Why not jumper one of the contacts with a clip lead. Then you can view scope during keyboard operation to see if that segment is clean. Should be able to see if problem is contacts or something else.

 


Ken S

-----Original Message-----
From: Eric Moore <mooreericnyc@gmail.com>
Sent: Aug 6, 2023 8:06 AM
To: Gerry Block <gblock@sbcglobal.net>
Cc: Greenkeys <greenkeys@mailman.qth.net>
Subject: Re: [GreenKeys] Model 14 keyboard adjustment

 

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@sbcglobal.net> wrote:
typo:  "60 ma"

Sent from my iPhone

On Aug 5, 2023, at 10:07 PM, Gerry Block <gblock@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@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@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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