Expanding the Range of the Clarifier   


During the early to mid 1970's, when 23 channel radios were the state of the art, the members of the "High Performance" CB crowd were always looking for ways to expand their radio's capabilities beyond what they were originally designed to do. This is no different from "Car Buffs" adding aftermarket parts to increase the horsepower of their engines, or to otherwise make the car handle better. Whatever the activity, there will always be people who want to push the design limits of whatever pastime they enjoy.  

I've already talked about adding Channel "22A", and swapping or adding extra crystals to expand frequency coverage.  For those who were also fortunate enough to have a SSB radio, there was yet another fairly easy to perform modification to gain a handful of additional frequencies.  Best of all it was relatively cheap, the mod required few additional parts, and mostly just involved time.  I'm talking about expanding the range of the fine tune, or clarifier circuit.  How does this mod yield more channels? I thought you'd never ask......

If you look at the original 23 channel band plan, you will see that the total number of individual 10 Khz channels, which occupy the space between the frequencies which make up Channel 1 and Channel 23 respectively, actually amounted to about 30. This span included the normal 23 class "D" CB channels, along with 2 business band channels (known as Channel 22"A" and 22"B") plus 5 class "C" radio control channels which were spaced evenly through the band between Channels 3 and 4, 7 and 8, 11 and 12, 15 and 16, and 19 and 20.  These "extra" channels were normally fairly clear and made for a good place to jump to for a little peace and quiet.  Since these "extras" were within the range of the "normal" channels, no decrease in radio or antenna performance would result from this modification. It was possible to install a crystal which would allow access to these 5 RC channels and 22"B", but SSB rigs could achieve this same benefit  (plus 1 channel below Channel 1 for a total of 7 extra channels), without the expense and hassle of a crystal purchase, and its associated switching arrangement, by simply expanding the clarifier range to "slide" down 10 Khz.

In the early to mid 70's, most 23 channel SSB radio clarifier circuits already moved both transmit and receive synchronously.  Normal range was usually about +/- 1 Khz, which was within the FCC's .005% frequency tolerance spec. The means for shifting the radio's frequency was primarily accomplished by either a variable capacitor which changed the capacitance of a bank of crystals (And thereby their frequency), or by a potentiometer which varied the voltage to a varactor diode, a special type of diode which exhibits a capacitance which changes proportionately with voltage.  This varactor circuit was also used to shift the capacitance of the crystal bank or a separate offset mixer crystal.  To increase the range that the clarifier covered, it was necessary to increase the amount of total capacitance change.  For radios where the shift was controlled by a varactor diode, this could be as simple as jumping out a resistor which ran from the low side of the clarifier potentiometer to ground. This would have the net effect of increasing the total voltage swing, which would directly translate to an increase in frequency shift.  Radios such as the Midland 13-893/895, and their same chassis cousins the Cobra 138/139, were good examples of radios which benefited by this simple change.  Once modified and aligned, the clarifier would typically drop about 12 Khz, which was enough to reach the "gap" channels, with a little extra left over for additional fine tuning range for SSB on those "drop" channels.  Another example of a radio which utilized the varactor diode clarifier mod was the Hy-Gain 623.  In this example of a clarifier on steroids, this radio, once modified, could drop over 200 Khz! This was far too much range for practical use, (fine tuning someone on SSB was an exercise in frustration, even with the standard 10 turn dial on the clarifier) although there were some who stubbornly tried to use it that way.  More practical people limited the down range to 50 Khz or less.  There were also some radios which were not as receptive to simple clarifier mods and more intensive changes had to be adopted.  Sometimes the varactor needed to be replaced with a different one with a greater capacitance change.  Sometimes a small inductor could be placed in series with the varactor, which helped to multiply the overall range. In extreme cases (like my Midland 13-885), the whole varactor circuit was replaced with a variable capacitor and inductor in series.  In any case, after a radio had the clarifier mod performed, the "center slot", or on-frequency point, would usually move from its normal 12 o'clock position to somewhere around 2 o'clock.  On a varactor circuit, this could be corrected by placing a resistor between the clarifier's potentiometer wiper and the high side of the pot. The exact value of the resistor would have to be determined through trial and error until the on-frequency spot was back at 12 o'clock.

Other radios, like the Lafayette Telsat SSB 50, the Realistic TRC-47 and TRC-48, and others, used a variable capacitor connected to a crystal bank to shift frequency. Modification to these radios usually involved placing a small inductor in series with the tuning cap, which would multiply the total range.  Once done, the range of drop would extend to 10 Khz or more. A realignment of the crystal bank would often be necessary to preserve enough positive range for proper SSB tuning operation, and prevent the oscillator from occasionally stopping.  Like on the varactor radios, the former 12 o'clock "center slot" position would shift to about 2 o'clock, but, unlike the varactor radios, there was no practical way to compensate for it.  Operators of these radios used markers, masking tape, or removed the knob and replaced it, all to help them remember where their on-frequency spot was.  Having a frequency counter was helpful as well.  But in the 1970's these were still very expensive and not too many people had them.

By the time 40 channel radios became the norm in the late 70's, the ease of expanding the PLL for these extra channels pretty much made the need for a >5 Khz clarifier drop unnecessary. But by then, the FCC had also changed the rules so as to limit the clarifier to only shifting the frequency of the receiver.  So for a 40 channel radio, the big clarifier "mod" was simply to restore the synchronous movement of both transmit and receive.  Expanding the range was usually limited to no more than +/- 5 khz, which was all that was needed to fill in any and all gaps.