Welcome to the hodge podge of the barely readable 561's IF-AF board
schematic. I haven't looked at it in ten years but I
meticulously dissected it back then and modified the hell out of it. The
piece you might be missing is the bottom left of the schematic: U3A/B and
U4A/B comprise the 750Hz bandpass filter and yes, there's a fader that
mixes unfiltered audio with bandpass filtered audio at connector 6.
GL/73,
Barry N1EU
On Fri, Mar 25, 2022 at 6:00 AM Shawn Upton via TenTec <
tentec@contesting.com> wrote:
> Sorry, my new-to-me radio didn't come with a manual. I'm glad that TenTec
> was nice enough to post pdf's but the copy isn't that great.
> From what I can tell, audio on the IF/AF board goes first to the notch
> filter. The AGC is then tapped off this, and the audio then goes to what I
> don't understand just yet.
> Looks like audio goes to the bandpass filter made of U3 and U4, and goes
> to connector 6. But the audio after the notch filter also goes to here.
> It almost looks like a fader control here? As in, you could turn the pot
> full one way and get one audio path; or full other, and get the other. Or
> mix (sum) the two in some way. That would make sense but then I have R64
> which looks like a sneak path around.
> [I did see the online mod for hiss reduction; I've ordered the OPA2134
> boards to try that out. Working on seeing if I can make as quiet as my
> Omni C. :) Am toying with getting the 1.8kHz filter for some large noise
> reduction but waiting on that as it's not cheap!]
>
> Shawn kb1ckt
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