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ELECRAFT Application Note
K3 Synthesizer ALC
Revision A, December 23, 2008
Copyright © 2008, Elecraft, Inc., All Rights Reserved
Background
The K3's synthesizer has an ALC loop that holds the VCO output relatively constant over a wide range of
frequencies and L/C ratios. At high operating temperatures, on some bands, the ALC loop may not provide
adequate current to the oscillator transistor, Q1. This can cause the PLL to lose lock, resulting in ERR VCO
during VCO CAL, or ERR PL1/ERR PL2 messages on the LCD display when tuning or changing bands.
Reducing the value of resistor R10 from 82 K to 56 K establishes a higher minimum VCO current. This keeps
the VCO output above 1.0 V pp even at very high operating temperatures (tested to > 50 C). The result is that
the PLL never loses lock.
This modification requires some simple soldering to install a resistor on the KSYN3 board. You may install a
conventional leaded 180 K resistor in parallel with the existing SMD resistor R10 or, if you have the tools, you
may replace the SMD resistor with a 56K unit. A leaded 180 K resistor can be provided by Elecraft.
The KRX3 subreceiver has a separate KSYN3 synthesizer board. If your K3 is equipped with the KRX3, you
should modify both synthesizers.
Was the Change Incorporated in My K3 At the Factory?
You can check your K3 by removing the top cover and inspecting the KSYN3 board(s). If a fixed resistor as
shown in Figure 3 is present, that KSYN3 board has been modified. If the resistor is not present, the KSYN3
board may have a 56 K SMD resistor installed at R10. Check the resistance across the solder pads where the
leaded resistor would have been installed. If the resistance is near 56 K, this modification is not required. Boards
that need to be modified will show a resistance near 82 K.
Procedure
Observe ESD precautions when working inside your K3. Wear an ESD wrist strap or touch an
unpainted, metal ground frequently while working.
Remove the K3 top cover as shown in Figure 1.
Sometimes one screw binds when the other screws are tightened. If one screw seems too tight
to loosen without damaging it, first loosen the other screws..
Figure 1. Removing the Top Cover.
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