[TenTec] CorsairII sidetone modification

[Carl Moreschi]

Wow, that's a pretty impressive before and after picture.  I like the 
simplicity of the mod.

Carl Moreschi N4PY
121 Little Bell Dr.
Hays, NC 28635
www.n4py.com

On 3/15/2013 4:37 PM, Steve Hunt wrote:
> I finally decided I needed to do something about the "raspy" sidetone on
> my CorsairII. Being a lazy guy I was looking for a simple and easy
> modification that made a significant improvement with minimal
> alterations to the radio. This is what I came up with - not sure if it's
> original?
>
> Audio waveform before the modification:
> http://www.karinya.net/g3txq/corsair_sidetone/before.jpg
>
> Audio waveform after the modification:
> http://www.karinya.net/g3txq/corsair_sidetone/after.jpg
>
> I simply added two components to form a high-Q tuned circuit across the
> Sidetone Level control. Here's the schematic:
> http://www.karinya.net/g3txq/corsair_sidetone/schematic.png
>
> Lmod and Cmod are the two new components - they resonate at 650Hz.
>
> This was the implementation:
>
> I took a small piece of PCB material and scored a gap along the centre -
> one side for ground and one side for the connection to R96,R97,C65. I
> bent a solder tag and soldered it to the ground side of the board:
> http://www.karinya.net/g3txq/corsair_sidetone/board.jpg
>
> I wound 122 turns of ecw on a high permeability toroid (I got tired at
> that stage) and measured the inductance as 40mH. I calculated I would
> need 1.5uF for resonance at 650Hz. Mounted the toroid on the board with
> a hot glue gun, and soldered a 1uF and a 0.47uF capacitor in parallel:
> http://www.karinya.net/g3txq/corsair_sidetone/circuit.jpg
>
> Then mounted the board at the front of the IF/AF board using the
> existing mounting screw between the AF Gain and Notch controls.
> Connected a short lead (yellow lead in the photo) between the board and
> the exposed end of R96; the board gets its ground through the mounting
> screw:
> http://www.karinya.net/g3txq/corsair_sidetone/installation.jpg
>
> Adjustment simply consists of adjusting the sidetone pitch to place it
> at the centre of the new filter - it peaks noticeably as you adjust it.
>
> Result: much nicer sounding sidetone; minimal changes to the radio;
> Sidetone Pitch and Level controls still work as normal.
>
> If you decide to replicate the mod, be aware that the inductor needs to
> be pretty high Q otherwise the output level drops considerably - I
> measured the Q of mine as 330. Initially I tried one of the small Toko
> inductors, but its Q was less than 10!
>
> The inductance is not critical - I tried values of 20mH to 100mH when I
> was experimenting - simply choose the matching capacitance value to
> resonate at your preferred sidetone frequency.
>
> Hope that may be of interest to some. In slower time I'll write it up as
> a page on my web site.
>
> 73,
> Steve G3TXQ
>
>
>
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