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

Sun Sep 7 18:35:18 EDT 2025

David,

Congratulations on your new M28!

The IRL FSK 1000 should be able to TX & RX with your M28.  I don't 
understand your comment that "the output is polar, +60ma/-60ma".  
Looking at the schematic (below), it shows a +170 V supply through an 
adjustable current limiting resistor to [LOOP OUT] a 3-circuit phone 
plug tip contact. The ring contact goes to the collector of Q4 and the 
emitter of Q4 is grounded.  So there is only positive current going to 
the TTY.

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-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 5:42 PM, David S wrote:
>
> 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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