The C-21 is nothing like the Corsair II. Ten-Tec calls the C-21 a "double
direct conversion" receiver.
The input stage has a diode ring mixer that has an antenna RF input and a
second input from the crystal Mixer board. The resulting IF output is a
broadband 5.0 to 5.5 Mhz (NOT 9 Mhz) signal. The detector board has
another diode ring mixer with the broadband IF as one input and the 5.0 to
5.5 Mhz PTO signal as the second input with resulting DSB audio output going
to an audio filter. There is NO AGC!
I have been told that some people have done a 30 meter conversion to the
C-21 but I would not try this.
The Century 22 does include the 30 Mhz band but I believe it uses a
different PTO frequency.
73, Bill - K6WLM
----- Original Message -----
From: "Dr. Gerald N. Johnson" <firstname.lastname@example.org>
Sent: Thursday, September 03, 2009 9:27 PM
Subject: Re: [TenTec] Century 21
> On Thu, 2009-09-03 at 19:30 -0800, Bill Miner wrote:
>> Sorry but I do not think it would be possible to modify the C-21 for 30
>> meter operation.
>> The PTO operates 5MHz to 5.5MHz with the second harmonic being on the
>> MHz band. Also the I.F. frequency is 5 -5.5 MHz. If Ten-Tec has used
>> different I.F. frequency then it would be possible to use the Aux
>> of the band switch for 30 Meters.
> The IF is 9 MHz. The VFO gets mixed with a crystal to produce 19-19.5
> then mixed with the 9 MHz VFO IF to produce 10-10.5. 9 will probably
> leak through and the second harmonic of the VFO can leak through the VFO
> mixer and the signal mixer. In the Corsair II, that crystal is on 13.99
> MHz and is also the low 10 meter crystal. Also the injection is the
> same, uses the same bandpass filter.
> So adding 30meter means using the low 10m oscillator mixing and filter
> and then adding a 10 MHz bandpass filter to the bandpass board. Can't be
> a stretched 7 MHz filter because of the 9 MHz leak through.
> If it works in the Corsair II and it does, it can work in the Century 21
> if you can sacrifice another band or can add a switch to switch in the
> 10 MHz bandpass filter in place of the rest of the bandpass filters.
>> If anyone else has some thoughts on this I would be interested.
>> Bill - K6WLM
> 73, Jerry, K0CQ
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