[TenTec] Century 22 filter mod idea

Tue Aug 22 14:25:56 EDT 2023

Thank you, Ken!  My thinking is that if the CF is lower you would have to
tune less far away from zero beat to get a sig into the center of the
filter.  So with the reduction spectrum you might encounter fewer
interfering sigs, on average.  In this case 750-500=250Hz closer to zero
beat on either side.  Diminishing returns perhaps (if I am even correct)!

My main objective is to lower the CF to a value pleasant to my ears!  Nice
to know there are others out there who like a low CW frequency!

73,

Garry

On Tue, Aug 22, 2023 at 1:27 PM Ken Brown <kenradiobrown at gmail.com> wrote:

> Hi Gary,
>
>      It is clear that you already know a lot more about those filters than
> I do. So I can't help you with your decision about changing R and C for
> filter center frequency, bandwidth and Q.
>
>      I just wanted to say that two bandpasses on opposite sides of zero
> beat give you a total receiving bandwidth of 2X the audio filter bandwidth,
> regardless of the filter center frequency. Selectivity would be the same as
> long as the bandwidth of the filter is the same at the two center
> frequencies you are comparing. Whether you will experience interference
> from signals on the opposite side of zero beat (in the other sideband) just
> depends on whether they happen to be on that frequency.
>      Maybe fewer operators will start up a QSO or call CQ only 1 kHz away
> than they will 1.5 kHz away, so perhaps you would experience a bit less QRM
> with the 500 Hz center frequency. Really though I think the deciding factor
> is the center frequency you like to listen to. I prefer about 450 to 500
> Hz. I'm glad my radio lets me change it at will, but I find myself keeping
> it at 450 Hz almost all of the time.
>
> Ken N6KB
>
> On Tue, Aug 22, 2023 at 4:51 AM Garry Nichols <garrnich at gmail.com> wrote:
>
> > Hi Folks;
> >
> > I have a C-22 (S/N: 666) and have verified that the AF filter has the
> .0022
> > uF caps in it and that the three resistors that set the filter
> > characteristics are the same as in the Ten-Tec circuit diagram.  This
> > supposedly gives about a 750 Hz center frequency and approximate testing
> by
> > me agrees with this number.
> >
> > Please Note:  the Ten-Tec circuit diagram indicates .0033 uF caps which
> are
> > the ones that gave a center frequency of 500 Hz.  I have heard that the
> > initial run of these radios were set up for 500 Hz.
> >
> > Does anyone know if the 500 Hz radios were intentional and that 500 Hz
> was
> > not received well by hams prompting a change to .0022 uF and 750 Hz?  Or
> > was it considered a goof up by Ten-Tec?
> >
> > My old ears do not like frequencies around 750 Hz.  So I am considering
> > converting the filter to 500 Hz for that reason.
> >
> > Also, it would seem that 500 Hz would give somewhat better selectivity
> > considering the "signals on both sides of zero beat" architecture of this
> > rig.  I reason that tuning out from zero beat 500 Hz on either side
> covers
> > a narrower spectrum than tuning out 750 Hz to a filter peak.  Is this
> sound
> > thinking???
> >
> > I have a print from somewhere on the internet of a mod that takes the
> > filter from 510 Hz center frequency to 600 Hz.  I don't know where I
> found
> > it but it is apparently by a Chinese ham.  He gives a link to an active
> > filter calculator which I have been playing around with:
> >
> > https://www.changpuak.ch/electronics/Narrow_Bandpass_2.php
> >
> > I've noticed that with a change to .0033 uF (the 500 Hz CF) and no change
> > to resistors the bandwidth reduces to 208 Hz.  Using the calculator with
> > .0022 uF the characteristics are likely the same as my current
> performance
> > and the calculator gives a bandwidth of 313 Hz.  I find the filter to be
> > fairly "peaky" at 313 Hz BW and am concerned that if narrowed to 208 Hz
> it
> > might have touchy tuning.  (I do realize this filter does not have the
> > steep sides of a crystal filter, but that is still pretty tight, I
> think.)
> >
> >
> > So I changed the Q of the filter from 2.4 (which is what gives my
> apparent
> > current performance with caps and resistors) down to 1.6 in steps and
> > continued to use the .0022 uF caps that are in it and wound up with a
> > change needed only to R3, the frequency determining resistor.  So that
> > would be 4 R component changes instead of 8 C changes and a calculated
> > bandwidth of 313 Hz which is what I have now with the current components.
> > The rolloff will be more gradual with the reduced Q, but being a newbie
> to
> > all this I'm not sure if it would be a detrimental rolloff.
> >
> > Does anyone have any experience with active filters or with C-22 filter
> > mods who can give any insight into my ideas?  This is a four section
> > filter.
> >
> > TU ES 73,
> >
> > Garry WA1GWH, near Syracuse, NY
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> >
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