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Cobra CB 19 DX4 EU Service Manual

CB 29 LX EU

Made by: Cobra
Type: Service Manual
Category: Radio
Pages: 14
Size: 0.62 MB

 

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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…………………………………………….…1

st

 IF: 10.695 MHz, 2

nd

 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   

 
 
 
 
 
 
 

 


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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. 

 


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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, 1

st

 IF section and 2

nd

 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. 

 

 


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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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Model No: 19 DX4 EU 

Cobra                                                                                                                                                                                                          Rev.: A 

Page: 6 / 13 

6

 

TEST SETUP 

 

A. TX test setup: 

 

 

      Spectrum                                                                                            Power 
   Analyzer                                              Supply 

                                                                                                                                                             
                           
             
      Modulation                Wattmeter                     Radio 
                Meter                                        10W                                                U.U.T. 
 
 
 
          Oscilloscope                                  Frequency                                  Audio Gen. 
                                 Counter 
 

 

 
B. RX test setup: 
 
 
            SINAD Meter                                  RF Signal                                  Power 
                                  Generator                 Supply 
 
 
 
 
            Distortion                                          8 ohm                                      Radio                               
                Meter                                              load                                        U.U.T.                             
 
 
   
           
      Oscilloscope                  
                                                                                           
 
 
 

Note:

 U.U.T. = device under test 

 
   

Filter with 
attenuation

 


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Model No: 19 DX4 EU 

Cobra                                                                                                                                                                                                          Rev.: A 

Page: 7 / 13 

7

 

ALIGNMENT PROCEDURE 

 

VCO Voltage 

1.

 

Test point TP1 connect to the multi-meter

set UUT enter test mode. Use channel “UP” key to select CH41 

(26.565MHz) of DE Band in FM mode. 

2.

 

Adjust IFT L17 for 1.0 

±

 0.1V on the Multi-meter read

and then checking CH40 (27.99125MHz) of UK Band 

should be less than 3.8V. 

 

RX Alignment 
Audio Level 

1.

 

UUT set to FM mode CH19

output of RF signal generator connect to TP2. Audio dummy load connect to 

EXT. Speaker. (@8

Ω

 load) 

2.

 

Set RF S.S.G Freq.: 10.695MHz

Mod.: 

±

1.2KHz

Fmod.: 1KHz, Output level:1mV. 

3.

 

Squelch Volume set to minimum position, volume control set to around middle position. 

4.

 

Adjust IFT L20 for maximum audio output amplitude. 

 

Sensitivity 

1.

 

UUT set to FM mode CH22, output of RF signal generator connect to TP5 (Antenna). 

2.

 

Set RF S.S.G Freq.: 27.225MHz, Mod.: 

±

1.2KHz

Fmod.: 1KHz, Output level:0.5uV. 

3.

 

Adjust the volume control for 2V output on distortion meter read. 

4.

 

Adjust L2 & L3 for more than 12dB at SINAD meter.   

 
T-Squelch Sensitivity 

1.

 

UUT set to AM CH22, output of RF signal generator to the antenna input terminal. 

2.

 

Set RF S.S.G Freq.: 27.225MHz

Mod.: 60%

Fmod.: 1KHz, Output level: -47dBm. 

3.

 

Volume control and Squelch control set to maximum position. 

4.

 

Adjust the RV5 to mute the speaker, then slowly turn RV5 to a position that the audio output on the speaker 

just appears from no output. Turn the RF S.S.G. output level to -57dBm, the audio must be off from the 

speaker. 

 

RX Signal Meter Display 

1.

 

UUT set to AM CH22

output of RF signal generator to antenna input terminal. RF Gain volume at maximum. 

Power switch turn on. 

2.

 

Turn the RF S.S.G output for 100uV without modulation. 

3.

 

Adjust RV6 for 9 characters displayed at the RX signal strength of the LCD. 

4.

 

Turn the RF S.S.G level output increase 30dB, the RX signal strength should up to +30 position. 

 

 

 


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Model No: 19 DX4 EU 

Cobra                                                                                                                                                                                                          Rev.: A 

Page: 8 / 13 

8

 
 

TX Alignment 

TX Output Power and Frequency 

1.

 

UUT set to FM EU CH19, Antenna connect to the RF power-meter input terminal.                              

2.

 

Set UUT to TX mode, adjust the IFT L8 and L9 for TX power output maximum, and then adjust the RV4 

check the output TX power for 3.5W. 

3.

 

Change to channel to EU CH19, Adjust the trimmer capacitor CT1 to 27.185000 +/- 500Hz on frequency 

counter. 

 

TX Modulation Deviation 

1.

 

UUT set to FM EU CH19, input of power-meter to antenna input terminal. Couple to MIC socket from the 

audio frequency signal generator output terminal.   

2.

 

Set audio frequency signal generator @ FREQ.: 1000Hz, Output level: 50mVrms. 

3.

 

Set UUT to TX mode, adjust RV1 for 1.5KHz deviation on modulation meter. 

4.

 

Set UUT to AM mode; Set audio frequency signal generator @ FREQ.: 1000Hz, Output level: 50mVrms.             

5.

 

Set UUT to TX mode, adjust RV3 for 65% deviation on modulation meter. 

 

 

 

 

 
 
 
 
 

 

 


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Model No: 19 DX4 EU 

Cobra                                                                                                                                                                                                          Rev.: A 

Page: 9 / 13 

9

 

VOLTAGE CHART 

 

Transistor 

Pin  

TX  

RX  

Transistor 

Pin  

TX  

RX 

Q2 E 

 0  0 

Q32 

E  

0 0 

 

C  

0.8 

 

C  

6.0 

 

B  

6.7 

 

B  

3.5 

Q2  

E  

Q37 

G  

 

C  

0.8 

 

S  

0.5 

7.9 

 

B  

6.7 

 

D  

2.8 

Q14 

E  

1.2 

Q40 

G  

0.7 

0.9 

 

C  

8.0 

 

S  

 

B  

2.0 

 

D  

6.1 

Q16 E 

 0 

 

   

 

 C 

 

8.5 

3.0 

 

   

 

B  

1.0 

 

 

 

 

Q17 E 

 0 

 

   

 

 C 

 

13.2 

2.0 

 

   

 

B  

13.0 

 

 

 

 

Q18 E 

 0  7.3 

 

   

 

 C 

 

0  0 

 

   

 

B  

0.5 

7.9 

 

 

 

 

Q19 E 

 

8.1 8.10 

 

   

 

 C 

 

7.9 0 

 

   

 

B  

7.3 

7.9 

 

 

 

 

Q28 E 

 

6.1  6.1 

 

   

 

 C 

 

8.1 

8.1 

 

   

 

B  

6.8 

6.8 

 

 

 

 

 

Diode 

Pin  

TX  

RX  

Diode 

Pin  

TX  

RX  

D1 

A  

0.5 

7.1 

DZ1 

A  

0.4 

0.4 

 

K  

6.1 

 

K  

8.7 

8.7 

D2 

A  

0.5 

7.1 

D16 

A  

2.5 

 

K  

6.1 

 

K  

2.2 

2.2 

D5 

A  

D19 

A  

7.9 

 K 

 

0 0   K 

 

7.3 0 

D6 A 

 

10.6 2.8 

 

 

 

 

 K 

 

9.7 

2.8         

D10 A 

 0 

 

 

 

 

 K 

 

12.8 

13.2        

D14 A 

 

7.9  0 

 

 

 

 

 K 

 

7.2 

0.6         

 

 


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Model No: 19 DX4 EU 

Cobra                                                                                                                                                                                                          Rev.: A 

Page: 10 / 13 

10

 
 

IC4  

PIN  

TX  

RX  

PIN  

TX  

RX  

1  

4.2 

4.2 

9  

3.6 

3.6 

2  

4.7 

4.7 

10  

4.0 

4.0 

3  

1.0 

1.7 

11  

4.5 

4.5 

4  

12  

0.7 

0.7 

5  

5.0 

5.0 

13  

6  

14  

7  

15  

4.7 

4.7 

8  

16  

4.1 

4.1 

 

IC3  

PIN  

TX  

RX  

PIN  

TX  

RX  

1  

3.2 

3.2 

8  

1.8 

2  

3.2 

3.2 

9  

1.8 

3  

3.2 

3.2 

10  

1.8 

4  

7.9 

7.9 

11  

5  

3.7 

3.7 

12  

6  

3.9 

3.9 

13  

1.7 

7  

3.9 

3.9 

14  

 

IC8  

PIN  

TX  

RX  

PIN  

TX  

RX  

1  

1.0 

1.0 

6.3 

6.3 

2  

0.8 

0.8 

13.2 

13.2 

3 0 0 

 

 

 

 

IC6 PIN  

TX  

RX  

PIN  

TX  

RX  

1  

5  

5  

3  

13.2 

13.2 

2  

0  

0  

 


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Model No: 19 DX4 EU 

Cobra                                                                                                                                                                                                          Rev.: A 

Page: 11 / 13 

11

 

IC7  

PIN  

TX  

RX  

PIN  

TX  

RX  

1  

5.0 

1.6 

2  

4.5 

10 

3  

4.5 

11 

5.0 

4  

4.5 

12 

5  

4.0 

13 

1.8 

6  

4.0 

14 

7 0 4.0 15 0  0 
8  

5.0 

16 

1.8 

 

IC5  

PIN  

TX  

RX  

PIN  

TX  

RX  

1  

33 

3.0 

3.0 

 1.4 1.4 34  3.5 3.5 

 2.3 2.3 35  3.5 3.5 

 

0 0 36 0 0 

 4.9 4.9 37  1.2 1.2 

 0.1 0.1 38  1.2 1.2 

 

0 0 39 0 0 

 4.2 4.2 40  1.2 1.2 

 3.2 3.2 41  1.5 1.5 

10 

 

5.0 5.0 42  1.5 1.5 

11  

43 

1.5 

1.5 

12  

44 

2.6 

2.6 

13  

45 

2.6 

2.6 

14  

4.9 

46 

2.6 

2.6 

15  

47 

2.6 

2.6 

16 

 

5.0 5.0 48  2.6 2.6 

17 

 

5.0 5.0 49  2.6 2.6 

18 

 

5.0 5.0 50  2.6 2.6 

19 

 

5.0 5.0 51  2.6 2.6 

20  

52 

2.6 

2.6 

21  

53 

2.6 

2.6 

22 

 

5.0 5.0 54  2.6 2.6 

23  

2.0 

55 

2.6 

2.6 

24  

56 

4.5 

4.5 

25  

57 

3.2 

3.2 

26 

 

4.8 4.8 58  1.8 1.8 

27 

 

0.3 4.8 59  1.8 1.8 

28  

0.4 

60 

2.6 

2.6 

29  

61 

0.1 

0.1 

30 

 

0 0 62 0 0 

31  

3.5 

63 

32  

3.5 

3.5 

64 

 
 

 

 


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