Cobra MR HH475 FLT BT/EU, Service Manual

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then is amplified by Q2 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 volume control VR3 and finally amplified by Audio amplifier U11
(NJM2070) and heard in the speaker.
The squelch control is also controlled by U1 (BA4116). The second IF passes through U1 (BA4116)
internal squelch control R90, C141, C142 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 R148, R149 is input to U7 Pin 11 for voltage level sample.
Voice recorder
Voice recorder consists of U5(4/4) LM324,U9(AT4SDBD41B)and MCU U7.Voice source from AF via
U5(4/4) LM324 amplifies then pass to MCU U7 pin 13,or voice signal from MIC via U4(1/4,2/4,3/4) then
pass to MCU U14,they will be transformed to voice data up to 20 seconds, U9(AT4SDBD41B) stores these
voice data. When recorder play, the voice come out form MCU U7 Pin43. Then pass to U4(4/4), after that
pass to U11(NJM2070) to amplify the recorder voice signal.
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 X1 20.95MHz and
U6 (KB8825). A phase Lock Loop (PLL) U6 (KB8825), TX VCO Q14 and RX VCO Q13. 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 U6 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
U9 is an EEPROM AT4SDB41B which 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 U7.
Bluetooth
Bluetooth circuit consists of U13(BT_MODULE)and CPU U7.When Bluetooth transmitting the voice from
U13 Pin7 and 5 input, then via U7 controls and U13 transforms to Bluetooth RF signal then radiates out by
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