Control Board; Theory Of Operation - Pyramid SVR-200 LA Service Manual

Receiver:
The receiver is a double-conversion superheterodyne type, designed for narrow band FM reception. The first
local oscillator is derived from the frequency synthesizer. The second LO is crystal controlled.
RF Stage: The incoming RF signal from the antenna jack is directed to the first RF bandpass filter to improve
selectivity and then to the input of the RF amplifier. The output of the RF amplifier is then presented to a second
bandpass filter.
First LO/Mixer: The first LO signal is developed by the synthesizer and is mixed with the incoming signal to
produce the first IF frequency (70 MHz high side injection). The IF frequency is filtered by FL1A and FL1B and
amplified by the IF amplifier before being presented to the second LO/Mixer IC.
Second LO/Mixer: The first I.F. signal is presented to the second IF IC (MC3371) which performs the functions
of second LO, second I.F. amp and mixer, FM discriminator and squelch. The second LO crystal (69.545 MHz)
is mixed with the 70 MHz first I.F. signal to produce the second I.F. frequency of 455 kHz. A 6 pole ceramic filter
provides selectivity for the 455 kHz signal.
Detector/Squelch: The MC3371 demodulates the 455 kHz signal via quadrature coil to produce the audio and noise
components. The output of the MC3371 is the recovered audio and the RSSI voltage (receiver signal strength
indicator) which is compared by the controller board with a threshold voltage level for squelch setting.
Transmitter
The output of the VCO is buffered by U5 and amplified by U6. The output of U6 drives the RF driver Q6.
The collector of Q6 is fed by the transmit 9V line from Q5. The final amp Q7 is a class C power amplifier and
drives the output lowpass and harmonic filte. U8 is the antenna switch. RF output power is controlled by changing
the bias on the gate of Q7 via the TX 9V line (pin 3) from the controller PCB.

Control Board

Power Supply: DC power comes from the mobile radio via P1 pins 1 and 5. Fuse F1 and MOV VAR1 provide
over current and voltage spike protection. Q3 is the remote enable/disable pass switch, controlled by Q1 and Q2
via P1 pin 3. Q3 output is switched 12VDC and is presented to audio amp U6, and voltage regulators U7 and U8.
Bias voltage for the op-amp circuits is provided by voltage divider R68, R69 and buffer amp U2A.
Transmit audio path: Receiver audio from the mobile is input to the mic amp portion of U4; PC programming of
the SVR-200 provides flat response or +6db/octave pre-emphasis. The output of the mic amp is internally connected
to the limiter and lowpass filter. When a condition to repeat exists (base-to-portable) U4 audio is switched on and
audio is presented to amplifier/limiter and lowpass filter to remove audio components above 3kHz. U4 provides
-48db/octave of attenuation to out of band signals. Transmit audio is output on pin 22 of U4 and passes through
the final lowpass filter U2C to remove any clock noise generated by U4's switched capacitive filters before being
presented to the RF module on P2 pin 6.
SVR-200 Service Manual

Theory of Operation

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