Amateur Radio (G3TXQ) - Ten-Tec Corsair II Sidetone

I finally decided I needed to do something about the "raspy" sidetone on my CorsairII. 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.

The left hand photo shows the sidetone audio waveform before modification, and the right hand photo shows the audio waveform after modification.

Here's the schematic. I simply added two components to form a high-Q tuned circuit across the Sidetone Level control. 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.

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.

Then I mounted the board at the front of the IF/AF board using the existing mounting screw between the AF Gain and Notch controls. I 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.

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. The result is: 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.