Possible K2 sidetone mod

[dw-4]

I don't have my K2 yet…but should have it in about a week..depending on
options.
I noticed in the schematic that the side-tone output(U10B pre-amp) looks
to
be connected as a secondary input to AF-amp(U9 LM380).
This makes me assume that side-tone volume is not fixed, but instead
changes along with, and is a percentage of AF volume.
If that is true, its a setup that I don't prefer.

My (off-the top of my head) solution would be to splice in a 2N3904 on
the output of U10B, and then move that output over to
AF-OUT.

That way, I always have a fixed side-tone volume, no mater what the AF
volume is set at.

Has anyone else done something like this already?

Thanks
Duane
N1BBR
--
 [hidden email]

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[Don Wilhelm-4]

Duane,

That input to the LM380 chip is not the normal audio input (check the
LM380 datasheet). The sidetone level is fixed by the menu parameters and
is not variable with any knob.

73,
Don W3FPR

dw wrote:

> I don't have my K2 yet…but should have it in about a week..depending on
> options.
> I noticed in the schematic that the side-tone output(U10B pre-amp) looks
> to
> be connected as a secondary input to AF-amp(U9 LM380).
> This makes me assume that side-tone volume is not fixed, but instead
> changes along with, and is a percentage of AF volume.
> If that is true, its a setup that I don't prefer.
>
> My (off-the top of my head) solution would be to splice in a 2N3904 on
> the output of U10B, and then move that output over to
> AF-OUT.
>
> That way, I always have a fixed side-tone volume, no mater what the AF
> volume is set at.
>
> Has anyone else done something like this already?
>
> Thanks
> Duane
> N1BBR
>  
>  

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[Bill Noyce]

"dw" wrote:
> I noticed in the schematic that the side-tone output(U10B pre-amp) looks to
> be connected as a secondary input to AF-amp(U9 LM380).
> This makes me assume that side-tone volume is not fixed, but instead
> changes along with, and is a percentage of AF volume.

No, the side-tone volume is adjustable independent of the AF gain setting.
The AF gain pot is between the product detector and the LM380.
Sidetone volume is set through the menu, and is controlled via pin 1
of the DAC (U8) on the control board.
Sidetone enters the LM380 without going through the AF gain pot.
The LM380 output then goes directly to the headphones or speaker (see
sheet 1 of the RF board schematic).

    -- Bill, AB1AV
(Proud parent of a week-old K2, #6636)
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