[GreenKeys] Current loop not providing (enough) current

Mon Sep 23 23:12:37 EDT 2019

The two 90ohm coils are wired in parallel so something in the neighborhood of 45ohms + a bit for the possibly iffy keyboard contacts (at low meter volts) seems right.

WayneWB4OGM

-----Original Message-----
From: Zach Zarzycki <shardsofaperture at gmail.com>
To: Jordan Spencer Cunningham <js at teletype.net>; GreenKeys <greenkeys at mailman.qth.net>
Sent: Mon, Sep 23, 2019 8:25 pm
Subject: Re: [GreenKeys] Current loop not providing (enough) current

Here is the 2 points I'm hooking up at - would be pins 15 and 26 (also my test points that show the 48 ohm) 
https://imgur.com/a/TGrywcghttps://imgur.com/FQKJe3C

On Mon, Sep 23, 2019 at 7:55 PM Zach Zarzycki <shardsofaperture at gmail.com> wrote:

 I'm thinking I need to boost the voltage overall. The darn loop only shows 48 ohms which feels wrong when I grab each end of the loop with the meter. The schematics are cryptic as all get out but I'm positive on this, where it shows the plug for the table, is where I need to wire my loop in. 
I'll have to snap a photo of the schematic I'm looking at, but seeing as how they keyboard works to TX to the computer terminal, I'm pretty sure I'm where I should be.
I went on mouser today and bought the linear 120vdc supply so when that comes in I'll try hooking that up though a 2k and see if it makes a difference.
I will say there are 2 5k resistors on the unit in different places, but they're not in the loop so to speak, especially as seeing I only read the 48 at the +/- twx line connection (where I hook my loop power supply and board into).  

|   From: js at teletype.netSent: 22 September 2019 12:26To: greenkeys at mailman.qth.netSubject: Re: [GreenKeys] Current loop not providing (enough) current |

 I'm certain that's the problem. I've done some testing on my Models 15 and 19 in the past, and if the 26 magnets are in the same realm of tolerances, they'll start to become useless at around 50mA, give or take. 25mA is much too low a current to have any effect.

The keyboard is keying the loop successfully since that's just mechanical, and the board can pick up signals even lower than 20mA, but the selector magnets can't.

The question is why. Looks like you expected 96 volts and so calculated 1600 ohms resistance to get 60mA. However, it sounds like there's more resistance in your loop. A few hundred ohms is expected, but you've got an additional 2.2k ohm and change. 

I'm willing to bet the loop may already have a 2k ohm resistor in series somewhere-- maybe installed within the machine itself if you have nothing else on the loop. Try measuring resistance in the full loop with and without your extra resistors, and I'd guess you'll see the same thing. As long as you see roughly 2000 ohms on the loop without your extra resistors, it will be safe to power it without the extra resistors. At 96 volts, however, you'll get a current of only around 40 - 45mA, which probably won't reliably trigger the magnets. But who knows, maybe it'll work fine-- it's always worth trying.

If not, you'll either need to locate and replace that 2k resistor with your 1600, or you'll need to boost the voltage.

-
Jordan Spencer Cunningham
teletype.net

On Sat, Sep 21, 2019, at 9:37 AM, Zach Zarzycki wrote:

Well - I have my 26 hooked up though the volpe board and have managed to get as far as when I type on the teletype's keys the terminal window on the laptop accurately displays what I've punched up.

However, typing on the teletype doesn't cause the keys to punch paper, and typing on the laptop keyboard won't cause the teletype to do anything.

I'm set up with a boost converter and a 12v 3amp power supply that SHOULD be supplying roughly 60mA at 97 volts though the loop after being boosted and then ran though about 1.6k wire wound resistor (600 ohm and a 1kohm in series, both 25W rated). 

The problem is when I read the voltage out of the booster, it shows  97v, but the most I've seen any amperage on any test point is 25ma, a far cry from 60ma. I'm wondering this this is possibly the reason for the computer keyboard not being able cause the keyboard close the magnet and select a letter then type. 

My gut says maybe? But I find it very odd though that I can make the greenkeys act as a keyboard, so maybe I'm missing something else.

Zach 
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