The Century 21 uses a 500 KHz wide 1st IF that is produced by heterodyning the
desired signal with a crystal oscillator for each band.
This 5.0 to 5.5 MHz IF signal is amplified via a broad band IF stage, then
mixed with the PTO via the product detector to produce the detected signal.
When Ten Tec put counters in most of these early radios, all they did was to
count the PTO frequency and add (or subtract) that count from a preset for each
band as determined by the oscillator crystal in use and whether the PTO is
added to or subtracted from the crystal frequency to get the detected signal.
The internal Ten Tec counters got this band crystal and add subtract
information directly from the band switch.
If you use a simple external frequency counter connected via loose coupling to
the PTO output line, you will only get an indication of the PTO frequency.
While you COULD simply start from the low end of a given band as indicated by
the band switch, then mentally add the reading to get actual operating
frequencies, this would only work on SOME bands. On the other bands, the low
end of the band is at the high end of the PTO frequency range.
That’s why people seldom try to use external counters with early Ten Tec radios.
A simpler alternative is a small counter that I have. This goes into the
antenna line and counts the carrier frequency in transmit TUNE mode. This gives
you a direct readout of actual transmitting frequency, but only when
transmitting.
Sorry I couldn’t make it easier…
Gary
W0DVN
> On Apr 12, 2017, at 11:31 AM, J. Fielden <gijoe1644@gmail.com> wrote:
>
> Where could I attach a freq counter to the C21?
> --
> 72/73, Jim - wa4ar
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