TTI Freequency TCB-770, Service Manual

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4-5-2. Receiver
Overload protection is provided to receiver’s semiconductors by diode D101. These diodes have
a little effect on the incoming signal from another CB station; they protect the receiver from stray
transmitter energy.
In the receiver mode of operation, Q923 transistor is turned on. Also bias voltage is applied to
Q101, Q102, Q103, Q106.
Q101 is a 27MHz RF input amplifier, and any excessive input signal is limited by diode D101.
CB receiver is dual conversion super-heterodyne type with the first IF 10.695MHz and the
second IF 455KHz.
Receiver is separated to blocks, 1’st IF section and 2’nd IF section. The PLL synthesizer
supplies 1’st local frequency 16.270MHz ~ 16.710MHz. The amplified 27MHz is mixed. With the
provide 1’st local frequencies Q102, Q103 mixes the incoming RF signal to generate 1’st IF signal. The
resulting first IF is 10.695MHz. Q102 and Q103 is the first converter, and 10.695MHz is sharply filtered
by L108 and crystal filter CF101. The first IF is again mixed with a second local oscillator of 10.24MHz.
With the 10.240MHz signal, IC101 FM IF IC converts the incoming signals to generate 2’nd IF
signal and recovered audio signals. 2’nd IF is filtered by a razor sharp ceramic filter CF101 coupled. The
455KHz signal from the 2’nd IF filter was amplified and limits internally. After amplification the signals
fed to the quardrature detector loop L109. Then could see the recovered signals Pin 11 of IC101.
With the amplitude of recovered signals, Q151 serves as an amplifier.
AM signals from the Pin 6 of IC101 were tapped with C161 and amplified two-stage amplifier
Q161, Q162. Q161 is a first 455KHz amplifier, and the Q161 being the last amplifier. D106 is a detector
diode witch produce audio signal as well as negative DC voltage for AGC action. The negative voltage
also provides forward biasing to the emitter of ANL clipping transistor Q164. The biasing voltage has a
time constant determined R174 and C176. Therefore any sharp negative going pulse from D161 will back
bias Q164 and clipped. The way to recover the AM information audio is envelope detector.