Ramsey Electronics FR220, Instruction Manual

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FR-220
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5
BUILDING YOUR FR220 KIT
There are just over 200 solder connections on the FR220 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.
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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.
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Carefully brush away wire cuttings so they don't lodge between
solder connections.
FR220 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 tunable 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 222-227 MHz signals, the
oscillator must tune 232.7 to 237.7 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.