Careful! You don't want an audio filter that connects outside of the
audio-derived AGC loop.
The SCAF-1 connects to the audio output of the radio, which is OUTSIDE of
the AGC loop.
The NESCAF appears to be similar.
These filters that were made to plug into the audio output of the
transceiver cannot simply be inserted into the low power audio chain of the
Argonaut. You need a filter designed to work at those signal levels.
If you use these two filters as they were designed, they will filter the
audio as they should but they will not prevent the receiver's AGC from
pumping due to strong neighboring signals. The proper filter type works
quite effectively at preventing this pumping.
73 - Rick, DJ0IP
(Nr. Frankfurt, Germany)
-----Original Message-----
From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of Brian
Sent: Tuesday, January 05, 2016 2:32 PM
To: ten tec list
Subject: [TenTec] re. 208 audio filter
This is from my own personal way-back machine (OK, I really just searching
the TT list archives for my old posting in 2001).Might be valid to the
person looking for the 208 audio filter.http://www.vectronics.com/ for one
source of a variant, but MFJ CWF filter is about the same if found
used.This was a common op-amp audio filter, same basically as audio filter
in older TT rigs inside OMNI C etc.
My personal Triton IV has an MFJ CWF board inside it for CW filtering,
installed by previous owner. I advise setting it such that it can be
switched in and out of the audio chain, as is possible with the 208 filter.
There are now other very good audio filters - the new CALF filter, Switched
Capacitor filters (NESCAF) and the Idiom Press SCAF-1 (Idiom Press' new
owners say that is back and available supposedly this month) as examples.
FWIWBrienVE3VAW
---------------This is the info I sent on the subject of the Triton IV
filter.
Some of you now or in the future (archived) might find it of use.
You have a good chance of achieving filtering this way:
In the QST review of the Triton IV analog rig version ( I believe) Doug
Demaw published the FULL schematic of the filter - which actually has 4
widths (and not just the one the TT filter made available by their front
panel switch). So if you build the filter and house it in a box (like the 2
width filter used by the Argonaut
509) you have a nice 4 width filter exactly as the Triton IV had (but which
did not use all available widths). This is really simple to build, in my
opinion, since it is just an LM747 dual opamp and some caps etc. Easy to
build with prototype board (copper strips and holes), or dead bug or
Manhattan style construction.
See QST of July 1979, page 46.
You could also buy the Vectronics CW filter, which is exactly the same
filter and schematic from what I can see ( I looked yesterday and the whole
assembly and parts manual for it is online at their site in Adobe .pdf
form). While they have it set up as an external audio filter to be used at
the headphones there is no reason one could not insert it into the audio
chain inside the Triton where it works before the audio amp stage.
This is the same design as TT - it was common at the time in various forms
and I think possibly also was in QRP Classics, an old ARRL compendium.
Now, if you want even MORE fun and performance, if you look at the QST for
Sept 1981, page 17, there is an article describing how to hook in a external
TenTec Model 217 500 HZ filter into the I.F. chain, giving the rig some good
narrow CW I.F. filtering. Very nice, taking the thing overall pretty much
into TT OMNI C territory (if I can go so far as to say that..) since one
would then have a switchable I.F. cw crystal filter in or out, as well as a
multiple width CW audio filter if you build and use the QST opamp-rc design
from the review's spec, or use the Vectronic filter.
The Vectronic filter is I think pretty much what the old MFJ CW-1, CW-2 etc.
external filters are/were and which are found at hamfests and other venues
of used ham gear.
Hope this helps you out.
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