Theory Of Operations - Cobra CB 19 DX4 EU Service Manual

Model No: 19 DX4 EU
Cobra Rev.: A

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.
st nd
Receiver is separated into two blocks, 1 IF section and 2 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
nd
ceramic filter). The 455 KHz signal from the 2 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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