on the rear panel. Direct
speaker away from the microphone to prevent acoustic feedback. Physical separa-
tion or isolation of the microphone and speaker is important when operating the
PA at high output levels.
8
Section III Installation (Continued)
TUNING THE ANTENNA FOR OPTIMUM SWR
Since there is such a wide variety of base and mobile antennas, this section will
strictly concern itself to the various types of mobile adjustable antennas.
Because antenna length is directly related to the channel frequency, it must be
tuned to resonate optimally all 40 channels of the transceiver. Channel 1 requires
a longer antenna than Channel 40 because it is a lower frequency.
Due to the various methods of adjusting antennas for proper SWR we have chosen
what we think is the optimum method:
A.Antennas with adjustable screws (set screws).
1.Start with the antenna extended and tighten the set screw lightly enough so
that the antenna can be lightly tapped with your finger for easy adjustment.
2.Set your COBRA 148GTL to Channel 21. Press the PTT (Push-to-Talk) switch,
and tap the antenna (making it shorter). The SWR meter will show a lower
reading each time the antenna is tapped. By continuing to shorten the anten-
na you will notice the SWR reading will reach a low point and then start ris-
ing again. This means that you have passed the optimum point for Channel
21. Extend the antenna a short distance and again follow the procedure
above.
When the lowest point has been reached, switch to Channel 1 and then to
Channel 40 and compare SWR readings. They should be almost equal.
B.Antennas  which must be cut to proper length.
1.Follow the same procedure as above, but adjust the length by cutting in 1/8”
increments until a good match is obtained.
2.
Be very careful not to cut too much at one time, as once it is cut, it can no
longer be lengthed.
3.The whip is easily cut by filing a notch all the way around and breaking the
piece off with a pliers.
NOTE
THE PROPER SETTING IS ACHIEVED WHEN THE SWR IS 
1.5 OR BELOW, AND WHEN IT HAS THE SAME READING 
FOR CHANNELS 1 AND 40.
If you are having difficulties in adjusting your antenna, check the following:
A.All doors must be closed when adjusting the antenna.
B.Make sure the antenna base is grounded.
C.Check your coaxial cable routing (it may be pinched when routed into the car).
7 
						

							10
Section IV Operation (Continued)
4.SWR CAL CONTROL (outer dual concentric). In order for you to achieve
maximum radiated power and the longest range, it is important that your
antenna be in good condition, properly adjusted and matched to your trans-
ceiver.  The Built-in SWR (standing wave ratio) meter lets you easily measure
your antenna condition. To operate this function, connect your antenna to
the transceiver antenna output connector. Select a channel near the middle
of the band such as 21 or the channel you plan to use most frequently. Turn
the power on and set the meter function switch to the CAL position. Press
and hold the microphone push-to-talk button and using the SWR CAL con-
trol, adjust the meter to read the CAL position indicated on the meter face.
Then, without releasing the microphone button, switch the meter function
switch to the SWR position and read the SWR indicated. The lower the fig-
ure, the better, with 1 being ideal. Generally speaking, readings up to 3 are
acceptable, but over 3 indicates that you are losing radiated power and
antenna adjustment may be advisable.
5.DYNAMIKE. Adjusts the microphone gain in the transmit and PA modes. This
controls the gain to the extent that full talk power is available several inches
away from the microphone. In the Public Address (PA) mode the control
functions as the volume control.
6.VOICE LOCK. Allows variation of the receiver operating frequencies above
and below the assigned frequency. Although this control is intended primari-
ly to tune in SSB signals, it may also be used to optimize AM signals as
described in the Operating Procedure paragraphs.
7.DIM/NOR/BRT SWITCH. Controls the brightness of the meter and LED
channel indicator for optimum intensity for day or night-time driving.
8.CHANNEL SELECTOR. This switch selects any one of the forty Citizens Band
channels desired. The selected channel appears on the LED readout directly
above the Channel Selector knob. Channel 9 has been reserved by the FCC
for emergency communications involving the immediate safety of life of indi-
viduals or immediate protection of property. Channel 9 may also be used to
render assistance to a motorist.
9.0FF/ANL/NB + ANL SWITCH.In the ANL position only the automatic 
noise limiter in the audio circuits is activated. When the switch is placed 
in the ANL + NB position, the RF noise blanker also is activated. The 
RF noise blanker is very effective for repetitive impulse noise such as 
ignition interference.
Section IV Operation
CONTROLS AND INDICATORS
There are thirteen controls and three indicators on the front panel of your COBRA
148GTL.
A. CONTROL FUNCTIONS
1.OFF/ON/VOLUME (inner dual concentric). Turn clockwise to apply power to
the unit and to set the desired listening level. During normal CB operation,
the VOLUME control is used to adjust the output level obtained either at the
transceiver speaker or the external speaker, if used.
2.SQUELCH (outer dual concentric). This control is used to cut off or eliminate
receiver background noise in the absence of an incoming signal. For maxi-
mum receiver sensitivity it is desired that the control be adjusted only to the
point where the receiver background noise or ambient background noise is
eliminated. Turn fully counterclockwise then slowly clockwise until the receiv-
er noise disappears. Any signal to be received must now be slightly stronger
than the average received noise. Further clockwise rotation will increase the
threshold level which a signal must overcome in order to be heard. Only
strong signals will be heard at a maximum clockwise setting.
3.RF GAIN CONTROL (inner dual concentric). Used to reduce the gain of the
RF amplifier under strong signal conditions.
9 
						

							Section IV Operation (Continued)
B. INDICATOR FUNCTIONS
1.S-METER.Swings proportionally to the strength of the incoming signal.
2.RF METER. Swings proportionally to the RF output power.
3.SWR METER. Swings proportionally to the ratio of standing wave voltage and RF
output. Used to properly adjusts the length of the antenna, and to monitor the qual-
ity of the coaxial cable and all RF electrical connections. If there is any degradation
whatsoever in any of the above, due to humidity, salt, spray, vibration or corrosion,
the SWR meter reading will rise, thereby indicating that a problem exists.
To calibrate, switch to the “CAL” position, transmit in AM Mode by pressing the (PTT)
mic switch, and adjust the SWR control to the “CAL” mark on the meter then switch
to “SWR” position for the SWR measurement (Note: CB must be in AM mode).
4.CHANNEL INDICATOR. Numbered LED indicates the selected channel you
wish to operate on.
5.RECEIVE/TRANSMIT INDICATOR. The receiver/transmit LED indicator is
located next to the channel indicator. When in receive, the LED will be
green. When in transmit the LED will be red.
6.PRESS-TO-TALK MICROPHONE. The receiver and transmitter are controlled
by the Press-To-Talk switch on the microphone. Press the switch and the
transmitter is activated, release the switch to receive. When transmitting, hold
the microphone two inches from the mouth and speak clearly in a normal
“voice”. The radio comes complete with low-impedance (500 ohm) dynamic
microphone. For installation instructions on the other microphones see
ALTERNATE MICROPHONES AND INSTALLATION section.
OPERATING PROCEDURE TO RECEIVE
1.Be sure that the power source, microphone and antenna are connected to the
proper connectors before going to the next step.
2.Set PA-CB Switch to the CB position and turn unit on by turning VOL control
clockwise on COBRA 148GTL.
3.Set the VOLUME for a comfortable listening level.
4.Set MODE switch to the desired mode.
5.Set the RF gain control fully clockwise for maximum RF gain.
6.Listen to the background noise from the speaker. Turn the SQUELCH control
slowly clockwise until the noise JUST disappears (no signal should be pre-
sent). Leave the control at this setting. The SQUELCH is now properly adjust-
ed. The receiver will remain quiet until a signal is actually received. Do not
advance the control too far, or some of the weaker signals will not be heard.
7.Set the CHANNEL selector switch to the desired channel.
8.Adjust the VOICE LOCK control to clarify the SSB signals or to optimize 
AM signals.
12
11
Section IV Operation (Continued)
10.PA/CB SWITCH. Selects the mode of operation. In the CB position, the PA func-
tion is disabled and the unit will transmit and receive on the speaker that is con-
nected. In the PA mode, incoming CB transmission will be heard through the PA
speaker. This allows you to monitor messages while outside of your vehicle.
To use the PA feature, a speaker having a voice coil impedance of 8 ohms and
a power handling capability of at least three watts should be used. This speaker
must be plugged into the PA SPKR jack at the rear of the transceiver. If the pub-
lic address feature is to be used primarily for outdoor applications, the use of a
weatherproof horn type public address speaker is recommended. The durability
of this type speaker plus the inherent efficiency of such a speaker will provide
more than adequate results when combined with the high audio output level
available from the COBRA 148GTL. With the PA speaker connected as out-
lined previously, be sure that there is physical separation between the micro-
phone and the speaker itself. If the speaker is located too close to the micro-
phone, acoustic feedback will result when the public address system is operat-
ed at high volume. A directional type outdoor speaker reduces the amount of
isolation required. Some experimentation will determine the minimum amount
of isolation required for a given sound level from the public address system.
NOTE
PA volume is controlled by adjusting the DYNAMIKE knob to
the desired volume.
11.S-RF/CAL/SWR SWITCH. When in the S-RF position, the meter swings propor-
tionally to the strength of the received signal. When transmitting, the meter indi-
cates relative RF output power.
When in the “CAL” position, the SWR meter can be calibrated by adjusting the
“SWR” control to the “CAL” mark on the meter face.
When in “SWR” position, the standing wave ratio is measured.
12.MODE (LSB/AM/USB) SWITCH. This switch is used to select AM, LSB or USB
mode of operation. Unless the station with which communication is desired is
equipped with SSB, the AM mode is normally used. The mode selector switch
changes the mode of operation of both transmitter and receiver simultaneously.
Turn to “Receiving SSB signals” for a further explanation of single sideband.
13.TONE SWITCH-HI/NOR/LOW. This switch is used to shape the audio
response to the operator’s preference. Bass is increased in the LOW position
and treble is increased in the HI position. 
						

							Section IV Operation (Continued)
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 con-
trol allows the operator to vary frequency above and below the exact-center fre-
quency 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.
14
Section IV Operation (Continued)
OPERATING PROCEDURE TO TRANSMIT
1.Select the desired channel of transmission.
2.Set the DYNAMIKE control fully clockwise.
3.If the channel is clear, depress the Push-To-Talk switch on the microphone
and speak in a normal voice.
RECEIVING SSB SIGNALS
There are three types of signals presently used for communications in the Citizens
Band: AM, USB, and LSB. When the MODE switch on your unit is placed in the
AM position, only standard double-sideband, full carrier signals will be detected.
An SSB signal may be recognized while in the AM mode by its characteristic
“Donald Duck” sound and the inability of the AM detector to produce an intelligi-
ble output. The USB and LSB modes will detect upper sideband and lower side-
band respectively, and standard AM signals.
SSB reception differs from standard AM reception in that SSB receiver does not
require a carrier or opposite sideband to produce an intelligible signal. A single-
sideband transmitted signal consists only of the upper or the lower sideband and
no carrier is transmitted. The elimination of the carrier from the AM signal helps to
eliminate the biggest cause of whistles and tones heard on channels which make
even moderately strong AM signals unreadable. Also, SSB takes only half of an AM
channel, therefore two SSB conversations will fit into each channel expanding the
40 AM channels to 80 SSB channels. The reduction in channel space required also
helps in the receiver because only 
half
of the noise and interference can be
received with 100% of the SSB signal.
An SSB signal may be received only when the listening  receiver is functioning in
the same mode. In other words, an upper sideband signal (USB) may be made
intelligible 
only
if the receiver is functioning in the USB position.
If a lower sideband (LSB) signal is heard when the receiver is in the USB  mode,
no amount  of tuning will make the signal intelligible. The reason for this may be
understood if you consider that  when the modulation is applied to the transmit-
ter’s microphone in the USB mode, the transmitter’s output frequency is increased
whereas in the LSB mode the transmitter’s output frequency is decreased. The
result in listening to the receiver is that when the MODE switch is in the proper
position (either USB or LSB), a true reproduction of single tone of modulation will
result, and if the tone is increased in frequency (such as a low-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.
13 
						

							Section IV Operation (Continued)
Fig. 2.Microphone Cable Preparation
To wire the microphone cable to the plug provided, proceed as follows.
Fig. 3. Microphone plug wiring.
1.Remove the retaining screw.
2.Unscrew the housing from the pin receptacle body.
3.Loosen the two cable clamp retainer screws.
4.Feed the microphone cable through the housing, knurled ring and washer as
shown Fig. 3B.
5.The wires must now be soldered to the pins as indicated in the above wiring
tables. If a vise or clamping tool is available it should be used to hold the pin
receptacle body during the soldering operation, so that both hands are free to
perform the soldering. If a vise or clamping tool  is not available, the pin recep-
tacle body can be held in a stationary position by inserting it into the micro-
phone jack on the front panel. The numbers of the pins of the microphone plug
are shown in Fig 4, as viewed from the back of the plug. Before soldering the
wire to the pins, pre-tin the wire receptacle of each pin of the plug.
16
34
2
1
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 NumberMic Cable Lead
1Grounding
2Audio Lead
3Transmit Control
4Receive 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” 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”
and tin the exposed wire.
Before beginning the actual wiring, read carefully, the circuit and wiring informa-
tion provided with the microphone you select. Use the minimum head required in
soldering the connections. Keep the exposed wire lengths to a minimum to avoid
shorting when the microphone plug is reassembled.
15 
						

							18
Section V Maintenance and AdjustmentThe COBRA 148GTL transceiver is specifically designed for the environment en-
countered in mobile installations. The use of all solid state circuitry and its light
weight result in high reliability. Should a failure occur, however, replace parts
only with identical parts. Do not substitute. Refer to the schematic diagram and
parts list.
NOTE
If the performance described in the OPERATION and MAINTENANCE 
AND ADJUSTMENT sections is not obtained, review the operating instru-
ctions to insure that proper procedures were followed. If a problem still 
exists, refer to WARRANTY SERVICE INSTRUCTIONS elsewhere in 
this manual.
FCC WARNING
Replacement or substitution of transistors, regular diodes or other parts of
a unique nature, with parts other than those recommended by Cobra,
may cause violation of the technical regulations of Part 95 of the FCC
Rules, or violation of Type Acceptance requirements of Part 2 of the Rules.
Citizens Band (CB) Radio operators are no longer required to obtain an FCC
license to operate their CB equipment or provide station identification.
Nevertheless, an operator of a CB radio station is still required to comply
with the communications act and with the rules of CB Radio Operation.
Section IV Operation (Continued)
Fig. 4. Microphone plug pin numbers viewed from rear of pin receptacle.
Be sure that the housing and the knurled ring of Fig. 3 are pushed back onto the
microphone cable before starting to solder. If the washer is not captive to the pin
receptacle body, make sure that it is  placed on the threaded portion of the pin
receptacle body before soldering.
If the microphone jack is used to hold the pin receptacle during soldering opera-
tion, best results are obtained when the connections to pins 1 and 3 are made first
and then the connections to pins 2 and 4. Use a minimum amount of solder and
be careful to prevent excessive solder accumulation on pins, which could cause a
short between the pin and the microphone plug housing.
6.When all soldering connections to the pins of the microphone are complete,
push the knurled ring and the housing forward and screw the housing onto
the threaded portion of the pin receptacle body. Note the location of the
screw clearance hole in the plug housing with respect to the threaded hole
in the pin receptacle body. When the housing is completely threaded into
the pin receptacle body, a final fraction of a turn either clockwise or coun-
terclockwise may be required to align the screw hole with the threaded hole
in the pin receptacle body. When these are aligned, the retaining screw is
then screwed into place to secure the housing to the pin receptacle body.
7.The two cable clamp retainer screws should now be tightened to secure the
housing to the microphone cord. If the cutting directions have been carefully
followed, the cable clamp should secure to the insulation jacket of the mi-
crophone cable.
8.Upon completion of the microphone plug wiring, connect and secure the
microphone plug in the transceiver.
17
3
2
4
1