Ramsey Electronics FR6 - Manual (Page 5)

FR6 • 5
BUILDING YOUR FR6 KIT
There are just over 200 solder connections on the FR6 printed circuit board.
That means your work could be 99% excellent and you could STILL have 2
or 3 cold solder points or solder bridges. Since this circuit is more
sophisticated than a direct-conversion HF receiver or a simple transmitter, a
beginner or casual amateur could have a harder time tracing a problem due
to a poor solder connection. Therefore, PLEASE take us seriously when we
say that good soldering is essential to the proper operation of your receiver!
ˆ
Use a 25-watt soldering pencil with a clean, sharp tip.
ˆ Use only rosin-core solder intended for electronics use.
ˆ Use bright lighting. A magnifying lamp or bench-style magnifier may be
helpful.
ˆ Do your work in stages, taking breaks to check your work.
ˆ Carefully brush away wire cuttings so they don't lodge between solder
connections.
FR6 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 single-package (8-pin DIP) mixer-
product detector-oscillator. The tuneable oscillator section of the SA602 is
aligned to operate at 10.7 MHz higher than the signal fed and amplified by
Q1. For example, to receive 50-54 MHz signals, the oscillator must tune 60.7
to 64.7 MHz 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