Cobra CB 19 DX4 EU, CB 29 LX EU Service Manual
Model No: 19 DX4 EU
Cobra Rev.: A
Page: 2 / 13
2
SPECIFICATIONS
1. GENERAL
Channels………………………...……………………..............................EU CH AM/FM 1W/4W
Frequency Range………………………………...………………………..26.565 to 27.99125 MHz
Temperature Range……………………..…………………….………..…-10°C / +55°C
Input Voltage………………………………...…………………………......13.2VDC ±15%
Size………………………………………….……………..………190(L) X 45(H) X 120(D) mm
Weight……………………………………………………..………………....Approx. 0.945 kg
2. RECEIVER
Receiving System………………………………………………………...Super Heterodyne
Frequency…………………………………………….…1st IF: 10.695 MHz, 2nd IF: 455 MHZ
Sensitivity……………………………………….....….0.5 µV for 12 dB SINAD in FM mode Audio
Distortion……………………………………..………..……..……………Less Than 8% @ 1 KHz
Current Drain at standby…………………………………………………300 mA
3. TRANSMITTER
Output Power………………………………………………..……….MAX. 4W @ 13.2 V DC
Modulation………………………………………………...………….FM: 1.7 KHz / AM 65%
Frequency response………………………………………………...From 400 Hz to 2.5 KHz
Output impedance…………………………………..……………… RF 50 ohm
MIN Current Drain……………………………………………………1500 mA
Model No: 19 DX4 EU
Cobra Rev.: A
Page: 3 / 13
3
OPERATION INSTRUCTIONS
Having properly installed your batteries and hooked-up the antenna, you are now ready to operate your
radio for optimum reception and voice transmission.
Turn the power “ON” with ON/OFF switch.
Set the desired channel in FM or AM.
Adjust the squelch control knob to reduce any undesirable background noise when no signal is being
received. To do this, select a channel where no signal are present, or wait until signals cease on your
channel. Then, rotate the squelch control knob clockwise to a point where the background noise
disappears.
Note: When the squelch is set properly, the speaker will remain quiet until a signal is received. In order to
receive weak signals, do not set the squelch too high.
Adjust the volume to the desired listening level.
To Transmit
Press and hold the push-to-talk button. Speak slowly and clearly in a normal voice two to three inches from
the microphone. A built-in modulation control circuit will automatically adjust the microphone input level.
There is no need to speak loudly.
To Receive
Release the push-to-talk button.
Note: Other operations mode see user manual.
Model No: 19 DX4 EU
Cobra Rev.: A
Page: 4 / 13
4
THEORY OF OPERATIONS
TRANSMITTER
A. RF Amplification
The output of double AMP Q14 is fed through tuning IFT L8 to the base of pre driver AMP Q15. The
output is then supplied through tuning circuit L9 to RF driver AMP Q16. The output of Q16 is supplied
with tuning circuit L10 and C116,C67,C95 and goes to the base of final RF AMP Q17. The output of Q17
is supplied to the antenna through L-C tuning circuit.
B. Modulation Control
i. FM
The mic input is fed to mic audio amplifier IC which drives modulation varicap diode D403 in the VCO
circuit. RV1 is used to limit the incoming modulation audio levels to inhibit over modulation.
ii. AM
Modulation signals are filtered with RC network and goes to the audio power AMP IC IC103 to make
nominal signal level to achieve wanted modulation. To control incoming audio signal, diode D7 and
corresponding ALC circuit is used to limit the incoming modulation.
C. Receiver
CB receiver is dual conversion super-heterodyne type with the first IF 10.695 MHz and second IF 455 KHz.
Receiver is separated into two blocks, 1st IF section and 2nd IF section. The PLL synthesizer supplies first
local frequency 37.26 ~ 38.686 MHz.(for EU) With the provided first local frequencies to Q2,Q3 mixes the
incoming RF signal to generate first IF signal. Mixed signals were filtered with the XF1 (10.695 MHz)
crystal filter and other tuning circuits. The filtered signal is then further mixed with the second local
frequency of 10.24 MHz to generate the second IF. Output signal of mixer is filtered with CF1 (455 KHz
ceramic filter). The 455 KHz signal from the 2
nd
IF filter was amplified and limits internally. After
amplification the signals fed the quadrate detector loop L20. Then we can see the recovered audio signals
on Pin 9 for FM of IC7. With the amplitude of recovered signals, Q10 serves as an audio amplifier. For
AM signal, it will be passed through the filter CF1 and be induced to Q6, Q7 respective and detected to
voice signal by D5.
Model No: 19 DX4 EU
Cobra Rev.: A
Page: 5 / 13
5
TROUBLESHOOTING HINTS
Symptom Possible Causes Remedy Suggestions
Unit does not work
at all
• Defective power switch
• Blown fuse
• Broken DC power cord
• Defective components
• Replace defective components
No output from
speaker at all
• Defective external speaker jack
• Poor connection on microphone connector
• Defective push switch on microphone
• Defective internal speaker
• Defective VR1 and IC8
• Replace defective components
No noise on
speaker
• Measure all the voltage of IC7 with voltage chart
• Defective squelch circuit components (RV5, VR2,
IC7)
• Replace defective components
Squelch does not
work
• Defective VR2, RV5, IC3-D.
• Replace defective components.
• Re-adjust Squelch sensitivity
No modulation
• Defective microphone.
• Poor audio output and defective IC3.
• Defective microphone connector components.
• Defective ALC circuit D8, D9
• Replace defective components.
• Re-adjust Tx modulation
LCD meter does
not work
• Defective D11, D22
• Defective IC5.
• Defective RV2. RV6
• Replace defective components.
• Re-adjust RX meter display
LCD display does
not work
• Defective RED wire fuse.
• Defective LCD module
• Replace defective components.
Channel selector
does not work
• Defective IC5. • Replace defective components.
EMG CH9.19 does
not work
• Defective IC5. • Replace defective components.
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