Good job of troubleshooting, Mikael. It's really
good you found this rather obscure problem. Who
would have guessed?
Perhaps there was a bad lot of rubber with
contaminants that you particular bumper was made from.
Mainly you now have the problem identified and can
correct it with a new part.
Phil w7ox
On 4/14/14, 11:15 PM, 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
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