[Elecraft] K2 CW TX frequency change: possible fix

[Wayne Burdick]

"Edmund H. Ramm" wrote:

> After I moved the VFO knob a tiny fraction the frequency crawl stopped
> regardless of how much I cared to hammer the trusty old Junker M.T. And
> I couldn't reproduce the phenomenon by turning the VFO knob back by what
> felt to be the same amount.


Hi Eddi,

Your posting was very helpful. I have been working very hard trying to
duplicate this problem on my lab K2--no luck so far. 

One of my theories has been that our off/on cycling of the encoder's
ground pin could be compromising its built-in hysteresis. We turn the
encoder off/on in receive mode to reduce RX mode current drain. But if
you were right on an edge (which is very difficult to find), you could
see the encoder tick in one direction or the other due to the slight
change in supply voltage or load that might occur during transmit.

Your observation about moving the VFO slightly stopping the up-tick
lends credence to this theory. And this may also be why I can't
duplicate the problem: my lab K2 is running experimental firmware that
leaves the encoder turned on at all times!

I would like to ask you and others who have noticed this problem to try
a very simple modification to the K2 front panel board. This mod will
keep the encoder powered on at all times, increasing current drain
slightly in receive mode. If encoder on/off cycling is causing the
problem, this will cure it.

Here are the modification instructions:

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 screws from the front panel module, and unplug it from the
RF board.
   (NOTE: 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
(TPIC6B595 or similar).
   (This pin goes to pin 4 of 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. Power up the K2 and make sure the encoder and other front panel functions
   are working normally.
8. Re-install any options, then the K2 top cover.

Please let me know right away if you see the problem continue to occur
despite this modification. If the mod *does* fix it, we'll make it
permanent, either in hardware or firmware. One possibility is to only
power-cycle the encoder in OPT=BATT mode, and only after a long period
of VFO inactivity. But the saved current drain would be minimal.

Thanks,
Wayne
N6KR