Cobra MR HH425 LI VP, Serivce Manual

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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
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