[GreenKeys] Help with a model 28

[John Nagle]

    You have basically the right idea. Others on here can tell you
more about Model 28 machines; I only work on the older pre-WWII
machines.  Model 28s have lots of wiring, accessories, and filters,
and there are lots of variations on that.

    The usual "60 mA" supply is a 120VDC supply with a 2K 10W resistor
in series.  That limits the current to 60mA.  The reason so much
voltage is needed is to overcome the inductance of the selector
magnet to get a fast enough rise time.  It only takes about 3-6VDC
to pull in the selector magnet, but the pull in will be so slow
that it won't type properly.

    However, once you find the proper connections to the selector
magnet, try applying a small 9V battery to them. That should
pull in the selector magnet, and the machine should stop
"running wildly", disengage the clutches, and idle quietly.
That's a good basic test to see if you're connected to the
right place.

    Here's what I do with Teletypes:

	http://www.aetherltd.com

				John Nagle


On 12/11/2014 08:08 AM, greenkeys-request at mailman.qth.net wrote:
> Message: 42 Date: Thu, 11 Dec 2014 03:41:04 -0500 From: "Baggio,
> Thomas"<TBaggio at telesat.com> To:"greenkeys at mailman.qth.net"
> <greenkeys at mailman.qth.net> Subject: [GreenKeys] Help with a model 28
> KSR Teletype!!! Message-ID:
> <A471B09E91CE2746A1DBC66163361A420B5D2B6BDD at APKEMAIL2.telesat.corp>
> Content-Type: text/plain; charset="us-ascii"
>
> I have recently acquired a model 28 KSR that I would like to get
> running and connect it to my PC and run the Heavy metal program to
> get TTY data to the Teletype.
>
> I have got the PC part working fine.
>
> When I turn the teletype on with nothing connected to it (as far as a
> current loop supply ) The teletype just turns away wildly.  I believe
> this is supposed to happen.
>
> What I can't figure out is where on the Teletype I would connect the
> output of the Current loop supply.  I have traced the wiring a bit
> and can see that there are two selector Coils wired in parallel with
> each other.  These coils are connected to connector that goes back to
> a terminal strip inside the cabinet.
>
> I guess what I am asking is is there anyone out there that can help
> me with this project.  I am having a very hard time finding any info
> out about how to get this hooked up.
>
> Basically, I want to do the following
>
> from the PC serial port (RS232) I would go into my current loop
> supply that I have constructed using transistors and optcouplers. I
> assume because the selector coils are wired in parallel, I need 60 mA
> to  "drive" the coils?  I am not really sure how this works . DO I
> just connect the output of the current loop supply right to the coil
> terminals?
>
> It also looks like there is no LESU in this unit. It was used by a
> local HAM radio club in the past.  It looks like they were missing
> the electrical service unit so they bypassed all the  wiring for it.
>
>
> I really need some help figuring this out.
>
> Any help you could provide or contacts would be greatly appreciated.
>
> I am new to Teletypes and as I understand it, in theory, I need to do
> the following
>
> I think what I am going to do is hook up my current loop driver (I am
> going to try at 20mA so I will need to wire the selector magnets in
> series as they are now in parallel.
>
> My idea is to go in with the current source on one side of the
> magnets .  The current should travel through the selector magnets
> over to the keyboard (signal) contacts, out of the signal contacts
> over to the "negative" side of my current loop.
>
> Does this sound like the basic idea.  I have included a crude diagram
> of how I think it should work.
>
> [cid:image001.png at 01D014F4.4A24B600]
>
> Thanks in advance
>
> Tom
>