Softening The Rock-Mite Sidetone
The Rock-Mite has a very raspy sidetone. This is due
to the nature of how the sidetone is generated.
U3 generates a square wave at about 700hz on pin 5
whenever the radio is keyed.
The square wave has the bad characteristic of sounding
very raspy because of the wide range of harmonics that are generated.
To soften the square wave a bit, Dave Benson recommends
the installation of a simple low pass filter composed of
a 10 ohm resistor and a 10uf capacitor.
This simple filter is also known as the 10-10 filter.
C8 is not removed from the radio.
Here's a schematic:
The first thing I did was to cut the trace between pin 5 on U3 and C8
using a dremill type tool. Then I used perma-bond to glue
the 10uf cap down to the back side of the PCB
taking care to orient the leads correctly.
Here's a pic that shows this stage completed.
I then soldered the 10 ohm resistor across the now open connection
between U3 pin 5 and C8. Then I soldered the + lead from the cap
to the junction between the new 10 ohm resistor and C8.
Then finally I tacked the - lead from the cap to a ground near by.
Here's a pic to show the final stage.
I then performed the smoke test - no smoke.
The sidetone is much smoother and pleasant now.