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'?