Elecraft K2ATOBKIT - Manual (Page 7)

7
4. 15/17 m & 20/30 m Bandpass Filters
See descriptions below. Band-pass filter realignment
will be done following all hardware and firmware
updates.
15/17 m Bandpass Filter
These optional changes narrow the 15/17 m bandpass
filter to improve image rejection. In Europe in
particular, strong shortwave broadcast stations in the
16 MHz range may produce low-level image responses
in the 15-m band with the original filter values.
Change C30 and C36 from 330 to 470 pF ("471").
Change C33 from 3 (or 3.3) pF to 2 (or 2.2) pF.
20/30 m Bandpass Filter
This optional change adds 12-pF capacitors in parallel
with each of the 20/30-m bandpass filter trim caps,
which should eliminate any difficulty in aligning the
bandpass filter on 20 meters. Some builders found that
the 20-meter trimmers "topped out" before the peak
was located.
Solder 12 pF capacitors across C21 and C23, on
the bottom of the board in the bandpass filter area.
(These capacitors are designated C28 and C29 on the
Rev. B RF board.)
Re-installing the Heat Sink
Inspect the PA thermal pads and re-install the heat sink
as described in the last two steps on page 6.
5. BFO Temperature Stability
This optional set of modifications reduces BFO
frequency drift an average of 70-80%. The parts and
documentation for the change are supplied with part #
BFOMDKT.
If you previously installed this mod kit, make sure
that R19 (soldered to the bottom of the board in the
PLL reference oscillator area) is 2.7 k. Some early
BFOMDKT kits were sent out with 220 ohms for use
at R19, which can cause PLL reference oscillator
instability. (1.8 to 3.3 k can be used.)
Note: Skip to step 5A, at right (PLL Upgrade) if
you've already installed BFOMDKT.
If you have an older K2 (Rev. A or XC), you may
have installed a large capacitor (150 pF or larger) at
C174 due to inadequate BFO range. Increasing only
the size of C174, without a proportional increase in
C173, may cause excessive BFO injection and is no
longer recommended. Restore C174 to its original
value if applicable (82 pF; supplied with
K2ATOBKT.)
Note: The crystals supplied with BFOMDKT should
have adequate range with the standard capacitor value
at C174 (82 pF). If the range is found to be slightly
low, two supplemental capacitors (47 pF and 120 pF),
which are supplied with BFOMDKT, can safely be
added in parallel with C174 and C173. This is
explained on page 12 as part of the BFO range test.
If you have an older K2 (Rev. A or XC), you
may have a 100 k RPACK at RP2. In this case, solder
10 k resistors (
R REFA , R REFB
) across RP2 pins 1-2 and
pins 3-4, on the bottom of the board in the VCO area.
(There's no need to remove RP2, which can be
difficult. The two paralleled resistors will suffice.)
Locate the instructions supplied with BFOMDKT.
Put an "X" by the instructions covering installation of
firmware, as well as BFO and PLL test and alignment.
These steps will be skipped for now.
Follow all instructions supplied with part #
BFOMDKT, except those you marked with an "X"
above.
5A. PLL Upgrade
The PLL Upgrade is a very important design change
that was recently added to the K2 (s/n 3446 and later).
It uses a thermistor to temperature-compensate the PLL
reference oscillator, reducing drift by a factor of 5 to
10. This upgrade kit should only be installed if you
have also made the BFO mod described above. The
parts, small PC board, and documentation for this
upgrade are supplied with part # E850138.
Follow all instructions supplied with the PLL
Upgrade kit, E850138, except those dealing with
alignment and test. This will be done later.