[GreenKeys] Current loop not providing (enough) current

Thu Sep 26 22:04:01 EDT 2019

Now that is strange. Your measurments seem to point to the board as causing the issue since current appears to drop to 30ma after connecting the board in series. I would suspect a bad component, but I tested each board before shipping on a Model 19. The couple of baddies I set aside, and they didn't work at all (send OR receive). Still, maybe something went bad afterward.

What's the resistance between the two terminals on the RX pair and the two terminals on the TX pair? I believe you should see about 150 ohms on RX and 22 ohms on TX, or about 170 ohms between the farthest RX and TX terminals (the board must be powered on from USB-- if powered off it'll be something like 36 kohms). This assumes the bridge wire between the closest RX/TX terminals is still in place, too.

With a 2 kohm resistor and the board and nothing else on the test circuit I have set up (no teletype right now), I measure about 56mA. It drove a teletype all day last Saturday.

Polarity shouldn't matter, though I did see a weird polarity problem on one of my prototype boards once. 

-
Jordan Spencer Cunningham
teletype.net

On Thu, Sep 26, 2019, at 6:20 PM, Zach Zarzycki wrote:
> Ok, so the power supply and resistors made 60 ma, when I hook the power supply up to the PC board, drops in half to 30 ma (no teletype on the loop). It was my understanding the polarity into the PC board didn't matter if it was hooked. I'll try to flip them. 
> 
> But at this point supply to resistor in a loop is 60mA!
> 
> 
> As an aside, the teletype does measure about 45 ohm.
> 
> 
> 
> Sent from my Android BlackBerry KEY2

> *From:* shardsofaperture at gmail.com
> *Sent:* 26 September 2019 20:10
> *To:* rmowery28146 at earthlink.net
> *Cc:* greenkeys at mailman.qth.net
> *Subject:* Re: [GreenKeys] Current loop not providing (enough) current
> Eric - I have found 2 resistors on the unit in different places, but both are stock. Every test point I've tried doesn't show anything more than 48, 177 or 250 ohm! I am keeping this in mind. Jordan did mention it before but the "hidden" factor makes me think theres going to end up being something to this. It was wired up completely different when I found it (jumpers on 11-12, 13-14 and the 2 wire cable with a cap wired to 13 and 16 which I don't understand why, I think it was original from 1945 and nothing to do with how I would want to use it.
> 
> 
> Ralph, 
> 
> I'm going to try this again from scratch and report back! Not sure if you do DMR but almost want to try and find you and talk to you about this haha!
> 
> 
> 
> Sent from my Android BlackBerry KEY2

> *From:* rmowery28146 at earthlink.net
> *Sent:* 26 September 2019 19:51
> *To:* shardsofaperture at gmail.com
> *Cc:* greenkeys at mailman.qth.net
> *Subject:* RE: [GreenKeys] Current loop not providing (enough) current
> 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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