ELECRAFT K2 BUILDER ALERT #16
Correcting K2 Frequency Shift on Transmit
26 February, 2002



APPLICABILITY
This builder alert applies to ALL K2s, and all revisions of K2 firmware prior to K2 firmware version 2.02E. 

Update as of June 5, 2002: All currently shipping K2s, and K2s with serial numbers 2696 and later have shipped with f/w version 2.02E (or later) and do not need this modification. (The problem is corrected in the firmware.) You may also have received 2.02E with a recent option addition to your K2. You can check the K2 f/w version by powering up the K2 while pressing any front panel key. The version of the main K2 firmare will appear on the left side of the LCD display.


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 microphone PTT closures. However, such a shift can also occur in SSB modes when PTT is being used. It affects ALL modes of operation (CW, SSB, data), with or without SPLIT enabled.

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 makes 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. (If your K2 has firmware version 2.02E or later this problem has been corrected in the firmware and you do not need to do this modification.)

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. If the KAF2 audio filter is installed, remove it and set it aside. Also remove the metal hex standoff used to secure the KAF2 to the Control Board.

  4. Remove the two screws holding the Control board to the Front Panel board.

  5. Remove the four screws from the front panel module.

  6. Carefully 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.) 

  7. Referring to Figure 1, on the back of the front panel PC board, locate pin 11 of U3 (the 20-pin IC. near the shaft encoder, Z1). Note than since the IC is now backwards from its normal position, pin #1 it at the top right corner of the 20-pin configuration, pin #10 is at the bottom right, and pin #11 is at the bottom left of the twenty IC pads..

  8. Referring to Figure #2, 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 Figure #1 causing any damage to the IC.)

  9. Plug the front panel module back in and secure it with four screws.

  10. Secure the front panel board to the control board with two screws.

  11. Power up the K2 and make sure the encoder and other front panel functions are working normally.

  12. Re-install any options and the top cover.

Figure 1

Figure 2


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).

Thank you to Tom, N0SS, for formatting this document.