[TenTec] re. 208 audio filter

[rick@dj0ip.de]

Hi Geoff,

HNY!

Unless the 544 had a significant change to the design of the 505, 509, and
Triton 540, that is not quite correct.

If you look at the block diagram of the rig (example Triton 2), it "looks
like" the ACC feedback tap is on the IF board and before the external CW
audio filter.  But that is not the case. You have to examine the schematic
of the IF board to see how it really works.

The board labeled "Audio Board" is only the high power stage of the audio.
The low level stages of audio amplification are physically located on the IF
board and feed the AGC.

The Product detector directly follows the two IF stages, then passes the low
level audio to "FILT IN" pin, then passes through the optional audio filter
board, then comes back into the "FILT OUT" on the IF board.  From there it
then passes through the audio op-amp IC1, THEN it feeds the AGC Darlington
stage consisting of Q4 and Q5, which generate the feedback AGC voltage to
control the gain of IF amps Q1 and Q2.

Thus the filter is "inside" of the AGC loop and behaves much like a crystal
IF filter behaves.
THIS IS A MAJOR DIFFERENCE to using an external filter outside of the AGC
loop!

I can't find a manual or schematic of the 544 but its successor, the OMNI A
still handled this the same way of generating AGC feedback voltage.

I recognize the 4 stage audio filter shown in the 544 review by Doug.
I once built one of those on a breadboard and used with my Argosy, since I
was unable to locate an original Argosy filter.

Sure brings back nice memories!

All in all, the Triton IV was one of the finest radios ever built!

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



-----Original Message-----
From: TenTec [mailto:tentec-bounces at contesting.com] On Behalf Of geoffrey
mendelson
Sent: Tuesday, January 05, 2016 3:10 PM
To: Discussion of Ten-Tec Equipment
Subject: Re: [TenTec] re. 208 audio filter

On 1/5/2016 3:50 PM, rick at dj0ip.de wrote:
> 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.
>
>

Look up Doug DeMaw's (I think he was still W1CER at the time) review of the
Ten-Tec 544. It was a few paragraphs of description, some photos and almost
a full page schematic of the optional internal SCAF filter. It was out of
the AGC loop. It also was also an unattributed copy of his design. He never
said that in the review.

Geoff.

--
Geoffrey S. Mendelson 4X1GM/N3OWJ
Jerusalem Israel.

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