Elecraft K2ATOBKIT - Manual (Page 8)

8
6. VFO ALC & 10/12 m Bandpass Filter
This set of modifications enhances the VFO ALC
circuit and narrows the 10/12 m filter. Without the
mods, a small increase in VFO spurious output will
sometimes occur when switching bands to or from 10
meters. This is usually significant only if the K2 is
driving a KPA100 or other amplifier, but we
recommend making the changes in all cases. The parts
and documentation for this mod are supplied with part
# E850093.
Follow all instructions supplied with the VFO
ALC mod kit, E850093, except 10/12 meter bandpass
filter alignment. Alignment will be done later.
If you have a rev. XC K2, you must also add R18
(1 M) to the VFO ALC circuit. Solder R18 on the
bottom of the board, directly across the pads of C59.
7. Second Crystal Filter Flatness
This change flattens the K2's second crystal filter
response, and allows the I.F. adjustment, L34, to be set
optimally for both CW and SSB. As a result, a much
better pitch match can be obtained on LSB and USB.
The parts and documentation for this mod are supplied
with part # XFILMDKT.
Follow all instructions supplied with
XFILMDKT, except those instructions dealing with
I.F. alignment, which will be done later.
8. First Crystal Filter Grounding
The instructions for grounding the crystals in the 5-
pole first CW crystal filter (X7-X11) originally called
for the wires to be soldered to the tops of the cans. A
few builders subsequently determined that soldering
the ground wires closer to the base of the cans resulted
in better ultimate attenuation. Some builders also
experimented with additional bypassing of filter
control lines using surface-mount capacitors, but this
appears to offer very little additional benefit, if any.
This change is optional. Most K2 builders who used
top-grounding of the crystals in the first crystal filter
found the ultimate attenuation to be completely
satisfactory.
i
In the following step, be careful not to
overheat the crystals. Use a temperature-controlled
iron, and limit soldering time to about 3 seconds per
soldering attempt.
There are two ground pads each for X7-X11, one
on either side. Use bare wires (10 total) for grounding
the crystals, but do not solder the wires to the tops of
the crystals. The wires must be soldered to the sides,
instead, about 1/4" (6 mm) up from the surface of the
PC board.
9. AF GAIN Control Smoothness
The original "2-wire" AF gain potentiometer circuit is
susceptible to mechanical potentiometer noise at the
counter-clockwise end of its rotation. The "3-wire"
modification used in the Rev. B K2 completely
eliminates this problem, making AF GAIN adjustment
smooth. The modification also allows the AF GAIN
pot to completely turn off the audio when rotated fully
counter-clockwise. In some cases, the old circuit would
permit some residual signal to circumvent the
potentiometer in this position.
This change is optional but recommended.
Follow all instructions in the application note,
except those dealing with testing the new circuit, which
will be done later. The application note can be found
at:
http://www.elecraft.com/Apps/K2_AF_pot/K2_AF_gai
n_app_note.htm (HTML)
or:
http://www.elecraft.com/Apps/K2_AF_pot/K2_AF_gai
n_app_note.pdf (PDF)