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FR6 • 22
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 tuneable 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 IC.
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!
Again, please be sure that you understand your warranty, and that you
understand your receiver and how to keep it working, before attempting any
modifications.
If you've enjoyed this kit, be sure to check out the entire Ramsey kit line. Call
or write for our catalog.