Sidetone Mod Working

[Darrell Bellerive-2]

Thanks to all for your assistance.

After playing around for quite a while with this mod, I finally have it
optimized for my K2. Sidetone is set to 580 Hz and measured at 589 Hz.

I went through over a dozen combinations before settling on the combination
listed below.

L1 was removed and replaced with a 4.7 uF capacitor.

C33 was removed and replaced with a 1 uF capacitor in series with L1.

The effects of this are:
1) Sidetone harmonics reduced as follows (ref to sidetone):
2nd harmonic from -31 to -43 dB (17 dB lower)
3rd harmonic from -27 to -50 dB (23 dB lower)
4th harmonic from -48 to -71 dB (23 dB lower)
5th harmonic from -40 to -68 dB (28 dB lower)
6th harmonic from -59 to -72 dB (13 dB lower)

The sidetone is now lower in volume for the same sidetone level setting. It
went from a level setting of 40 to 94 for the same volume level.

Overall the sidetone sounds less rough. While the level has been reduced, it
is still plenty loud for me at a setting of 255. Be nice to drop the third
harmonic further as this should give a little warmer sound. An active filter
in the sidetone line might be a solution.

It didn't seem to matter whether L1 was before or after the capacitor. I left
it after.

Also a resistor in series with L1 was tried. I used a 200 ohm trimmer and
tried adjusting it over its range with various capacitor configurations. At
no time did I find increasing the resistance above zero ohms to be
beneficial. In fact it was quite the opposite. Increasing resistance
decreased the sidetone level and increased the strength of the harmonics in
all cases.

Also, some combinations of capacitors increased the receive to transmit thump.
My T/R delay is set to 0.01. It appears the the T/R thump comes from both the
sidetone and the receiver audio chain. Although most is from the sidetone
line. Perhaps an active high pass filter on the sidetone line would be the
solution. A little noise also comes from muting FETs Q6 and Q7.

Also C29 was changed to 80 uF which reduced the thump level as well. The value
of C29 is highly dependant on the impedance of the speaker. I use an external
8 ohm speaker.

Please do not use the values of capacitors listed above in your mods without
careful testing. The values I ended up with are very specific to my sidetone
pitch, volume, and external speaker. In fact changing the sidetone pitch will
change the intensity of the sidetone and it's harmonics. With testing one can
optimize their K2 to their specific liking, but it comes at a price. I
sacrificed the ability to change the sidetone pitch for a better sounding
sidetone and less T/R thump.

Sure is fun tinkering with the K2 though!

--
Darrell Bellerive
Amateur Radio Stations VA7TO and VE7CLA
Grand Forks, British Columbia, Canada
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