LA3ZA Unofficial Guide to Elecraft K2 Modifications

Basic K2: Improved Performance

× Better grounding of  crystals X7-X11

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.

KO0B, KI6WX

× 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.

Elecraft

¤ 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.

WJ4P

×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.

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

5.

Elecraft

×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 is also available.

Note that a too low value for R19 was recommended for a while

5.

×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.

Elecraft

×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

7.

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.

Elecraft

Basic K2: Intermodulation, image rejection, spurious output

×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.

Elecraft

×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)

Change C30, C33, C36

4.

Elecraft

×Change 12/10m band filter

Narrower 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.

(Included with KPA100)

Elecraft

×VFO ALC mod

Improved Automatic Level Control in VFO for reduced spurious output

Change/add a few components

6.

(Included with KPA100)

Elecraft

Basic K2: Miscellaneous

× 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.

(Capacitor included with KPA100)

Elecraft

× Low-drop reverse polarity diode for D10

Improves tx efficiency and allows use of lower supply voltage for charging battery

Replace D10

3.

Elecraft

×Series resistor for ESD protection of microcontroller VRFDET input.

820 ohms series resistor from pin 10, P4 to pin 2, U6.

Cut one trace on PCB

10.

×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. & 10.

Elecraft

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.

Elecraft

Eliminate alignment difficulty on 20m

12 pF in parallel with 20/30 m band-pass filter trimmers

Add two capacitors

4.

Elecraft

>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.

Elecraft

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)

Elecraft

×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)

Elecraft

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)

Elecraft