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Author Topic: Optimize your key-contact characteristics!  (Read 1257 times)

Posts: 57


« on: November 23, 2004, 06:49:39 AM »

   At FDIM last year, I was displaying my Begali
Magnetic Classic paddle, which led to a meeting and
discussion with Bob Crane, W8SX. After the requisite
Hogan/Schultz banter, I learned that Bob's field was
involved with the characteristics of surfaces in
low-force electrical contacts! Son of a gun!

   A fascinating discussion ensued, during which I
shared that it seemed that some days, I could set my
paddle contact gap so close that no movement, or
"click," could be perceived, while on other days they
had to be much farther apart. He said that studies had
shown that the surfaces involved were often very
rough, and somewhat flat, and that their
characteristics were affected by changes in
atmospheric pressure, temperature, humidity, and other
variables. Made sense to me.

   A couple of months later, Bob wrote to me about his
work on improving the contact characteristics on his
Magnetic Classics, and here's an excerpt:

  "...I decided to look at the contact surfaces
carefully. What I saw on all of them was surprising,
to say the least. They were all very rough from a
quick machining process. In fact, I estimate that they
were too rough to make a consistent electrical
contact. In addition, the surfaces on the post
contacts were machined flat. I carefully used some old
metallographic polishing paper and created a smooth,
to a 5x magnifier, hemispherical surface. I also
smoothed the surfaces on the contact on the paddle
arms. This made a big difference in the consistency of
the keying characteristics of the paddle...These
contacts are where the action is and should be
hemispherical in shape and have a mirror-like finish.
If these two conditions are met, then the electrical
contact characteristics of the paddle would be much
improved and keying would be consistent...The most
important thing that I have learned over the past
couple of years in our work on low-force contacts is
geometry of the contact surface is of paramount
importance to their performance."

   A bit later than that, at my request, Bob sent me
three small swatches of polishing paper: fine, really
fine, and practically smooth, along with directions
for using them. Finally, last night, I had a chance to
give it a try.

   My present paddle is a Begali Signature Edition. I
had been hesitant to try to disassemble it, for fear
I'd ruin it, but my fears were unfounded. I found that
one screw released each contact post, and I was able
to find the ~2mm ball-bearing from the first one,
after it skittered away on the floor. Using a 10X
loupe for magnification, I carefully set to work,
beginning with the coarsest paper, and trying for a
slightly convex surface.

  After a bit of pleasant work on all four contacts, I
had really altered the surface topography of the
contacts! I didn't have the right stuff to get a
mirror finish, but I did change them from flattish,
rough-looking surfaces to smooth, slightly rounded
ones. The contacts on the arms were different from the
ones on the posts: larger and flatter, they had a sort
of a raised spiral on their surfaces. I don't know if
I was able to round them much, but I was able to
noticeably smooth their surfaces, and possibly
increased the available contact area by widening the
lands of the spiral.

      The entire process didn't take much time, and
when I reassembled the paddle and tried it out, the
result was immensely gratifying.  I could adjust both
gaps as close, or closer, than ever before, and I
expect that there will be far less variability in the
minimum gap possible from changes in day-to-day


Posts: 6

« Reply #1 on: August 21, 2007, 07:08:39 PM »

I just finished a FB qso with Bob (W8SX 01:35Z 40m 7040.00khz CW 8/21/07) and we were complimenting each other on the sound of our rigs. I am using a Vibroplex Brass Racer. Should have asked him about the contacts on it. Hee....
His little QRP 5 wts was punching a big hole in the ozone here to Iowa.
Next time I qso with him I will ask him about that.

Bill W0NBP
Ottumwa, Iowa
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