[GreenKeys] Current loop not providing (enough) current

Thu Sep 26 19:25:56 EDT 2019

This has probably been gone over already, but i've skipped over a lot of
this discussion. 

I discovered that my model 19's table has current limiting resistors fairly well 
hidden inside it in series with some of the loop circuits. Maybe you have a
similar situation, and there's already a 2k resistor in there somwhere? 

Maybe you could try measuring the DC resistance of your whole loop with no power
supply connected, and use a jumper clip to short out all the selector magnets?
If you still see more than a few ohms you can then go looking for secret
resistors.

eric

On Thu, Sep 26, 2019 at 07:20:33PM -0400, Zach Zarzycki wrote:
> 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???
> 
> On Mon, Sep 23, 2019 at 11:12 PM <hwhall at compuserve.com> wrote:
> 
> > 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.
> >
> > Wayne
> > WB4OGM
> >
> > -----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/TGrywcg
> > https://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.net
> > *Sent:* 22 September 2019 12:26
> > *To:* greenkeys at mailman.qth.net
> > *Subject:* 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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