(This information provided by Joe, N4YG)
The Corsair II has a lousy CW sidetone. I tried using a twin T oscillator, which had a nice sine wave, but I could never get smooth keying in the Corsair II. So I decided to try an idea which I had several years ago, using DDS. That’s right, DDS applied to audio frequencies. It’s a lot easier at audio frequencies since things are slowed down. The circuit shown in the schematic has only one active component (other than a 5 volt regulator), an 18-pin 16F88. This chip with its internal 8 MHz clock is fast enough to complete all the calculations in 28 microseconds, which is the DDS cycle time I selected. This provides about 50 analog samples per cycle at 700 Hz output. This is more than ample. Digital-to-analog (D/A) conversion is accomplished with a R, 2R, 4R, 8R, … 128R resistor ladder at the output, port B. The output amplitude is 5 volts peak-to-peak, far more than needed. The really nice thing is that there are no frequency determining components, timing being provided by the internal clock in the 16F88. You will notice no chirp when keyed. The very best feature is that leading and trailing edges of the keyed waveform are shaped in firmware, providing optimum shape. This is not possible using resistor-capacitor networks. There are no clicks or thumps when this sidetone is keyed and the output is a near-perfect sine wave. (Note: The Corsair has some audio thumps which are an artifact of the QSK circuit.).
All you need to do to put this into the Corsair II is to construct the circuit shown in the schematic.
You should be able to purchase all the components for under $10. You should use 1 percent resistors for minimum distortion although 5 percent will work. I constructed mine on a small piece of Veroboard and mounted it along the side chassis above the IF/AF board. I placed a 4-pin header with a connector to provide 12 volts, ground, key voltage and audio output. Hex Code is provided here for the 16F88. Source Code is also provided in case there is someone who wants to know how it is done or wants to play around with the code.
To complete the installation:
Construct the board.
With the Corsair II upside down, mount the board along the side chassis above the IF/AF board.
Disable the internal sidetone keying circuit by shorting the end of resistor R89 to ground. You can accomplish the same thing by shorting the base and emitter leads of Q11 together.
Connect the wire coming pin 1 to pin ST of connector 90. I did this by unpluging the connector and attaching a small piece of wirewrap wire around the exposed pin, just a little so that the connector can be reconnected. The wirewrap wire can then be connected to the wire coming from pin 1.
Connect the wire coming from pin 2 to the lead of capacitor C56, the one nearest U5. You may want to remove the heat sink from U5 first. You will have to clear away some of the coating on the lead of the capacitor near the board. Once this is done, the wire can be soldered to the exposed lead.
Connect the wires coming from pins 3 and 4 to convenient sources of ground and +12 or +13.6 volts.