[Elecraft] Improved 2nd Xtal Filter Flatness

[Mike Harris]

Hi,

This was the primary task of the weekend, the KI6WX stage two mod.

I took before and after Spectogram screen dumps.  It is vitally important to
take the audio from the K2 from the external speaker jack rather than the
headphone jack, there is a significant HF roll-off with the latter.  Also
essential to switch out the AF filter with its own switch otherwise there is
another source of HF roll-off.  For those with the KPA100 installed a handy
access port to the AF filter switch is just to remove the panel in front of
the heat sink.  Actually, in this configuration the filter didn't look at
all bad, except for a bit much ripple.  AF from the headphone jack and the
AF filter switched in will show about a 12dB slope on the trace and can make
you believe, as I did initially, that the filters are pretty rubbish when
they weren't.

Having previously completed the original filter flattening mod albeit with a
100pf capacitor (nearest value I had) instead of a 120pf capacitor I only
had to install the 5.6k ohm resistor from U11 pin 1 to 2.  Again with care
it doesn't look out of place, at least to my eye and I'm pretty picky.

The result was a slight decrease in the ripple and after tweaking the BFO
settings almost no change in noise when switching from USB to LSB, a common
finding.

Following that was a play with filter and BFO settings in anticipation of a
new set of matched crystals.  Actually until I see a significantly better
Spectogram plot from someone who has done the switch, I'm not in any
particular hurry.

One thing I would be interested to learn is how wide can the CW filter with
new crystals go before the top starts to break up into serious peaks and
troughs.  With my current set it deteriorates badly after about 1200Hz.

For CW I presently use OPt-1, 700Hz, 400Hz and 150Hz.  Thinking of changing
to 1200Hz, 800Hz, 400Hz and 200Hz.  Maybe we could have a pole as to the
most popular CW filter settings.

Regards,

Mike VP8NO (IOTA SA-002)
K2/100 #1400