[TenTec] MOD: Corsair Sinewave Sidetone

[Steve N4LQ]

TenTec Corsair sinewave sidetone modification:

The sidetone in the Corsair produces a sawtooth wave, rich in harmonics and is unpleasant to listen to for most of us. Fortunately, there is a very easy solution.
The Corsair has two audio oscillators. One is the sidetone, activated when you close the key and the other is the spot feature, activated by pushing the spot button. Push the spot button and notice how pure it sounds. This is a sine wave oscillator. We are going to use this for our cw sidetone.
The process is very simple:
1. Turn the Corsair over. Remove the Corsair's bottom cover.
2. With the Corsair's front panel facing you, locate the IF/AF board. It is on the right, front side of the rig.
3. Locate connector 28. It is located on the left, front side of the IF/AF board.
4. Using a piece of wire, short the two outer pins of connector 28 together. I pulled the connector off, stuck the fine wire inside each outer hole and plugged it back in taking care not to allow the jumper to contact the center pin. This could be done from the top of the plug. It's you're preference.
5. Turn the volume down completely on the original sidetone.
6. Key the rig and adjust R2 for proper pitch and R3 for desired volume.

Further refinements:
1. To clean up some of the audio thumps, solder a .22uf (224) capacitor between the center wiper of R2 and ground. I obtained ground from the lug on the PTO.
2. The original sidetone will leak some of it's nasty tone even when turned down to minimum. I grounded the front end of R4 which totally kills the beast. Use the PTO lug for ground. There are other solutions but this one is easily undone.

Side effects:
The sidetone will now sound very clean. Adjusting the pitch and volume will not be as easy unless you knock out a hole in the chassis under R2 and R3. 
Pushing the spot button will still produce the tone but it will be slightly different from the sidetone's pitch. This could be compensated for very easily by installing a trimmer pot in series with the spot button however I seldom have need for the spot button and the difference is very slight.

Corsair II.
The Corsair II's spot button functions totally differently. Instead of keying another oscillator, it changes the frequency of the BFO. So from what I can tell by the schematic this mod will only work with the older Corsair.

Note: When I aligned the BFO oscillators, I set the offset for 600hz instead of the factory 750hz because I prefer the lower pitch. To adjust the new sidetone, you can connect a frequency counter to the speaker and adjust for 600hz a pitch. This is strictly user preference. 
Note 2. Using an MFJ-281 ClearTone speaker will produce beautiful cw and reatly reduce some of the anoying thumps associated with the Corsair's agc and qsk system.

73

Steve Ellington N4LQ