Elecraft K2 PLL User Manual
K2 Temperature-Compensated PLL Reference Upgrade
Rev. E, March 24, 2005
Description
This upgrade kit significantly improves the stability of the K2’s VFO, reducing drift per unit of
temperature by as much as 90%. It is especially useful with a K2/100, since operation at higher power will
cause faster heating inside the cabinet. The circuit, designed by John Grebenkemper, KI6WX, uses a
thermistor to temperature-compensate the varactor diodes in the PLL reference oscillator.
All parts used in the upgrade are installed on a small PC board that substitutes for resistor network RP3 on
the RF board. (See photo of installed thermistor board on page 4.) You can assemble this small board now
(see page 2), or when it is needed during assembly of the K2. All related manual changes are shown below.
If your K2 is not yet assembled, make the following changes to the Owner's manual:
On page 47, right column, last step: Make a note to install the thermistor PCB assembly at the
location of RP3, rather than the original resistor network.
On page 50, left column, first step: Make a note to use only the 12.096 MHz crystal supplied with
this upgrade kit, which is marked ECS 12.09-CD (there may also be a lot number, which can be
ignored). Your K2 may have been supplied with 1 or 2 crystals of a different type. Only one crystal is
now needed (X1). X2 will not be used.
On page 54, right column, second step: Make a note to substitute a 12-H inductor (supplied with
this upgrade kit) for the original 10-H unit supplied with your K2 for use at L31.
If your K2 serial number is lower than 3000, you should first install the BFO mod kit (Elecraft part
number BFOMDKT). The BFO modifications are already present in K2s with serial number 3000 and
higher. NOTE: The BFO mod kit originally included a 12.096 MHz crystal for use in the PLL reference
oscillator. This crystal is now supplied with the PLL upgrade kit. However, if you installed the crystal from
the BFO mod kit, you will probably not have to remove it. This is covered in the PLL upgrade kit
instructions.
Upgrade Kit (E850138) Parts List
Ref Description Qty Part #
RF-L31 12H Shielded Inductor 1 E690019
RA 33K 1/4W Resistor (org-org-org) 1 E500057
RB 12K 1/8W Resistor (brn-red-org) 1 E500089
RC 2.2K 1/8W Resistor (red-red-red) 1 E500088
RD 1.8K 1/8W Resistor (brn-gray-red) 1 E500087
RE, RF 10K 1/8W Resistor (brn-blk-org) 2 E500092
THM Thermistor, 10K, 3% (small orange body
with black and brown stripes near leads)
RF-X1 Crystal, ECS 12.09-CD 1 E850007
MISC Thermistor PC Board 1 E100152
MISC #24 Green Wire, Insulated 1 ft (30
1 E500091
E760008
cm)
Thermistor PC Board Assembly
Locate the thermistor printed circuit board. One side will be labeled with THM and the letters A through F. All
components will be installed on the labeled side, and soldered on the other side.
Install 1/8th-watt resistors RB, RC, RD, RE, and RF in their respective positions as shown on the board. (Be sure
to check the color codes carefully.) Do not pull on the resistor leads, as they may be fragile. Solder all of the leads,
then trim off as much of the excess length as possible.
Install resistor RA (33 k, 1/4 watt). Solder and trim its leads.
The thermistor has an orange body with black and brown stripes. The thermistor is fragile, so avoid bending its
leads any more than necessary. Insert the thermistor at THM (the part is symmetrical and can be installed in either
orientation). With the thermistor body touching the circuit board, solder and trim its leads.
The remaining 8 holes on one edge of the board are used as leads to replace RP3. Strip a 6” (15 cm) length of
green hookup wire. Cut the bare wire into 8 pieces of about 0.75” (2 cm) in length. Solder each wire into one of
the holes so that most of the lead protrudes from the component side.
On the component side, bend each lead to a 90-degree angle so that it points away from the board.
The circuit board is now complete and ready to be installed on the K2 RF board.
Thermistor board installation at time of K2 assembly
Complete K2 assembly through page 47 of the manual.
Locate the position of resistor network RP3 on the RF board. Slide each of the 8 leads of the thermistor board
through the corresponding holes at RP3. The components on the circuit board should be facing U6, and the
thermistor should be oriented toward the position of crystal X1. (See photo, page 4.)
Once all of the leads are inserted, slide the board down so that it is flush against the RF Board. It may be
necessary to bend the thermistor board toward U6 so that the solder joints on the bottom of the RF board can clear
C87. The thermistor board and its parts should not be touching U6 or its pads.
Make sure that the edge of the thermistor board is contacting the RF Board along its full length, or there may not
be enough clearance for an audio filter option (which mounts on the Control board). Solder the eight connections
on the bottom side of the RF Board.
This completes installation of the PLL reference oscillator upgrade. Check your work to make sure there are no
shorts between the thermistor board and other components.
Continue with K2 assembly and test as described in the K2 owner's manual. You will not use the Test
instructions in this document. However, you should keep this instruction sheet for reference. You may wish to read
the Theory of Operation section (page 4).
Thermistor board installation in an assembled K2
Remove the top, bottom, and left side covers from the K2.
Remove the Control Board from the K2.
Locate inductor L31, which is near the left front side of the RF Board, mounted on the bottom side. Cut the
inductor's leads. Straighten the leads with long-nose pliers, then use a soldering iron to remove the leads from
each hole. Do not pull on a lead until after the solder has melted; otherwise you may damage the plated-through
hole in the PC board. Note: The holes for L31 and other components to be removed will be cleaned out later.
Locate crystals X1 and X2, which are mounted on top side of the RF Board near L31. Only X1 will be present if
your K2 is s/n 3000 or higher, or if you installed the crystal supplied with the BFO mod kit.
If your board already has only one crystal (at X1), check its part number. If the part number contains ECS 12.09-
CD, you can skip the instructions below that deal with removing and replacing the crystals. (There may also be a
lot number on the crystal, which may be ignored.)
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