Sidetone mod

Ashhar Farhan VU2ESE, designer of the uBITx  recommends replacing R253 (a resistor) with a 0.1uf capacitor to get the sidetone to work properly if you are encountering issues with it (mostly on a v5 uBITx).  You can also replace the 1K resistor at R250 with a  10 K resistor to increase the sidetone volume.
Ashhar was fooling around with an upgrade to the Raduino, (more to come shortly) when he noticed that the sidetone was not working properlt. After a day’s frustration, he put a scope probe on the sidetone line and saw that as the sidetone made a square wave of 0 to +5v and back, the average voltage rose to 2.5 v.  This was directly coupled  to the LM386 audio amp’s volume control. Replacing the final resistor(R253) in the low pass filter with a 0.1uf capacitor fixes the problem.
The problem was that the sidetone would work for “dits” satisfactorily, but the sidetone for a “dash” would vanish after an initial blip. That’s because the LM386 (in the v5 uBITx) was being pushed to the rail by the 2.5v DC on the input!
Reference

Leave a Reply

Your email address will not be published. Required fields are marked *