[Elecraft] K2 Frequency shift on transmit: FIXED

[Wayne Burdick]

K2 BUILDER ALERT: Correcting K2 Frequency Shift on Transmit

APPLICABILITY

This builder alert applies to ALL K2s, and all revisions of K2 firmware.
It affects ALL modes of operation (CW, SSB, data), with or without SPLIT enabled.

BACKGROUND

Some K2 operators have reported seeing the operating frequency shift up
or down one VFO step at a time (10, 50, or 1000 Hz) as the rig is keyed.
It may move continuously in one direction as long as keying continues.
This symptom has only been reported in CW mode, probably because CW key
closures happen much more frequently than mic PTT closures. However,
such a shift can also occur in SSB modes when PTT is being used.

We have now identified the cause of the problem: on/off cycling of power
to the optical shaft encoder. If the encoder is positioned such that it
is on the border between two counts, this power cycling can cause the
encoder to lose its hysteresis (analog position memory). When this
happens in conjunction with keying of the DOT or PTT line, the small
amount of noise injected into the encoder's logic circuitry can cause
the encoder to switch states. This make the firmware think the VFO knob
has been moved. The reason why the condition is so rarely seen is that
it is extremely difficult to position the encoder on one of these count
boundaries. 

Fortunately there is an easy fix for this in hardware. We recommend that
all K2 owners make the simple modification described below. A single
short jumper must be added to the Front Panel board. 

MODIFICATION INSTRUCTIONS

NOTE: These instructions will be posted on our web site soon, and we'll
be adding a photo showing exactly where to install the short jumper. If
you're anxious to make the mod right away, just make sure you have
correctly identified U3 pins 11 and 14.

1. Turn the K2 off. Also set the internal battery switch to OFF 
   if applicable.
2. Remove the top cover (disconnect all option cables).
3. Remove the four screws from the front panel module, as well as 
   the two screws holding the control board to the front panel board. 
   Then unplug the front panel module from the RF board. (There's 
   no need to remove the front panel knobs or separate the board from 
   the sheet metal.)
4. On the back of the front panel PC board, locate pin 14 of U3 (the 20-pin
   I.C. near the shaft encoder, Z1).
5. Install a short jumper between pins 11 and 14 of U3. (This will 
   permanently enable the shaft encoder. Pin 14 is an open-drain output, 
   so it can be shorted to ground without causing any damage to the IC.)
6. Plug the front panel module back in and secure it with four screws. 
7. Secure the front panel board to the control board with two screws.
8. Power up the K2 and make sure the encoder and other front panel functions
   are working normally.
9. Re-install any options and the top cover.

TECHNICAL NOTE

Power to the optical encoder was cycled on/off to save a few milliamps
of receive-mode current drain between encoder reads. However, encoder
reads are done at a fast rate anyway, so the current saved was only
about 10-15 mA (varying somewhat from unit to unit).