[Elecraft] K1 pot transplant - don't try the 3541H...

[Wallace, Andy]

My K1's tuning was fairly solid with the original Bourns 3590 
ten turn pot installed. But in looking over the email list archives,
I saw that people recommended the metal shaft 3540 pot, about 
$13, and worth trying if you want an improvement in shaft play
and runout and other nits to pick...

Well, being smarter than the average bear, I did some looking at
Bourns' site:

http://www.bourns.com/pdf/3540.pdf

...and found that the recommended 3540 has a 3541 big brother which
has a so-called HYBRITRON element. I then saw that 
www.mouser.com has them both, and the 3541 Hybritron is about
$30... Well, says I, that must be so much better than the 3540. I'll
get 'em both and see...

Don't try this at home!

The 3541, which Bourns told me had an element made of conductive plastic
(as opposed to plain wirewound 3540/3590). The problem this seemed
to cause, once I installed it, was that at the high end of the bands things
would
get jittery and noisy when tuning -- almost like the good old days when the
wiper
contact of a variable cap got dirty. When you stop tuning, things are fine,
but I
did not like the garbage that was heard while tuning.

So I took it out, installed the $13 3540, and have set the 3541 aside for
some 
head scratching. My speculation is that the plastic element -- at the limit
of
travel -- tends to make resistance variations more pronounced when used in a

circuit like a VFO. The wirewound contact wiper doesn't experience quite so
much jitter as it's tuned, probably because the wiper is touching at least
two
windings at all times. This was disappointing as the 3541 DOES feel even 
more smooth and solid than the 3540. I emailed Bourns with my concern and
a request for help, but they never answered. Perhaps I can rig up the 3541
to
a battery and an oscilloscope to see what happens when it's near the limits.

It could be that the effect could be reduced by putting a small cap in the
circuit
to smooth out the tuning jitter. 

Just thought I'd pass this along in case someone else is as "smart" as I am!

-Andy

(Here's the 3590 specs: http://www.bourns.com/pdf/3590.pdf )