Modifications to bring the Elecraft K2 up to date with serial numbers 3000+

Part 2 of LA3ZA Unofficial Guide to Elecraft K2 Modifications

Starting with serial number 3000 in October 2002, a major update to the K2 transceiver took place. The main modifications required to bring an older K2 up to date are described in Elecraft’s memo and in the older short list. The numbers in the second last column below refer to the modification step in the first memo "Upgrading the Elecraft K2 to revision B". It is recommended that the order of that memo is followed when modifications are done.

The Elecraft kit of parts for these modifications is K2ATOBKT. In addition one should get the BFOMDKT,  E850093, and the XFILMDKT as indicated below (Updated April 2014).

The following table shows these modifications in an alternative view, adds some minor modifications taken from a comparison between the schematics, and offers some of the background for the modifications as well.

Basic K2: Improved Performance

Reference Description Comment Complexity Elecraft mod. step. Source
× Better grounding of  crystals X7-X11, see Assembly, Part II in manual rev I, page 54. Improved IF stop-band attenuation Important, real perceivable improvement of receiver’s stopband attenuation Tricky soldering of ground wires to case of crystals. Risk of ruining a crystal. 8. First Crystal Filter Grounding
× Rewiring from balanced to unbalanced audio pot Removes scratch noise. Important for removing the nuisance of a scratchy AF pot, also gives an AF control with smoother characteristics, and makes it possible to turn AF completely off. Requires
modifications to all three circuit boards.
9. AF GAIN Control Smoothness
¤ Modification of sidetone circuit More sinusoidal waveform for easier zero-beating Filters out harmonics of otherwise square wave sidetone. See also TF3MA’s improved filter 1 resistor, and an inductance (47k + 82mH) 10. Misc. Control Board Changes
× More accurate current metering Some
RF gets into current sense circuit on 160/80 m
Simple to do (control board) Make C42 0.1 µF (100 nF) 10. Misc. Control Board Changes Elecraft
(There was a C44 parallel to R7 up to serial #3077, but it should be removed)
× Reduced
temperature drift in BFO
Replace L33 with one wound on T44-7 core // 5.1M resistor Reduces BFO temperature drift to about 25%. New L33 can be
purchased from Elecraft
(BFOMDKT) Updated April 2014
5. PLL and BFO Temperature Stability
× Reduced
temperature drift in PLL reference oscillator
Remove X2, parallel RP2 with R19=2.7k, check that pulling range is >10 kHz and recalibrate Reduces PLL/VFO temperature drift to less than half.

Note that a new TCXO kit, E850138, that improves this further is available Updated April 2014

Note that a too low value for R19 was recommended for a while 5. PLL and BFO Temperature Stability
× Too sensitive AGC AGC activates on noise only Need for this mod is greatest for K2’s with an 8V supply on the high side Decrease
R1 on control board or use a pot to adjust U2 pin 5 to 3.90 V.
10. Misc. Control Board Changes
× Improved
version of 2. stage crystal filter mod
'Flatter' SSB filter response Important, gives real improvement of SSB receive quality Remove one C, add 2 Cs, 2 Rs, and a transistor
Updated April 2014
7. Second Crystal Filter Flatness KI6WX
× Interaction
between VFO and PLL reference oscillator
Weird behavior around 7185 kHz. Add L and C to Vcc of MC145170 Important and easy to do Add two components (Replaced RFC11 may be reused)
2. PLL Frequency Stability at 7182 kHz


Basic K2: Intermodulation, image rejection, spurious output

Reference Description Comment Complexity Elecraft mod. step. Source
× Change of RFC11 to a toroid   Less radiated RF from post-mixer amplifier into IF signal path The first measure to try against intermodulation from strong stations

3. Misc. RF Board Changes

× Change 15/17m band filter
Narrower bandpass filter, better stopband attenuation
Reduces intermodulation from 18m broadcast band, also increases image rejection in 15m band (from 25m broadcast band)
C30, C33, C36
4. 15/17 meter and 20/30 meter Band-pass Filters
× Change
12/10m band filter

bandpass filter, better stopband attenuation
Decreases spurious output, and increases image rejection in 12m band. Makes the 10m band 1.2 MHz wide (e.g. satellite operation will suffer some sensitivity, See alternative 10m filter options)
Change C42, C45, C48
6. VFO ALC and 10/12 meter Band-pass Filter
(Included with KPA100)

×VFO ALC mod Improved Automatic Level Control in VFO for reduced spurious output
Change/add a few components
Mod kit E850093
Updated April 2014
6. VFO ALC and 10/12 meter Band-pass Filter
with KPA100


Basic K2: Miscellaneous

Reference Description Comment Complexity Elecraft mod. step. Source
× DC blocking capacitor for the aux RF connector
Prevents damage to the output stage if the battery or aux power connector is plugged into the RF connector by mistake Cut PCB trace and put a 0.047 µF (47 nF)/50V cap in series with aux RF connector. 3. Misc. RF Board Changes

(Capacitor included with KPA100)

× Low-drop reverse polarity diode for D10 Improves tx efficiency and allows use of lower supply voltage for charging battery

Replace D10
3. Misc. RF Board Changes
× Series resistor for ESD protection of micro-controller VRFDET input. 820 ohms series resistor from pin 10, P4 to pin 2, U6.
Cut one trace on PCB 10. Misc. Control Board Changes
× Improved Aux bus filtering for increased noise immunity Improved filtering out of MCU and into RF board IOC Prevents aux bus problems with KDSP2 + KAT100 for e.g. KNB2 Change R6 to 100 ohms, C12 to 0.0027 uF (ctrl), R64 to 100 ohm (rf) 3. Misc. RF Board Changes & 10. Misc. Control Board Changes
Improved keyer noise immunity. Improved auto-detect for straight key.
× Changed some time before #3000 Change R1, R2 on RF board to 220 ohms 3. Misc. RF Board Changes
Eliminate alignment difficulty on 20m 12 pF in parallel with 20/30 m band-pass filter trimmers
Add two capacitors 4. 15/17 meter and 20/30 meter Band-pass Filters
Sidetone source is permanently at U8-4 + added RS232 TxD line Sidetone source had previously to be changed from control board U6-25 to U8-4 for the KIO2/KPA100 U8-4 is now sidetone source and U6-25 is TxD line as required by the KIO2 and the KPA100 Connect U8-4 to gate of Q5 + connect U6-25 to P4-1 (Aux I/O) and bypass w/ 0.001 uF. 10. Misc. Control Board Changes
Change 40m bandpass filter to combined 40/60m operation + add varactors to VFO Adapt LP filter and tuning range of VFO to combined 40 and 60 m operation
Ready for 60m option K60XV.
Change C225, 226, 227 and L25 and L26. Replace C70 with D19, D20 and R29 on RF board
Not in list

× Replace voltage selector switch on control board with jumper block
It was too easy to move the switch by accident, and then lose the power supply voltage reading Simple to do. Not in list 
R60 100 ohm pot replaced with 100 ohm resistor
There was no need to adjust bias of PA stage, so the pot is not required.
Not in list

Items marked with × are implemented in my K2, while ¤ denotes modifications that I have tried, and then removed.

First version 8 Nov 2002, last update 9 April 2014.

73, Sverre Holm, LA3ZA

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