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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