[sf-lug] SF-LUG meeting notes for Sunday December 3, 2017

[Michael Paoli]

> From: jim <jim at well.com>
> Subject: Re: [sf-lug] SF-LUG meeting notes for Sunday December 3, 2017
> Date: Thu, 7 Dec 2017 03:43:13 +0000

>
>     600VDC? with a 10MOhm resistor. Great
> tip!

And really only a "great tip" if one knows what the heck one is
doing, and also only quite applicable to the repair of a
sealed magnetic reed relay switch - where one does not and
cannot have physical access to the electrical closure contacts
of the switch itself.  If one can actually reasonably get
access to the physical electronic switch closure contact
points (or similar for relay), then there are generally other
highly preferable methods for dealing with non-conductivity at
physical closure on contacts due to oxides or pitting or the
like.

Random: that sealed magnetic reed switch keyboard - probably only
keyboard I have that *may* be safe to use in an explosive gas
environment.  *May*?  Well, that would also depend upon additional
factors such as other materials used (e.g. sufficiently anti-static /
static resistant materials, etc.).  With magnetic reed relay switch,
the actual switch closure contacts are inside a sealed glass envelope,
so no environmental exposure on those, thus the slightest of potential
spark there is isolated from surrounding environment ... no KABOOM! from
switch contacts opening/closing in explosive gas environment.
And remember, smell gas?  Don't switch anything on or off - and that includes
most flashlights, etc., and certainly don't strike a match or lighter or
the like.  And also take precautions to avoid static discharge.
Most all switches aren't sealed like magnetic reed relay switches.

https://en.wikipedia.org/wiki/Reed_switch

Another type likewise sealed:
https://en.wikipedia.org/wiki/Mercury_switch
But those are much less common these days - notably due to their
containing mercury (toxic).  Also, just because you have an older
"silent" type switch, doesn't mean it's a mercury switch or
the like - some "silent" type switches just use a much quieter
switch closure mechanism, and no environmental isolation of the
switch contact points.  But *some* older "silent" type switches
may in fact be mercury switches.  Sometimes they're also used
for their gravitational dependency/orientation - e.g. as an
anti-tip switch to shut off an electric heater if it's knocked
over.  But probably relatively useless for, e.g. keyboard,
but magnetic reed relay switches work fine there ... though
not a particularly economical way to manufacture a keyboard
certainly at least by today's standards and technologies.

> On 12/07/2017 03:23 AM, Michael Paoli wrote:

>> oh, and I did have one key on a Cromemco 3102 terminal which I'd received
>> which didn't work (magnetic reed relay switch ... and ... how'd I repair it?
>> switch physically closed contacts, but not electrically - sealed in a glass
>> envelope - apply a rather high voltage (about 600+ VAC) very limited current
>> (about a 10+M Ohm resister in series) ... burnt off whatever (oxide?) was
>> preventing electrical contact, without welding the contacts together - has
>> worked fine ever since) ... and after that on the Zareason,