Cobra MR HH425 LI VP Instruction manual

Page
Specifications .................................................................................……………………….………...……2
Theory of Operation .………………...................................................………..........……………….....….4
Block Diagram .............................................................................……...…………………………………7
Disassembly Instructions ...........................................................……….........………………………...…8
Alignment Procedures ...……………...........................................…….................…...…………………9
Adjustment Procedure …….....…………...................................................................…………………10
Wiring Diagram ……......................................................................…………..…………………….……12
Printed Circuit Board ....................................................................……………....……………………...13
Troubleshooting …...................................................................................………...……………………16
Exploded View ......................................................................…………………………..………………..18
Mechanical Parts List .....................................................................…………………….…………….…21
Electrical & Packing & Other Parts List ..............................................................…………………...….23
IC Lead Identification and Internal Diagram ............………………………………………........………..46
Transistor Lead Identification …………………………………………………………….……...……….54
Voltage Chart ...............................................................................………………….……………...……56
Schematic Diagram..........................................................................………………………………...….63
Version:01 Date: Jan 03,2008

Measurement Conditions
Frequency range:transmit MHz 156.025 To 157.425
Frequency range:receive MHz 156.050 To 162.025
Number of channels 59 INT Channels
57 USA Channels
57 Canada Channels
Memory channels All marine channels
Oscillate mode PLL
Modulation FM(16K0G3E)
Channel spacing KHz 25
Frequency stabllity PPM ±10
Stantard operation temperature °C -20 To+55
Normal working voltage V 7.2 (With 6 AA Ni MH Battery)
Low limit working voltage V 6.3
Battery lifetime(Tx5%/Rx5%/Standby90%) Hour 12.0 ( 7.2V / 1500mAH )
Memory retented EEPROM
Antenna socket TNC
Display LCD With Backlight
Built-ln speaker Diamter 40mm / Impedance 8 Ohm
Dimension: 64mm(W)×135mm(H)×38.5mm(D)
Weight (without battery): 250g
RECEIVER UNIT LIMIT
Sensitivity for 12dB sinan dBm ≤-118
Squelch
a) squelch threshold dBm ≤-120
b) squelch tight dBm -113 ~ -107
c) hysteresis ( tight ) dB 2 ~ 7
Rated audio output
@5% Thd speaker mW ≥250
Deviation sens.for rated output (VR @ max.) ±kHz ≤2.5
Max.S/N ratio @ 1mV dB ≥40
Audio frequency resp.
@300HZ dB 9 ± 3
@2500HZ dB -11 ± 3
Spurious response rejection 10MHz to 1GHz
SPECIFICATIONS
2

78.4MHz dB ≥46
39.2/ 146.8/ 154.4/ 155.9/
157.8/ 158.2/ 163.9/ 249.4MHz dB ≥50
Other frequency dB ≥70
Adjacent ch.rejection dB ≥55
Image rejection dB ≥65
Intermod rejection ( 3-Gen.Method ) dB ≥60
Scan time. per channel ms ≤200
Speaker output:8Ωload @ max vol. mW ≥400
StandBy current mA ≤50
Receiver current @ max vol. mA ≤200
Charging current mA 180 mA ± 50
TRANSMITTER UNIT LIMIT
Carrier power(no mod)
Hi mode W 5±0.8
Mid mode W 3±0.5
Lo mode W 0.8±0.2
Carrier frequency tolerance ppm ± 10
Modulation limiting ±kHz 4.3~ 4.9
Modulation sens.for ±3 kHz dev. mV 7±3
Audio frequency response (1KHz/0dB ref )
@300HZ dB -12 ± 3
@2500HZ dB +6 ± 3
Audio distortion @ 2.5 khz dew. %≤5
Hum and noise ratio dB ≥35
Spurious emission dBm ≤- 36
Current drain (TX)
Hi mode A≤2
Mid mode A≤1.5
Lo mode A≤0.9
Mic modulation sen.(at sound level of 94 dBA ) ±kHz ≥1.5
Mic modulation limiting(at sound level of 110 dBA ) ±kHz 4.0 ~ 4.9
Note: All units should be able to approximate these specs. Some will exceed these specs, and others
may drop slightly below them. Limit specs represent the absolute worst condition that might still be
considered acceptable. A unit never fail to meet limit specs.
3

THEORY OF OPERATION
This is a PLL - controlled VHF and UHF dual band marine transceiver provides an accurate and stable
multi-channel operation.
The transceiver consists of 8 main sections:
 Transmitter
 Receiver
 Low voltage detection
 Vibration Alert
 Voice recorder
 VOX
 Local oscillator PLL (Phase Lock Loop) Circuit
 Memory backup
 Transmitter
The audio is picked up from the internal MIC, the audio signal is then amplified by Audio Amplifier, U9
LM324 (4/4), U9 (1/4) and filtered by a low pass filter U9 (2/4). The audio is divide into two paths VHF and
UHF.
UHF
The audio is adjusted with VR2 to obtain a suitable Audio frequency response, and then modulated with
the carrier by VCO, through Q13. When the unit is transmitting, the audio signal is added to the TX VCO
Q13. The capacitance of Cbc varied following the audio signal and when mixed with the carrier to form the
modulated signal. The modulated signal output from the VCO is pre-amplified by Q11, D14, Q10, Q9 and
D5. Then it is amplified by Q2, Q1 .The amplified signal then passes through a match network that
consists of C79, L3, and the antenna switching circuit D1, L37, D7. The signal is filtered by another
low-pass filter circuit which consists of C55, L10, C59, C63, L2, C58, C1, L1 and C54. These low pass
filters are necessary to suppress the second and third harmonics. The signal is then fed into the antenna
input and radiated out.
VHF
The audio is adjusted with VR3 to obtain a suitable Audio frequency response, and then modulated
with the carrier by VCO, through D15. When the unit is transmitting, the audio signal is added to the
TX VCO D15. The capacitance of D15 varied following the audio signal and when mixed with the
carrier to form the modulated signal. The modulated signal output from the VCO is pre-amplified by
Q12, D12, Q5, Q4 and D5. Then it is amplified by Q2, Q1 .The amplified signal then passes through a
low-pass network that consists of L14, C77, and the antenna switching circuit D3, L16, D4. The signal
4

is filtered by another low-pass filter circuit which consists of C91, L13, C84, C80, L12, C83, C70, L11
and C67. These low pass filters are necessary to suppress the second and third harmonics. The
signal is then fed into the antenna input and radiated out. TX current converts into a voltage signal,
then fed into U3 LM2904 (1/2) for amplifying. After that the signal passes to U3 (2/2) to control the TX
power.
 Receiver
The receiver uses a double frequency super-heterodyne circuit. The first Immediate Frequency (IF) is 21.4
MHz and the second is 450 kHz.
UHF
The RF signal is received by the antenna, and passes through a low-pass filter network L1, C54, C1, L2,
C58, C63, L10, C59, C55, and L37 to filter out the unwanted signals, the antenna switching circuit D7
switching circuit to receive. Then passes through a hi-pass filter network C164, L38, C165 to filter
out-of-band signals. The RF amplifier Q7 amplifies signals within the range of the frequency coverage
and passes though a saw filter F6 to further filter out-of-band signals. The RF signal then is mixed with the
local oscillation frequency by the mixer Q8. A first IF (Immediate Frequency) 21.4 MHz is produced.
The IF is passed through a band switch D13 and a pair of crystal filter F3 (1/2), F4 (2/2) to further filter
other unwanted signals.
VHF
The RF signal is received by the antenna, and passes through a low-pass filter network L11, C67, C70,
L12, C83, C80, L13, C84, C91, L16, C108 to filter out the unwanted signals, the antenna switching circuit
D4 switching circuit to receive. Then passes through a match network C85, L19 of F1. F1 filters the
unwanted signals. The RF amplifier Q3 amplifies signals within the range of the frequency coverage and
passes though a saw filter F2 to further filter out-of-band signals. The RF signal then is mixed with the
local oscillation frequency by the mixer Q15. A first IF (Immediate Frequency) 21.4 MHz is produced.
The IF is passed through a band switch D13 and a pair of crystal filter F3 (1/2), F4 (2/2) to further filter
other unwanted signals.
The first IF then is amplified by Q6 and the IF amplifier U1 (BA4116). U1 is a integrated RF amplifier
which consists of a local oscillator, a demodulator, a second mixer, squelch control circuit, and RF
amplifier. The 21.4 MHz IF then is mixed here with second mixer and converted into 2nd Immediate
Frequency (IF) 450 kHz. The 2nd IF passes through a ceramic filter F5 to filter out the residue unwanted
signal at pin 5 of U1 (BA4116) output this final IF signal and the Audio signal is output at pin 9 of U1
(BA4116). The audio signal is fed through a 300Hz hi-pass U8 (LM324), after that passes to a switch U13
(NJM4053). When switch U13 passes audio to volume control VR4, the audio finally amplified by Audio
amplifier U14 (NJM2070) and heard in the speaker. The squelch control is also controlled by U1 (BA4116).
5

The second IF passes through U1 (BA4116) internal squelch control R166, C138, C139 form as a squelch
amplifier. The ceramic filter produces a squelch signal (RF noise). Pin 14 of U1 sends the digital squelch
control signal to the CPU mute the audio speaker path. Pin 12 of U1 output a RSSI level to the CPU.
 Low Voltage Detection
The battery voltage, divided by R221, R222 is input to U7 Pin 6 for voltage level sample.
 Vibration Alert
When vibration alert is on, Pin54 of U7 output HI level, Q23 switch on, M1 MOTORS vibrates.
 Voice recorder
Voice recorder consists of U13 NJM4053 and U402 ISD1730. U13 Pin14 select voice source from P13
(MIC) or P12 (RX voice). The voice signal comes to U402 Pin9. U402 store the voice in it up to 20 seconds.
When U402 play, the voice come out form U402 Pin17. Then pass to U13 Pin15, after that pass to Pin1
(speaker) or Pin2 (TX).
 VOX
The audio is picked up from the internal MIC, the audio signal is then amplified by Audio Amplifier, U9
LM324 (4/4), U9 (1/4) and passes to D25. D25 convert audio to a DC signal, then passes to U5 Pin1 DAC
input. There is another VOX path which consist of Q30, U7 (2/4), U7 (3/4). This is a 300Hz low-pass filter.
After that the audio passes to U5 Pin2 DAC input. Two VOX signals let the VOX works well in a noisy
place.
 PLL (Phase Lock Loop) Circuit:
The receiver and transmitter both share the same PLL (Phase Lock Loop) Circuit to produce the carrier or
the receive frequency. The local oscillator consists of a fundamental frequency oscillator X2 20.95MHz and
U2 (KB8825). A phase Lock Loop (PLL) U6 (KB8825), VHF TX VCO Q20, VHF RX VCO Q16 and UHF
VCO Q13, Q14. The fundamental frequency is frequency divided by U6 and a 12.5 kHz signal is produced.
When the VCO frequency applied to and frequency divided by U2 produces a frequency comparable to
12.5 kHz, PLL will control the VCO. When these two frequencies are matched, a constant control voltage
is output from PLL to lock VCO in desired frequency. The PLL also will output a lock indication to MCU to
indicate the PLL is in frequency lock state.
 Memory Backup
U11 is an EEPROM AT24 C16 that acts as a memory backup for the working channel code and the
system parameters. Every time when the unit is switched on, the MCU will reset the system, clear the
RAM, and recall in the memory from the EEPROM to refresh the RAM in CPU U5.
6

BLOCK DIAGRAM
BACKLIGHT
BACKLIGHT
7

DISASSEMBLY INSTRUCTIONS
To remove the front and rear panels from the main chassis:
1. Remove the rubber antenna, SPK/MIC knob and the VOL/SQL knob from the main unit.
2. Remove 4 screws from the rear housing of main unit.
3. Detach the die cast from the front housing.
1
2
3
8

ALIGNMENT AND PROCEDURES
This unit has been fully aligned at the factory before shipment and does not normally require further adjustment.
When necessary, however, the unit may be aligned as indicated below.
Do not adjust any circuit in this radiotelephone unless you understand the circuit operation and have
experience in adjusting the radiotelephone. Tampering with the radiotelephone may upset the alignment and
lower its performance.
The test equipment listed below are used for the test setup shown in Fig. 3.1.
This test setup used either partially or totally during the following adjustments.
Test Equipment
1) DC Power Supply (7.2V DC) 0 - 15V 3A max.
2) RF Power Meter 10 W 50 Ohm 100-200 MHz
3) RF Signal Generator 100-200 MHz, 50 ohm termination
4) FM Linear Detector (FMLD) 100-200 MHz
5) Frequency Counter 1-500 MHz
6) Oscilloscope 20 MHz
7) Distortion Meter
8) Sinadder
9) Audio Oscillator
10) Toggle Switch (for use as PTT switch).
Fig. 3.1
RF POWER
METER
DC POWER
SUPPLY
AUDIO OSC
PTT
SWITCH
UNIT
UNDER TEST
OSCILLOSCOPE
SINADDER
FMLD
FREQ
COUNTER
DISTORT
METER
RF SG
SPEAKER
JACK
MIC
ANT
RX TX
COUPLER
9

ADJUSTMENT PROCEDURE
Step Adjustment Test Point Procedure
1 L44 VHF Receive TP1
1. Connect digital voltmeter to TP1 on RF PCB.
2. Set VHF CH01.
3. Adjust L44.
4. TP1 voltage 1.2~1.6V DC.
2 L46 VHF Transmit TP1
1. Connect a digital voltmeter to TP1 on RF PCB.
2. Set VHF CH01.
3. Adjust L46.
4. TP1 voltage 1.2~1.6V DC.
3 L45 UHF Receiver TP2
1. Connect a digital voltmeter to TP2 on RF PCB.
2. Set UHF CH01.
3. Adjust L45.
4. TP1 voltage 1.2~1.6V DC.
4 VC1
1. Connect the antenna coupler output to a frequency counter.
2. Set channel to VHF CH01 (156.050 MHz).
3. Adjust VC1 to obtain a frequency reading 156.050 MHz±200Hz.
5 VR2 UHF
Modulation
1. Connect the antenna coupler output to an FM linear detector.
2. Connect Audio Oscillator to Microphone Jack.
3. Set unit to transmit mode.
4. Set audio oscillator output to -23dBm 1 kHz.
5. Adjust VR2 to obtain ±2.3 kHz deviation.
6 VR3 VHF
Modulation
1. Connect the antenna coupler output to an FM linear detector.
2. Connect Audio Oscillator to Microphone Jack.
3. Set unit to transmit mode.
4. Set audio oscillator output to -23dBm 1 kHz.
5. Adjust VR3 to obtain ±4.5 kHz deviation.
10

Main PCB
Top View Bottom View
VR3
VR2
VC1
L44
L46
SQL
L45
11

WIRING DIAGRAM
CHARGER+ CHARGER-
12

Main PCB Top View
PRINTED CIRCUIT BOARD
SQL
13

Main PCB Bottom View
PRINTED CIRCUIT BOARD
14

Charger PCB
PRINTED CIRCUIT BOARD
Top View
Bottom View
LCD PCB
Bottom View
Top View
15

TROUBLE SHOOTING
Item Symptom Possible Cause
1 Unit does not turn on.
 Defective power switches VR4.
 Check the battery voltage.
 Defective regulator U12.
2
Speaker no sound with
AF signal applied to
volume control
 Defective volume control.
 Defective speaker.
 Defective U14 and/or associated components.
3 Squelch circuit inoperative  Check squelches control.
 Defective U1 and/or associated circuitry between pin 9, 10 and 11.
4 UHF&VHF No receive (RX)
 Defective regulator U12.
 Defective regulator U14
 Check second OSC 20.95MHz, pin9 of U2;
 Defective Q6.
 Check U1 audio output voltage at pin 9.
 Defective F1 and F2;
5 Only UHF No receive (RX)
 Check TP2 0.5 – 4.0V
 Defective Q8, Q7.
 Defective F6
6 Only VHF No receive (RX)
 Check TP1 0.5 – 4.0V
 Defective Q15, Q3.
 Defective F1, F2
7 UHF&VHF Low receiver
sensitivity
 Check antenna and connector for possible corrosion or bad connection.
 Defective F3, F4
 Defective U1
 Check the output level of local OSC.
8 Only UHF Low receiver
sensitivity
 Check antenna is a dual band type;
 Failure of the output from Q7, Q8;
 Defective F6;
9 Only VHF Low receiver
sensitivity
 Check antenna is a dual band type;
 Failure of the output from Q3, Q15;
 Defective F1, F2;
10 UHF& VHF No transmit
(TX)
 Defective PTT switch.
 Defective regulator U12.
 Check power transmits circuit Q1, Q2;
 Check power control circuit Q33, Q38;
16

11 Only UHF No transmit (TX)  Check TP2 voltage 0.5 – 4V;
 Defective D14, Q10, Q9;
12 Only VHF No transmit (TX)  Check TP1 voltage 0.5 – 4V;
 Defective D12, Q5, Q4;
13 Poor or no modulation
 Defective microphone.
 Defective U9 and/or its associated components.
 Defective VR2, VR3
14 Deviation of transmit
frequency  Check crystal X2 and VC1.
17

MAIN UNIT EXPLODED VIEW
18

CHARGER UNIT EXPLODED VIEW
19

BATTERY TRAY UNIT EXPLODED VIEW
20