Use bright lighting. A magnifying lamp or
Do your work in stages, taking breaks to check your work.
Carefully brush away wire cuttings so they don't lodge between solder connections.
FR146 CIRCUIT DESCRIPTION
VHF signals from the antenna are amplified through the tuned input circuit (L1, C3 and L2,C4) by Q1, a microwave bipolar transistor. Q1's output is fed to the input of the SA602 IC, an efficient
158.7MHz in order for the SA602's mixing capability to produce a steady 10.7 MHz output signal to the rest of the circuit. The oscillator frequency is determined by L3 and its associated capacitors, and varied by the varactor tuning network using D1 and varied by R1.
The output from pin 4 of the SA602 passes through a ceramic 10.7 MHz filter, amplified by transistor Q2 and applied to input pin 18 of U2.
Q3 provides AFC (Automatic Frequency Control) by keeping the local oscillator of U1 from drifting away from an incoming signal. This is accomplished by tuning the varactor circuit in the direction opposite the drift.
The MC3359 IC has an internal oscillator controlled by the 10.24 MHz crystal. The 10.24 MHz signal is mixed with the 10.7 MHz input from U1 to convert down to the low IF of 450 KHz. The 450 KHz IF is filtered by FL2 and then amplified by a limiting amplifier in U2. Audio demodulation takes place in the quadrature detector, with L4 adjusting the detector.
The audio modulation is detected in U2, and the background noise is used to control the squelch. U3 amplifies the audio output from pin 10 of U2 to a practical level for speaker or headphone operation. Pin 16 of U2 will ground the input of U3 when the squelch is closed.
L4, a 450 KHz IF coil, permits adjustment of the
FR146 • 5
