[ARC5] Etching Crystals and WHINK Revisited - contact resistance.
Kenneth G. Gordon
kgordon2006 at frontier.com
Tue Nov 4 16:49:32 EST 2014
On 4 Nov 2014 at 8:33, David Stinson wrote:
> I started with a 3146 KC Marine crystal. Put about an
> ounce of WHINK in a sealed 2" square plastic container
> and in with the blank.
> Ken had reported 70 cycles a minute.
Yes, but that was a WWII FT-243 and quite old. I have been pretty certain for
along time that these had been ground to frequency. Every one I have
etched with Whink really "took off" at first. Then as the process proceeded, it
slowed down....drastically. By this activity, I suspected that, as Dennis
mentioned, the surfaces were just full of dirt or fragments of crystal "dust"
which was dissolved first.
I suspect your Marine crystal had been etched to frequency, not ground, and
was therefore "clean" to begin with.
In order to move those along a bit faster, you need a much stronger solution
of HF. Although the Whink will move it, it will be very slow.
Or, you could grind it "up" towards the frequency you want, THEN, when it is
within, say, 5 Khz, use Whink to thoroughly clean it and bring to your final
frequency.
One more thing concerning our old crystals which I learned from a recent
article in Electric Radio Magazine: usually there are two plates, usually
copper, which have "tails" which are soldered into the pins. These plates
contact two other metal plates with raised edges which then sandwich the
crystal between them.
These other plates look like plain steel to me, but they MAY be stainless
steel.
What the author of the other article discovered is that there is a very thin
layer of some sort of non-conductive oxide that has built up over the years
on the surfaces of the steel plates which SEVERELY reduces the good
contact between the two copper plates and those two steel plates, raising the
crystal's " internal resistance" very high, and in some cases, absolutely
preventing the crystal from working at all.
He discovered that thoroughly cleaning the surfaces of the steel plates and
of the copper plates, using a hard plastic "scrubby pad" and some sort of
liquid, alcohol, maybe, vastly lowered the contact-resistance and crystals
which wouldn't work at all before he used this procedure, became very active
and worked just fine.
It is certainly something to consider.
Ken W7EKB
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