[GreenKeys] Current loop not providing (enough) current

[Ralph Mowery]

Take the computer interface out of the circuit, or just short it out. 

If this does not make the current go to about 60 ma then:

 

 

Starting from scratch.  

 

Disconnect everything.  Measure the resistance of the teletype.  Should be about 45 ohms.  Measure the 2000 ohm resistor.  Check voltage of supply with just the resistor connected across it.  Should be close to 120 volts .  Measure current of just resistor and power supply.   Should be close to 60 ma.  Insert teletype machine in circuit.  Current should just barely drop a ma or three.  At this point you should be able to type on the keyboard and see it on the teletype machine.  If not, you may have to reverse the wires on just one of the coils.  If wired backwards, they tend to cancel  out the pulling of each other.  

 

If working at this point,  look for about a 2000 ohm resistor in the interface.  If not, there may be a bad keying transistor/optical isolator. Or they are not driven hard enough to put them into saturation.

 

Ralph ku4pt

 

 

 

From: greenkeys-bounces at mailman.qth.net [mailto:greenkeys-bounces at mailman.qth.net] On Behalf Of Zach Zarzycki
Sent: Thursday, September 26, 2019 7:21 PM
Cc: Green Keys
Subject: Re: [GreenKeys] Current loop not providing (enough) current

 

https://i.imgur.com/LGKpUOs.jpg shows the loop hooked up and only 31mA

https://i.imgur.com/mxLXj7q.jpg resistor tip to tip 2.004k.

 

I can't for the life of me get the current up to 60mA - I now have a 120v linear, measuring 120v post to post. When I put it on the loop it only shows 30mA on the meter. 

 

On the the teletype itself, it  will still register keys on the computer. Between the 15 and 26 without any loop connected registers 48.6 ohms. I tried to hook up the wiring on the teletype like I had it according to the non line-relay contacts to see if it would drawn anymore current / cause anything to happen and it does not. The way I have it wired up between 15 and 26 is as follows -  https://i.imgur.com/FQKJe3C.jpg for the assuming the wiring for "table plug" is what I'm going for and wiring the loop ends on the 26 itself to contacts 15 and 26  following the "M2DE cord" https://i.imgur.com/YQPuMJ7.jpg ). 

 

Thoughts from anyone???

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