[Elecraft] K2 BFO drift problem

[TheMG .]

Wow, you'll never believe what I just found out!

There is absolutely nothing wrong at all with L33. It turns out it's
the rubber bumper that is at fault!

I found this out by pressing down on the stem of the bumper with the
plastic tuning tool, instead of pressing on the inductor. Guess what?
Frequency started to drift!!!

On a hunch, I decided to get my multimeter and measure on the bottom
of the rubber bumper. It's slightly conductive! Starts at about 3
megohms and if I hold the multimeter probes in place the resistance
slowly starts to drop down to 1 megohm and below. After a couple
minutes holding the probes in place about 1/4 inch apart on the bottom
of the bumper, the resistance got down to 500k!

Since the solder pads for X3 BFO crystal are underneath the rubber
bumper, it is effectively like putting a resistor in parallel with the
crystal, which obviously has undesirable effects on the operation of
the BFO oscillator.

The slowly decreasing resistance as the meter probes are held in
contact with the rubber is consistent with the fact that the BFO
drifts over time.

I suspected contaminants on the surface of the rubber, but after
cleaning it with iso alcohol, there is no change. Something in the
rubber is causing it to be conductive.

I wonder if somewhere out there, there are hams with K2s experiencing
BFO drift related to the same cause? If I got a conductive rubber
bumper in my kit, it's highly likely someone else has also.

Anyways, it's getting a little late right now. I'll communicate my
findings to Elecraft tomorrow (in case they aren't already reading
this).

73,
Mikael
VE8MT

On Sun, Apr 13, 2014 at 9:50 PM, Don Wilhelm <w3fpr at embarqmail.com> wrote:
> Get a replacement L33 and see if that takes care of it.  You might wan to
> request a replacement mounting resistor as well.
>
> 73,
> Don W3FPR
>
>
> On 4/13/2014 9:23 PM, TheMG . wrote:
>>
>> Leads under L33 are flush cut as flat as possible and it is mounted on
>> the rubber bumper with half the stem cut off exactly as per the
>> instructions. The resistor was pushed down into the center to keep L33
>> secure and to prevent the resistor body from being pressed on by the
>> bottom cover.
>>
>> No luck on resoldering L33 or the crystals underneath it. However I
>> did notice that when I pull up on the resistor so it isn't pressing
>> down on L33 (the inductor is at this point completely loose and free
>> to move around. I have the K2 upside down with the bottom cover
>> removed and it has been running this way for several hours with
>> absolutely no drift. BFO range is 4912.31 to 4917.85 and stays within
>> 0.01 of that as it warms up.
>>
>>
>