[TenTec] CorsairII sidetone modification

[Steve Hunt]

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