[CW] Eric KE6US on Scratchy Dits

[D.J.J. Ring, Jr.]

Eric KE6US said on the SKCC email list.  (Stolen blatantly by DR.)

The discussion mostly was about a particular electronic keyer cleaning up
the output when a bug was used.

However, this circuit works with varying success on bugs I've tried it
with. I bought it several years ago to try, but I don't actually use it
with any of my bugs:

http://www.wb9kzy.com/bugde3.htm

Silver tarnish is not silver oxide. Silver does not react with the air we
breathe. Silver oxides can exist, but not from contact with air.

What we call tarnish is silver sulfide. It is conductive, but not near as
conductive as clean silver. At the low voltages and currents in the keyer
path of modern transceivers, it is a significant problem.

As I said in the post you quoted, gold is not as good a conductor as silver
(it's third after copper), but it doesn't tarnish so that is the best
material to use for keys intended for modern transceivers, just as Begali
has done on your paddle. Platinum would be good. Some relays use palladium
(chemically related to platinum), and I think it's used a lot for
electrodes in ICs.

Just for fun, I looked up market prices. You can see at a glance why silver
is the most common on relays and keys.

Palladium ~$1900/oz

Gold         ~$1700/oz

Silver       ~$17/oz
It transfers exactly to keys. They are manually operated relays/switches.
No difference between an open frame relay and an open frame key. Also
depends on the service. There are relays meant to switch high voltage high
current loads and there are relays mean to switch low voltage, low current
loads. The demands are different. Same with keys except keys are made by
traditionalists, not engineers. The guy who bought Vibroplex a few years
back  builds them exactly the way they were built in the 1920's at the dawn
of wireless communication when keys could carry lethal voltages. Very
little has changed in key design since then, especially in relatively
inexpensive ones. I can't think of anything relating to the contacts that
wouldn't make your relay expertise equivalent to key expertise. In fact,
you should share that experience. The only different is one is
electromechanical actuation and the other is manual. The electrical side of
it is identical.

OK, I posted how to clean the contacts to reduce scratchiness. Let me tell
you how to make your contacts really scratchy. Here's your worst case list
of things to do:

1. Use a modern solid-state rig. My K2 runs about 600 microamps through the
contacts. That’s not always enough to punch through the buildup of crud on
the contacts. The crud is only microns thick, but it’s there and causes
scratchiness if it isn’t removed by the key design or manual cleaning. We
had the problem with tube rigs, but for other reasons. Silver is the best
conductor of all, but it is LOUSY in low voltage, low current (say, less
than 25 volts or so) circuits. The silver sulfide tarnish (no, it isn't
silver oxide) conducts, but not as good as pure silver and it isn't that
good at low voltage/current. Gold isn't as good a conductor, but it doesn't
tarnish so it's best for low voltage/current. But who can afford that?

2. Big contacts: The bigger, the better--less resistance, right? The
problem isn’t electrical resistance. It’s the physical size. Larger
contacts have a lower contact pressure per square inch. They don’t develop
enough pressure to punch through that crud. They also spread any wiping
action over a larger area making it less effective. That’s another source
of scratchiness. How should it be done? The Begali Intrepid bug ($500+)
uses a small pointed spring-loaded contact against a flat one. Lots more
pressure at the point of contact. That’s a bug designed in the modern age,
not the Golden Age of high voltage/current tubes like almost every other
bug out there.

3. Perfectly aligned flat contacts. Seems like that should be a good thing.
It’s not. There’s no wiping action at all, The contacts collide and bounce
back off whatever crud is on the contacts. More scratchiness. That’s why
many contacts are rounded, not flat. Better still, they set them to contact
off-center, not perfectly aligned. When they make contact, they ramp down
against each other wiping off the crud and exposing clean metal with each
closure. Silver is soft and responds to a wiping action.

Sometimes scratchiness can be eliminated by turning the stationary contact
post a few degrees one way or the other so the contacts intentionally don’t
hit square against one another. It adds some wiping action that will keep
them cleaner longer. Easy to do on a bug or cootie. Not so easy on a
straight key.

There isn't much guidance from old telegraph books. They didn't have to
deal with low current loops. But modern relay manufacturers do deal with
it, and have been for many decades. They learned all this stuff and passed
it on. Not a lot of hams hear of it.

Fact is, you shouldn't use ANY abrasive on key contacts. Not even paper.
That's one of the reasons for persistent scratchiness. Solid metal contacts
will hold up a little better, but some are just plated and that comes off
easily. No paper. No metal polishes. Brasso is for belt buckles and ship's
bells. Keep it for polishing those.

The scratchiness problem isn't tarnish. It's DIRT. You don't abrade dirt
off, you clean it off. One thing that will work is GLOSSY PAPER about as
thick as a magazine cover. Glossy paper is glossy because it has a fine
coating of clay applied under high pressure. Use glossy paper with no ink
on it. It should be pure white. Get a sample from a local copy center, and
use that.

Soak a strip of it in contact cleaner (non-abrasive) and run that between
the contacts. That will remove the microscopically thin layer of dirt and
debris that is causing the scratchiness. The clay gently polishes the
contact. The clay also binds the paper fibers so they aren't deposited on
your contacts.

When you're done, remove all traces of the contact cleaner from the
contacts with alcohol. You want clean dry contacts. No lubricants even if
they're made for contacts. That might be OK for an enclosed switch. But
with the open frame design of a key, lubricants (and probably currency ink)
attract more microscopic dirt and debris that cause the problem in the
first place.

We're not cleaning up the points on a 1960 Ford Falcon to get it running.
We're trying to get precision closures from a fine instrument.

Eric KE6US

CRAIG GØKVL

I use this circuit which works fabulous, it's based on a 555 timer, what
looks like pin 7 & 8 at the top is actually pins 4 & 8.[image:
db555schematic.png]
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