[GreenKeys] GreenKeys Digest, Vol 138, Issue 3

[John Nagle]

    Yes.  You do need a contact burnishing tool.  I've looked at
Model 15 contact behavior on a scope, and seen it get much better after
burnishing the contacts.  Use the right tool; contact burnishing
tools are just hard enough to take off dirt and carbon without
sanding down the contact surfaces.

    Also, Model 14/15 contacts need a higher voltage than a logic
level to work properly. Because they're running in an oily
environment, an oil film will probably get on the contacts
after a while.  If you have 120VDC, the voltage will burn through
the oil film.  If you only have 5VDC and low current, it won't.
This was a big problem when cars started using electronics
and switches in an oily environment were switching very low
energy signals.  Modern automotive switches are designed to
make contact with a wiping motion to self-clean.  Teletype
keyboard contacts, at least from the 14/15 era, are not.

    My little Teletype interface board

(http://www.aetherltd.com/connecting.html)

runs keyboards at 120VDC, but only about 12mA.  That's enough
to reliably burn through oil films.  I also have a 1000pf 400V ceramic
capacitor in series with a 1K resistor across the contacts
as a snubber, to prevent arcing.  If the contacts are allowed
to arc (which early Teletypes without RF noise filtering caps
routinely did), you'll have to burnish them occasionally.

    I've run a Model 15 at seven steampunk conventions and
a Maker Faire without problems from the Teletype.  Because
I turn operations over to costumed theater people, I want
the machines to run without problems.  This requires
extra attention to reliability issues like this.

				John Nagle

On 07/05/2015 08:36 AM, greenkeys-request at mailman.qth.net wrote:
> Message: 1 Date: Sat, 4 Jul 2015 12:02:20 -0500 From: Jeffrey D
> Angus<jdangus at att.net> To:greenkeys at mailman.qth.net Subject: Re:
> [GreenKeys] Distortion Testing of my AN/FGC-25
> Message-ID:<5598119C.6090107 at att.net> Content-Type: text/plain;
> charset=windows-1252; format=flowed
>
> On 7/4/2015 11:43 AM, Don Sentz via GreenKeys wrote:
>>> Thank you for your feedback about the glitches.
> <http://www.vetco.net/catalog/product_info.php?manufacturers_id=22&products_id=12021>
>
>
And a better view of it:
> <http://www.vetco.net/catalog/images/GC-9337-3.jpg>
>
> The problem is, ALL mechanical contacts tend to bounce a little bit.
> Notably from open to closed (space to mark). Hence the suggestion of
> adding a "de-bouncing" circuit between the mechanical contacts and
> any solid state inputs.
>
> Some thoughts on contact bounce:
> <http://pdfserv.maximintegrated.com/en/an/AN764.pdf>
>
>
> -- Jeff-1.0 wa6fwi http://www.foxsmercantile.com