MorseMaster II Sidetone Tryout

Continued from yesterday:

https://wb9kzy.blogspot.com/2026/06/morsemaster-ii-shaped-code.html 

I did try the MorseMaster II sidetone on the solderless breadboard.  It actually works pretty well.  Here is an short pulse from the LM380 output with the RC filtering:

It seems to me that I tried this circuit before but found it thumpy.  This time the thumping is minimal.  The two high pass RC pairs may account for this - I may have not used the spec'd parts before.  The only problem, noticed with headphones, is backwave.  There is a low level audio signal coming from the 555 chip which is running all the time - think of it as an on frequency QRP audio station which is key down all the time !

The backwave can be reduced by adding a series resistor to the headphones, I tried 180 ohms but more experimentation might help.

One way around this problem would be to key the 555.  But that's not quite so simple since most of the 555 CPO circuits out there use a variation of this circuit from the 1973 ARRL Handbook:

The keying is done from the positive supply to the power pin of the 555.  Kit plug: any of the Keyall kits could do this.  But ideally a switch closure to ground would be the way to go to key the 555.  I suppose that the connection from ground to the ground pin of the 555 could be opened and the key could be inserted there.  The Accu-Keyer suggested sidetone did it differently:

The Accu-keyer U7 output goes high when keying.  In my MorseMaster II Q4 was connected as an inverter (no shaping) to provide this keying to the 555.

I tried something similar for the shaped circuit.  I disconnected pin 4 of the 555 from +V and reconnected it through an 18k ohm resistor to the collector of Q4.  This does work however it increases the thumping :(  Possibly a different resistor value might help ?

Basically either live with the backwave present or live with the thumping, probably just depends on whether a speaker or headphones are used.

I don't think I'll use this circuit in my MorseMaster II, I'll leave it a plain 555 CPO for now and keep looking for something better :)

Best Regards,
Chuck, WB9KZY
http://wb9kzy.com/ham.htm