[GreenKeys] Model 14 keyboard adjustment

Sun Aug 6 12:43:39 EDT 2023

 All--Jeffrey's admonition re not using sandpaper (or crocus cloth or emery paper, for that matter) on the keyboard contacts should be heeded.  There is a contact burnishing tool in the usual Teletype tool kit for this purpose.  See the tool listing on RTTY.com. The keyboard contacts are made from a special silver-bearing alloy and can be irreparably damaged if mistreated.  (The contacts of a telephone No. 5 crossbar switch is a similar case in point.)
Use of brown paper such as that from a brown paper bag with some contact cleaner or rubbing alcohol can be used.  Fold a small strip of the paper in half and saturate it with cleaner.  Place the folded strip between the contacts and pull it through while squeezing the contacts together with your fingers. Several tries may be needed to clean the contacts adequately. The wood fibers in the brown paper act as a wiping mechanism.
An old mentor of mine (long gone, unfortunately) introduced me to the concept of Sealing (or Wetting) current.  This is the amount of minimum current needed to make sure contacts carry the signal through them adequately. This notion applies to toggle switches, relay contacts, push button switches, etc. (Try Googling Sealing Current; you might find some useful information there.)  It is not clear to me what sort of circuit arrangement is being used in this situation, but the recommendations of using a standard 60 mA loop with a compliance voltage of 80 - 120 volts should do the trick. I used this set-up and the cleaning procedure above on all my mentor's many wire service machines with success.
Good luck!George, W6BN
    On Sunday, August 6, 2023 at 06:27:25 AM PDT, Jeffrey Golas <jeffg at junknet.net> wrote:  

 Dont use sandpaper! The MA is important as it does affect the waveforms. The machine wants to see 60ma whether it's 120vdc or 15vdc. Set the scope to be wide enough to get an entire character if you havent already, it almost looks like the pic may just be showing the stop bit (33ms?).
Dial in the loop first, verify its 60ma at idle.
Jeff

On Aug 6, 2023, at 9:05 AM, Eric Moore <mooreericnyc at gmail.com> wrote:

https://deramp.com/downloads/teletype/Model%2015/loop%20interface%20board/Schematic%20Rev%20A.pdf
Verified the printer loop at 60ma. The keyboard loop runs at 15V, not sure on the ma, but not sure it matters much as it just opens and closes without driving any coils or similar.
I am fairly certain my issue is the contacts somehow. I see voltage (and continuity) up to those contacts, but despite cleaning fairly extensively with contact cleaner, they are still too high resistance. I am going to try some sandpaper today and see what is what...
-Eric
On Sun, Aug 6, 2023, 12:10 AM Gerry Block <gblock at sbcglobal.net> wrote:

typo:  "60 ma"

Sent from my iPhone

On Aug 5, 2023, at 10:07 PM, Gerry Block <gblock at sbcglobal.net> wrote:

eric
what does your loop schematic look like?  sketch it out and enclose photo. what is the loop power supply voltage. what is the series resistance to make the 60ms. are you sure it is running at 60 ms?where is the scope probe grounded and where is the loop supply grounded?i doubt it is the contacts.  
gerry

Sent from my iPhone

On Aug 5, 2023, at 5:49 PM, Eric Moore <mooreericnyc at gmail.com> wrote:

I have cleaned with contact cleaner and paper tape pretty extensively, but can always go another round.
I do not think that is a good waveform, but similarly, no idea.
-Eric
On Sat, Aug 5, 2023, 7:23 PM Jeffrey Golas <jeffg at junknet.net> wrote:

Correct me if Im wrong but could just be "dirty"; as the cam rubs against the switch youre seeing noise like a dirty volume pot.

Id use some alcohol and a business card and scrub the contacts (plus Im not sure thats even a terrible signal for a 1940s device. Going machine to machine I dont think that tiny bit of leakage woulda mattered.

Again, correct if wrong, Im here learning too.

Jeff KC3GJX

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