ELECRAFT K2 - Page 59

58 ELECRAFT
VCO Alignment
In the following steps you’ll adjust the VCO inductor (L30) so that
the VCO control voltage is in the proper range.
Disconnect the internal frequency counter probe and remove
it completely from the K2.
Select 80 meters, and set the VFO for about 4000 kHz.
Connect a DMM (digital multimeter) to the left end of resistor
R30 (near the center of the synthesizer area of the RF board) and
ground. Use a small alligator clip to ensure a good connection. (You
can also use the built-in voltmeter to measure the VCO control
voltage. Refer to Voltmeter Probe Assembly in Part I.)
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It is possible to damage the slugs in slug-tuned inductors if
you use a metal tool or if you tune the slug too far in or out. The
tuning tool provided will not damage the slugs.
Using the wide end of the plastic tuning tool, adjust the slug in
inductor L30 until the voltage at R30 reads 6.0 V. If the voltmeter
reading does not change at all as you tune L30 through its full
range, refer to Troubleshooting. If the voltage changes but you
cannot get to 6.0 V, you have probably wound the VCO inductor
(T5) incorrectly or have installed the wrong value at L30 or C72.
Set the VFO for approximately 3500 kHz.
Measure and write down the VCO control voltage at this
frequency in Table 6-1 (using pencil).
For each remaining band, set the VFO to the low and high
frequencies listed in Table 6-1 and write down the VCO control
voltages.
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(You can tune quickly to the approximate frequencies in
the table by selecting the 1-kHz tuning rate.)
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Usable VFO coverage extends well beyond the ranges given in the table. 15
MHz is used as the upper boundary on 20 meters to allow reception of WWV
at this frequency.
Table 6-1. VCO Voltage Readings
Band Low Freq.
Voltage High Freq. Voltage
80 m 3500 ______ 4000 ______
40 m 7000 ______ 7300 ______
30 m 10000 ______ 10150 ______
20 m 14000 ______ 15000 ______
17 m 18000 ______ 18200 ______
15 m 21000 ______ 21450 ______
12 m 24800 ______ 25000 ______
10 m 28000 ______ 28800 ______
If some VCO control voltage readings above are < 1.5 V, or
some of them are > 7.5 V, you may be able to shift the entire set of
readings so that they are all within the 1.5 to 7.5 V range. Switch to
the band (and frequency) that had the highest or lowest voltage,
then adjust L30 to bring that reading into range. Then re-measure
all of the voltages to make sure they're in range.
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If you have some voltages that are < 1.5 V and others that
are > 7.5 V, you have probably installed the wrong value at one or
more of the VCO capacitors (C71-C74) or varactor diodes (D21-
D26). Another possibility is that T5 has the wrong number of turns
or that you installed the wrong type of slug-tuned inductor at L30.
If you change any of these components, repeat the VCO alignment
procedure.
Disconnect the DMM from R30.
Connect the internal frequency counter probe to the BFO test
point, TP2 (right side of the RF board, near the crystal filter).