[GreenKeys] New (to me) Model 28, "Bell System"

Tue Sep 9 18:26:53 EDT 2025

Is the LINE/LOCAL button OUT for LINE operation?? If it is IN, then the
meter/lamps will indicate but not key the TTY.

Make sure the TTY signal line is hooked up to the 3-conductor plug to TIP
and RING only.
The SLEEVE is for ground and should NOT be connected to any signal line.
The FSK-1000 keys the RING to ground, so if your TTY is hooked TIP to
SLEEVE then the keying transistor will be bypassed and the loop will be
always MARKING which is the condition you have described.

Along that line, a blown keying transistor will produce the same symptoms.

Nick England K4NYW
www.navy-radio.com

On Tue, Sep 9, 2025 at 2:24 PM David S <david at stickelman.net> wrote:

> Thank you very much for answering, apologies for my lack of understanding
> on some of these terms
> On 9/7/2025 5:35 PM, Duncan Brown wrote:
>
> David,
>
> I don't understand your comment that "the output is polar, +60ma/-60ma".
>
> I read this somewhere and I may fundamentally misunderstand what a polar
> loop is vs a neutral loop.  I have seen these terms before, but don't have
> a good grasp of what these terms mean and I can't find where I read this
> description previously (im getting lost in all the stuff I am reading).
> The FSK manual (and label on the device) states it is a polar loop.  I am
> not sure what the M28 I am working with expects, how to distinguish it, or
> if this distinction is even relevant.
>
>
> What kind of connector did you use from the M28 to the FSK-1000?  If it
> was only a 2-circuit plug, the M28 would run closed, since you are just
> connecting to +170V & ground.  You need a 3-circuit plug to allow Q4 to
> control the loop.
>
> The FSK manual insists that only the TRS (3 circuit) plug be used so I
> have been sticking with that.  It does not run open (striking the same
> blank key over and over) with the FSK plugged in, but does not print ITTY
> even though the mark/space lights flicker like I have seen in some youtube
> vids.  I can type on the keyboard and get printing on the M28 itself when
> the M28 is plugged into the FSK-1000.
>
> The Mainline ST-3 does not appear to give a loop since the M28 runs open
> with a 3 or 2 connector cable.  The meter on the Mainline ST-3 bounces like
> I would expect and the mark space lights flicker much like the FSK, so I
> think its correctly receiving ITTY, at least in audio, but since the
> machine runs open it isnt printing the ITTY.
>
>
> The FSK-1000 had an optional AFSK board. If you have that board, you can
> run the audio (low tones) [AFSK OUT] into the Audio Input your transceiver.
> Your radio may also take low-level data to generate FSK. If so the FSK-1000
> can generate that [DATA OUT] and would be the more reliable connection.
>
> Good luck & have fun,
>
> Duncan
> K2OEQ
>
>
> On 9/7/2025 5:40 PM, Nick England wrote:
>
>
> Looking at the IRL FSK-1000 manual
> https://www.navy-radio.com/rtty/irl/fsk-1000.pdf
> It has a loop supply with + output on the tip and - connection on the ring
> of the 3-conductor phone plug,
> So it is not +/- 60ma (polar), but switched 0/60ma (neutral) which is what
> almost all model 28 machines expect.
> Depending on the LESU that is installed in your 28KSR, it may be polarity
> sensitive, so if you have the connection the wrong way around, the printer
> will run open. Try swapping the leads. Caution - this circuit has +160 on
> the leads all the time, so remember to turn off the power!
>
> It does not run open, and I can type on the machine and get local copy
> when I do so it seems to be partially working.  The manual calls the loop a
> "+170 polarized loop", maybe they mean something else by this term?  When
> feeding the M28 ITTY via the FSK-1000 I am not hearing the selector magnets
> clicking around.  And yes, I am being very cautious with what I am
> touching, I am sure 170v at 60ma isn't pleasant.  I am completely
> unplugging, waiting a for awhile, and only then messing with the
> connections.
>
> I am using the procedure in the operation section of the manual, save the
> radio tuning part, and the M28 does not print ITTY.
>
> I have put a shorting plug on the serial in connection since the FSK-1000
> manual specifies this, but it states neglecting this will cause the de
> modulator to not function, but I think the demodulator is functioning since
> the mark/space light flicker like I think they should.
>
>
> The IRL provides an input (SERIAL IN) that can be used to drive an audio
> tone generator (DATA OUT) or to drive a transceiver that has a built-in
> tone generator.
> There was an optional AFSK tone generator for the IRL - check if you have
> that
>
> Here's the ST-3 article
> https://www.rtty.com/development/tu/st-3/st-3.htm
>
> While the ST-3 did not include an AFSK output for transmit, there were
> boards and kits available, and someone may very well have added it to the
> unit you have.
>
>
> I did get a full set of schematics with the FSK (and a nicely printed
> manual), ill have to see if I can identify the optional AFSK board.  Same
> for the ST-3, ill have to see if it has option boards on it.
>
> Perhaps my problem is the the FSK (and standard ST-3) is inherently FSK
> (without the optional boards), and ITTY is AFSK?  The two signal patterns
> do appear different, at least according to Not Black Magic: AFSK
> <https://www.notblackmagic.com/bitsnpieces/afsk/> and other articles
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