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

Tue Sep 9 18:05:55 EDT 2025

The more I think about it this may be a home brew ST-6.

On 9/9/2025 4:09 PM, John, W9DDD wrote:
>
> 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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>

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The above comments or recommendations are SWAG. Use at your own risk.

John, W9DDD