I guess some folks do not believe the Ten Tec C21 manual and its schematic...
The 5.0 to 5.5 MHz IF goes into a product detector which the PTO performs the
role of BFO.
If you look closely at the 80356 audio board, you will see “IF input” and “PTO
input”. No premix appears on this board, where true product detection takes
place. If you trace the “PTO input” signal back to its source, it comes
directly from the PTO, not through and sort of premixer.
If you look at the 80358 mixer board, you will see that the receiver signal
comes from the input bandpass tuning, then into a mixer, whose injection is the
band crystal. An IF of 5.0 to 5.5 MHz results, and this passes on to the 80356
Audio board as a 5.0 to 5.5 mHz IF.
This is nothing different that what Heathkit did for the first IF in many of
their radios, most notably the SB-104. They heterodyned the incoming signals to
band crystals to produce a very broad IF (about 1 MHz wide) and then used the
VFO to heterodyne the broad IF down to the ~3.3 MHz IF where the narrow filters
were. This is a true dual conversion receiver. The C21 skips the second IF and
uses the PTOP to provide BFO for the product detector.
The C21 is simply a single conversion of the same thing. Using the PTO as the
BFO in a product detector is clever, and cheap.
Its advantage over direct conversion is freedom from hum and images (wrong
sideband). Also, one needs to be extravagant in shielding the premix in a
direct conversion receiver as it can leak directly into the antenna input, with
obvious adverse results.
The premix process (mixing the band crystals and the PTO) IS used in the C21
but this is what becomes the TRANSMIT signal in a C21, since the PTO and the
band crystals are summed (or subtracted) to produce the on-frequency
transmitted signal. In receive, there is NO on-frequency signal present
anywhere in a C21. Not true in a C22.
> On Dec 29, 2017, at 6:15 PM, Shawn Upton via TenTec <email@example.com>
> I don't think the DC receiver matters for qrp as much as for high band noise
> or crowded band conditions. It should have a 500Hz filter--but it is an audio
> filter, not crystal.
> If you can live with the bump in noise then the only thing I would be
> concerned with is the PTO. I contacted TenTec a couple months ago and they
> didn't have PTO rebuild kits. Should look into the finals and see if they are
> I forget if the C22 has AGC. I believe the C21 did not. Oh, and the C21 had a
> funky sidetone, iirc it was fixed in freq and rather low at that--and a
> sawtooth too. Not sure what bits carried over. But the C21 had a built in
> power supply, while the C22 does not.
> shawn kb1ckt
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