[Elecraft] K2 BFO drift problem

[Don Wilhelm]

Mikael,

If you can, please do a test.  See if you can loosen L33 from the rubber 
bumper just a little by raising the bumper.  then slide a strip cut from 
paper stock (like a business card) between the board and the bumper.
See if the BFO still drifts.

If it stops, then your assumption is correct and you should get a new 
rubber bumper.

Good troubleshooting!

73,
Don W3FPR

On 4/15/2014 2:15 AM, TheMG . wrote:
> 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.
>>>
>>>
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