[TenTec] re. 208 audio filter - more

[rick@dj0ip.de]

Never touch a running system.

The circuit Doug described on page 46 of the july 1979 QST is identical to
that of the Model 208 (external) filter except 4 resistors change slightly
in value (from 21,8K to 22K).  Seems strange, but 
Seems they used the same circuit for as long as they were using
Audio-derived AGC.

73 - Rick, DJ0IP
(Nr. Frankfurt, Germany)



-----Original Message-----
From: TenTec [mailto:tentec-bounces at contesting.com] On Behalf Of Brian
Sent: Tuesday, January 05, 2016 4:43 PM
To: ten tec list
Subject: Re: [TenTec] re. 208 audio filter - more

Thanks to the person who pointed out the difference in the use of a filter
inside the AGC loop (as is the 208 filter) and a an audio filter used at the
audio output (headphones jack etc).The reference  I had in my original email
re. DeMaw circuit I reiterate here, since it was also mentioned in a later
mail:See QST of July 1979, page 46.If you 'act today' you can see the
schematic of the thing (or something that is really darn close to it)  at
this site (who knows how long this link will
work).http://www.k4icy.com/cwfilter.htmOther filters that might be useful
(you can determine yourself if possibly suitable inside AGC loop or used
externally):http://kc9on.com/calf-the-cw-audio-limiter-filter/http://www.4sq
rp.com/HiPerMite.php
I have myself used the SuperSCAF and the Datong audio filters externally,
both to good effect, in the past.I have no interest in any of this except to
confer an variety of possible sources and ieeas.I cannot comment at this
time on any variant there there might or not might be in regard to the MFJ
"CWF" filter board that is inside my Triton IV (and I assume in the AGC
loop, but who knows) versus the external CWF-2 box that MFJ marketed as an
external 'plug into the headphone jack' filter.BrienVE3VAWFrom:
brianpepperdine at sympatico.ca
To: tentec at contesting.com
Subject: re. 208 audio filter
Date: Tue, 5 Jan 2016 13:31:51 +0000




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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