[Elecraft] K2 BFO drift problem

[TheMG .]

I've put a piece of paper under the bumper and re-installed L33 held
tightly down against the board (pushing the resistor down so it holds
it tightly).

No observed drift in frequency after a couple hours other than the
normal temperature-related drift (about 10Hz downward).

This does seem to confirm that the bumper conductivity was in fact the problem.

Looks like I will need a new rubber bumper. I'm not sure what happened
to this one to make it conductive, but it's definitely not something I
did as I did not use any cleaning chemicals when initially building
the K2 (not even alcohol).

I'm also considering just securing L33 in place with a dab of
electronics-grade silicone and leaving it at that. Although Elecraft
does not recommend it I don't see how it would be a problem if the
proper type of silicone is used that is non-corrosive, non-conductive,
designed specifically for electronics use. If it's good enough for use
in aircraft electronics... it ought to be fine for this. Of course, it
would make it slightly more difficult to remove L33 in the future if
for some reason it was ever necessary to remove it, but not
impossible. I'm not too worried about that.

73,
Mikael
VE8MT

On Tue, Apr 15, 2014 at 7:20 AM, Don Wilhelm <w3fpr at embarqmail.com> wrote:
> 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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>