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The Ramsey FR-series FM receivers are among our most widely-used
and popular kits. The circuit itself, assembly and use are "cut-and-dried."
If your receiver does not work, there is probably either an error in
assembly or a defective part. Following is some help in case you
experience difficulty.
Problem: Receiver completely "dead"- no sound at all.
Check the following:
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First, the cable or plug adapter leading to speaker or phones!
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Battery condition and battery wire polarity and soldering at S1.
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Connections associated with U3, the LM386 audio output IC.
Problem: Receiver "hisses", but NO signals are heard, not even a signal
generator 2 inches away.
First, you need to verify that the local oscillator (NE602, L3, etc.) is
operating. Use another VHF receiver or a frequency counter to verify
oscillator operation. Next, make sure that the 10.24 MHz crystal oscillator
is running by tuning a general purpose HF ("shortwave") receiver to that
frequency. If either oscillator is not running, re-check correctness of parts
and solder joints associated with both oscillator circuits. Further signal-
path tracing may be necessary to find your problem.
Problem: Very weak signals
If Q1 is defective or incorrectly installed, very little signal energy will
reach the NE602. If Q2 is defective or installed incorrectly, very little of
the NE602's 10.7 MHz output is getting to the MC3359. Also, be sure that
L4 is peaked correctly.
Problem: Considerable frequency drift or shifting
First, make sure that L3 is soldered securely to the PC board. If you plan
to use the receiver "on the go", you may wish to secure the base of the
coil to the board with reinforcing glue. If the receiver drifts, check all
wiring and components associated with AFC Q3 and pin 11 of the
MC3359.
Problem: Squelch does not function
Re-check all connections and components associated with pins 10
through 16 of the MC3359. Check polarity of C27 and D2. C27 sets the
time constant for the squelch circuit.
SOME NOTES FOR ADVANCED USERS
Unlike Ramsey HF receivers and transmitters, the FR-series of VHF
FR-10 ? 24
crystal and two ceramic filters, there are nearly 60 capacitors and
resistors. Surely, all that should result in a decent receiver! You could
easily spend twice the money plus hours of time trying to gather the
equivalent parts from catalogs and still need to make your own circuit
board.
BUILDING YOUR FR-10 KIT
There are just over 200 solder connections on the FR-10 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!
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Use a 25-watt soldering pencil with a clean, sharp tip.
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Use only rosin-core solder intended for electronics use.
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Use bright lighting. A magnifying lamp or bench-style magnifier may
be helpful.
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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.
FR-10 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 NE602 IC, an efficient single-package (8-
pin DIP) mixer-product detector-oscillator. The tunable oscillator section
of the NE602 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 NE602'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 NE602 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
FR-10 ? 5
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