Elecraft K2 PLL - Manual (Page 2)

Page 2 of 4
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.)