[TenTec] Century 22 filter mod idea

[Garry Nichols]

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