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

Tue Sep 9 17:09:30 EDT 2025

I don't know how much documentation you've found for the ST-3, but you 
may find this useful.  There is description of how it works.

http://www.familytree.k7tty.com/development/tu/st-3/st-3.htm

It would seem from the picture, that your ST-3 has a lot of enhancements 
which may complicate trouble shooting.  I note there are more torroids 
than a ST-3, perhaps it was based on a later model (ST5-5?)  I would 
expect there's a loop supply there somewhere, perhaps blown fuse?. I 
note that referenced schematic uses 2 pole jack (TS). Which means the 
shield would have to be isolated from ground.  In a similar TU that I 
have it uses the 3 conductor jack and the selector magnet is connected 
to Tip and Ring.  The sleeve then is at chassis ground.  I would check 
your system to see if either the T or R measure any higher voltage to 
common.  If not you might have an open resistor or blown fuse in the 
loop supply.  Or maybe the output transistor is blown, but don't they 
tend to short rather than open?  I forget, I don't think I ever let the 
smoke out of one of those..

(I've been a touch typist for 70+ years, lately I believe my keyboard 
has developed dyslexia)

On 9/9/2025 1:23 PM, David S 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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The above comments or recommendations are SWAG. Use at your own risk.

John, W9DDD