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

[David S]

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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