I've successfully made the following mods to two K2/100's, (both latest
spec/upgrades). The QSK mods are based on previous posts on the Elecraft
reflector.
QSK. Firstly, on the control board, cut the short track between U6 pin 17
(RC2 /MUTE) and the junction of C23/R17. Solder a 220k resistor across the
cut, i.e. in series with the /MUTE line. Without this resistor, the mute
circuit itself generates audible clicks on both key up and key down.
Secondly, replace R17 (3M3) with a 1M5 resistor, or solder a 3M3 resistor in
parallel with the existing R17. This shortens the hang time. With these
two changes and TR set to 0.00 (latest firmware), it is possible to receive
between dots at 20wpm and between characters at 40wpm. In practice this is
adequately fast, and the QSK is reasonably smooth.
RX audio. In some circumstances the K2 could benefit from a little extra
audio gain. At times I've found myself with the AF gain fully up when
listening to a weak signal on a quiet band. If you have the KAF2 option
installed it is easy to increase the audio gain. On the KAF2 board, cut the
short track between RP1 pin 5 and U1a pin 1. Solder an 8k2 resistor across
the cut. This gives +10dB of audio gain, (alternatively a 3k9 resistor will
give +6dB). This mod does not alter the audio bandwidth, and the additional
gain is present regardless of the selected bandwidth. Obviously the maximum
audio output power (1W) is unchanged, but I find the additional gain useful
at times. The sidetone level is unaffected.
Finally, I often use a very simple passive filter in the headphone lead.
This comprises a 47mH Toko 10RB series miniature inductor, in series with
1.2uF (1U//220n miniature poly box caps). This series LC is connected in
series with the headphones, and gives a broad peak centered on about 660Hz.
It's best to use the speaker output on the rear panel for this, and connect
stereo headphones so that Left and Right are in parallel, so typically
presenting a 16R load. This filter improves the K2 sidetone and further
cleans up the QSK transients. The Q is low so it does not have a dramatic
effect on the receive audio bandwidth, which can still be selected on the
K2.
I find the above mods together make a significant improvement.
73 Fraser G4BJM #4368
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