I received some "brain wave" transmissions from certain Blog Readers about not wasting their read time with CW stuff. Afterall, the Newington Nerds have managed to eliminate anything regarding CW from the licensing requirements so why bother to talk about it. Out of respect to the 20 or so souls who do read this blog -- this is the last posting on the CW subject.
Today's subject is sidetone. The first obvious question is why have it. The first obvious answer so you can monitor what is sent or mis-sent. I almost suspect when I hear crappy CW being sent -- No Sidetone. Many of the early common commercial CW transmitters had no sidetone which created a whole series of schemes to do that function as an add on external accessory.
Some of these schemes were pretty innovative. Friend N2CQR when he fires up his DX-100 once a year on Straight Key Night (SKN) at New Years Eve, Bill listens to the power transformer hum as full current is being drawn from the power supply. Other sidetone approaches included RF actuated sidetone monitors so in essence RF floating around the shack was detected and further processed to produce an audio tone. Look Ma no wires.
The ARRL Handbook (circa 1961) in one of their somewhat unsafe tube transmitter designs used a NE-2 Neon glow tube as an oscillator to produce a sidetone. I call those designs unsafe as most of the time their designs had no fuse in the power supply. Who does that? Yes, it is the Newington Nerds!
So how was sidetone generated in this transceiver? Built into the design is a 2N3819 FET switch to mute the Audio when you are transmitting. If you look at yesterday's posting on the NE555 you will see a dotted diode in the keying circuit that says Mute Switch. Yes, you missed that part.
Often my audio amp stage would include a 2N3904 as an audio pre-amp but in this case, there is plenty of audio to drive a small loudspeaker fitted to a plastic cup. The mute switch was included because if absent and you are using earbuds, you will hear an ear shattering sound in your ears.
Initially I found out that the mute switch did work but it was not a "full muting". I was a bit disappointed as this was the 1st time I used that circuit. But then the flashing light went on as now I had Sidetone.
There was another bonus in that this lash up enabled me to set the proper frequency of the transmit mixer crystal. All I had to do was put the rig in CW and adjust the trimmer while listening with the cans (an OT term for earphones). What sounded most pleasing for me to copy would be a great starting place to set the trimmer. Indeed, this is a twofer.
So, what I thought was a not so good mute switch was indeed perfect. Not so perfect from a safety perspective is shared below -- no fuse but this does show the NE-2 Sidetone.
There done and no more CW stuff.
73's
Pete N6QW