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SOME NOTES FOR ADVANCED USERS
Unlike Ramsey HF receivers and transmitters, the FR-series of VHF
receivers do not offer much PC board space or real need for
customization or other modifications. These receivers do what they are
intended to do at the lowest price that we are aware of for a tunable
VHF FM receiver with squelch, AFC and generous audio output.
Therefore, while we are certainly interested in hearing about significant
performance improvements, we do not recommend casual modification
attempts.
However, there is one practical customization which could be
undertaken by experienced VHF experimenters who wish to take
advantage of the SA602's ability to accept a crystal resonator instead of
the existing L-C tank circuit. The goal would be a one channel repeater
monitor or dedicated packet receiver. Inductor L3 and associated
capacitors would be replaced by the appropriate crystal tank
components. Remember that the crystal oscillator should be planned to
operate 10.7 MHz above the desired receiving frequency. Please do not
attempt such a project unless you have a clear knowledge of oscillator
design and have reviewed and understood the terms of your Ramsey Kit
Warranty. We regret that the factory cannot offer any assistance in
performing this or any other modification, so... you're on your own, but
that's what experimenting is all about!
A helpful article on the Signetics NE602 IC, including a discussion of
crystal-controlled oscillator configurations, was written by Michael A.
Covington with acknowledged assistance from Signetics personnel,
published with good circuit illustrations in Radio-Electronics magazine
for April 1990 (Vol 61, No 4). This will help you understand your SA602.
A less complex approach to the same general goal, eliminating the
expense of a crystal and the logistics of adding it to the PC board, is to
"undo" the excellent tuning range we designed into your receiver! This
would require selecting appropriate values for C9,C10 and C12 and
reducing the effect of the varactor tuning to perhaps a 100 KHz swing
for fine tuning purposes only. Try reducing C12 to 1 or 2 pF for starters.
Finally, the only pin of the MC3359 not used in our circuit is pin 15, for
scan control. Perhaps you will come up with some fascinating idea for it!
FR-10 • 25
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