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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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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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.
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.
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
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
typo: "60 ma"
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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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