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

[jim]

     yeah, but now the challenge for me is
to see if/how to apply some AC voltage to
the secondary and see what appears on the
primary. I'll use my Variac to adjust the
applied voltage.
     There's an olde tyme trick of using
identical transformers, tying the two
secondaries together, then applying VAC
to the primary of one and getting more or
less the same voltage at the primary of
the other, a DIY isolation transformer.

     More practically, given contacts that
are accessible, what are good solvents to
clean them? Acetone? Ethyl alcohol (I use
vodka)? other?




On 12/07/2017 04:40 PM, Michael Paoli wrote:
>> 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,
>
>
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