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

[David S]

I bought a beautifully maintained Model 28 floor model geared for 
60wpm.  It was fairly recently used on air based on the printed pages in 
it.  I got it cleaned up and lubricated and it runs beautifully locally, 
except hanging up a bit on the first characters in a new line.

It came with a paper winder which is a fascinating feature, I have not 
seen this accessory previously.

It also came with a Mainline ST-3 demodulator, which apparently was not 
manufactured, but plans were designed by Irvin M. Hoff in 1970 who was, 
at the time, W6FFC.  This call sign appears to be used by someone else 
currently.  Thus, someone homebrewed this device, but did a great job 
since it appears to be factory made equipment in quality and 
appearance.  It turns on and when sending audio to it from ITTY, appears 
to pulse/respond like other terminal units I have seen.  I do not 
believe this has a loop supply, it does not keep the Model 28 from 
running open when connected.

I also have an iRL FSK-1000 terminal unit. The manual for this states it 
gives polar (+60ma/-60ma) signaling.  It keeps the Model 28 from running 
open, and permits local typing, but when sending ITTY to the iRL, it 
does not signal the Model 28 to type and the typing unit is just idle.  
Hopefully doing this wasn’t a mistake, given polar (+60ma/-60ma) vs 
neutral (+60ma/0ma) signaling, assuming I understand that correctly.  If 
this Model 28 is neutral signaling, I cant imagine sending it -60ma 
would be ideal if it wasn’t set up for this.

The Model 28 has a ‘Bell System’ disk on it which probably indicates 
what kind of signaling it would use?

It also has a switch on the keyboard, going to a pair of regular wires.  
I assume this is how the operator keyed his radio when it was in use 
previously.

My intent is to get this Model 28 properly on the air, which should be 
possible.  The typed paper that was in it indicated a large handful of 
QSOs, so it was functioning on air at some recent point.

I have another Model 28 functioning, but it is 100wpm and not really 
usable on the air since 60wpm is common.  Thus I have not had to 
confront transmission of signal so far, just receive.

My understanding, which may not be accurate, is that to receive on the 
60wpm machine, I need the demodulator.  The mainline ST-3 likely does 
not have a loop supply, so I think I would need this.  This demodulator 
is likely FSK (frequency shift keying).  The Mainline ST-3 would act 
like a switch for a loop supply, and the loop supply is something like 
160ish volts at 60ma.  This voltage level I am reading is necessary to 
overcome the inductance of the selector magnets.

There are two types of signaling I am finding, the polar (+60ma/-60ma) 
and neutral (+60ma/0ma).  I am not sure what devices would use what type 
of signaling, or what the consequence of getting this wrong would be.  
Would using wrong signaling be damaging or just not work?

Reception I think will be the easier of the two challenges.  The other 
challenge will be taking the signal generated from the signal generator 
on the keyboard and getting this modulated and sent to a radio.  I have 
an Icom 7300 and 7200 available.  This is the part I have no idea where 
to start with.  I suppose I need a 'modulator'?
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