it's fun beating a dead horse.  
i think everything everyone is saying is true.  earlier in this thread somebody used the magic term " compliance voltage" which is the key to driving the coil inductance. a constant current supply with a zero load (open) output voltage of 15v doesn't solve much. 
when i was running machines i used a 150v hp lab supply set to "constant current".  so the "compliance voltage" was 150v - until the load dynamically needed less to stay at 60ma. 
this is not much better than a REC loop supply with a wire wound resistor but is smaller and weighs less and generates little heat and impresses your neighbors. 

i apologize in advance. i was working these machines this when the machines were pretty new and had never seen a machine in storage for 50 years. 
i have kept only a model 12 and i swear i will lubricate it and get it working.  Btw i had a model 28asr that was NEW!
enough. ...



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On Aug 6, 2023, at 1:23 PM, Gerry Block <[email protected]> wrote:

paul is absolutely correct.  the contacts are not designed for low level switching.  
if you use a normal loop your problems related to dirt and oil will go away. 
i didn't catch it - what device is this thing driving?

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On Aug 6, 2023, at 6:37 AM, David V. Corbin <[email protected]> wrote:


Paul, trying to wrap my head around this. Ohms law. If the state is static, then to keep a 60mA flow, the voltage will be a constant based on the resistance of the loop. Now when the state transitions to “open” it does not matter how high the voltage goes, there will not be current. Now when the circuit initially closes, inductance May raise the voltage needed for achieving the target current, but then it will lower to the steady state value.

This is why I use programmable current supplies rather than supplies where one sets the voltage. 
From: [email protected] <[email protected]> on behalf of Paul Heller <[email protected]>
Sent: Sunday, August 6, 2023 9:30:50 AM
To: Jeffrey Golas <[email protected]>
Cc: Gerry Block <[email protected]>; Greenkeys <[email protected]>
Subject: Re: [GreenKeys] Model 14 keyboard adjustment
 
I remember a discussion years ago where a number of folks said high voltage was important for the contacts. The design voltage was much higher than 15v. You might try a loop source that delivers higher voltage and 60ma. That might work better with your contacts. 

Paul
W2TTY

ITTY: HTTP://INTERNET-TTY.NET:8000/ITTY
ITTY 100:  HTTP://INTERNET-TTY.NET:8010/ITTY100

On Aug 6, 2023, at 7:27 AM, Jeffrey Golas <[email protected]> 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 <[email protected]> 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 <[email protected]> wrote:
typo:  "60 ma"

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On Aug 5, 2023, at 10:07 PM, Gerry Block <[email protected]> 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

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On Aug 5, 2023, at 5:49 PM, Eric Moore <[email protected]> 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 <[email protected]> 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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