Ramsey Electronics FR6 manual Some Notes for Advanced Users

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.

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