S Meter Preamp
The S meter preamp was designed specifically for the Uniden/Audioline range of rigs which suffer from "a lazy needle". The LM358 op amp is used because it is able to work from a single-rail supply. Silicon diodes are used in the feedback path to give a logarithmic response which more closely approaches the response required for a true S meter. The 4k7 resistor sets the overall gain of the circuit at low signal levels.
When the circuit is used in the Audioline, the meter connections must be reversed as the output from the amplifier is positive-going. D5 must be reversed so that power output can still be read. RT1 must be disconnected from RT3 and from the meter and the free end wired to the -ve input of the amplifier. The output from the amplifier is connected to the S meter +ve terminal via a 47k preset. RT1 and this preset can then be juggled to give the optimum setting on received signals, bearing in mind that the aerial efficiency will have a big influence on the final settings. Although the two adjustments are interdependent, RT1 determines the low-level signal readings, and the 47k preset determines the high-level readings.
Uniden "S" meter Amp
The R.F. Preamp was designed for general use in CB radios but, so far, has been used only in Audioline/Uniden rigs. The circuit may be built on veroboard but connections should be kept short and unused sections of track be disconnected to avoid a build up of capacitance. VR1 may be any value, but R1 should be changed accordingly to maintain the same ratio. R3 is essential to lower the "Q" of the circuit and prevent oscillation. The supply to R4 should be taken from the RX supply rail which comes from TR17 emitter.
Disconnect C4 from the transformer L1 in the Audioline. Connect G1 of the preamp to the junction of C1 and L1 in the rig. Connect D of the preamp FET to the free end of C4 in the rig. Adjust the inductor of the preamp for maximum mid-band signal.
In the Audioline, the "preamp" was found most useful as a means of decreasing, rather than increasing, the signal in order to reduce "bleedover" (where this rig already scores highly).
The squelch control was replaced with a dual ganged control from a scrap rig.
With a little ingenuity this preamp should work in most rigs. (Who's going to be first to try it in a Cobra 148 ?)
R.F. Pre-amp for Uniden rigs