[GreenKeys] Grounding lines in 60ma loops

Harold Hallikainen harold at w6iwi.org
Sun Dec 3 21:16:43 EST 2023 

I agree the TU-170 can be used with an external loop supply (negative side
to ground, positive through equipment to ring of loop jack). One concern
is the voltage of the external loop supply. With the 84V internal loop
supply, I found a flyback voltage of 346 V when driving a model 15. The
loop key transistor is rated at 300 V. Also, instead of a series resistor
to limit the loop current, the loop current is limited by the keying
transistor (Q6). As the loop supply voltage is increased, the dissipation
in this transistor increases. Of course, an external resistor could also
limit the current, taking over some of the dissipation from the
transistor.

On flyback voltage: To avoid overstressing Q6, a zener clamp has been
added. A 200 V zener (NTE5105A) and a 1N4004 diode were placed in series
between the collector of Q6 and the positive side of the loop supply
(between tip and ring of the loop jack J3). This should clamp the flyback
voltage to about 285 V. https://w6iwi.org/rtty/tu170/ shows the flyback
voltage was limited to 294 volts by adding the clamp.

Harold
https://w6iwi.org


On Sun, December 3, 2023 6:58 pm, Daniel Jones wrote:
> The TU-170 can be used with an external loop supply also.  That’s how I
> run mine with a PP-3494A/UG loop supply.
>
> Daniel
>
> www.K6YIC.com
> DJones at K6YIC.com
> HH#11973
>
>> On Dec 3, 2023, at 17:07, Harold Hallikainen via GreenKeys
>> <greenkeys at mailman.qth.net> wrote:
>>
>> Flesher TU-170 has internal loop supply with negative side ended.
>> Current
>> is set using an active current sink transistor between the loop and
>> ground.
>>
>> The DSP TU (https://w6iwi.org/rtty/DspTU2/ ) has a floating loop
>> interface. It does not care about ground or polarity. It looks a bit
>> like
>> a printer with selector magnets in series with a keyboard.
>>
>> I really like isolated loop interfaces!
>>
>> Harold
>> https://w6iwi.org
>>
>>
>>
>>> On Sun, December 3, 2023 12:35 pm, Nick England wrote:
>>> As usual, I'm sure I'll screw something up so additions and corrections
>>> are
>>> welcomed.
>>>
>>> When hooking up multiple units to a multi-loop patch panel it is
>>> possible
>>> to inadvertently short something out because a connected device has one
>>> side of the line grounded.
>>> Typically the minus side of the loop supply will be grounded and that
>>> is a
>>> return for all loops. Each loop is fed by the positive side of the
>>> supply
>>> through a current limiting resistor and then through the LOOPING jacks
>>> and
>>> then the SET jacks (TTY printer, TD, reperf, etc.) and then to the LOOP
>>> DEVICE (keyer/converter/etc.) and then back to minus/ground. This
>>> arrangement works if NO LINE TO A TTY IS GROUNDED
>>>
>>> A note from hard-won experience - some TTY sets are polarity sensitive
>>> -
>>> M28 RO, KSR, and ASR may have a LESU with a line relay or selector
>>> magnet
>>> driver - you must apply proper loop polarity to the line inputs.
>>> Western
>>> Union devices might have one line grounded and you'll need to isolate
>>> that.
>>>
>>> Some RTTY converters and modems have isolated loop inputs and outputs
>>> but
>>> many do not. Here's a quick list of what I have found -
>>> FSK Receive Converters (with internal loop supply)
>>> Dovetron - negative side of loop is ground
>>>
>>> FSK Receive Converters (no internal loop supply)
>>> AN/URA-8A, 8B (CV-89) - switches loop (pin A) to ground (pin B)
>>> AN/URA-17 (CV-483) - switches loop (pin A) to ground (pin B)
>>> AN/URA-17A (CV-483A) - switches loop (pin A) to ground (pin B)
>>> AN/URA-17B (CV-483B) - switches loop (pin A) to ground (pin B)
>>> AN/URA-17C (CV-483C) - switches loop (pin A) to ground (pin B)
>>> AN/URA-17D (CV-483D) - switches loop (J8/A) to ground (J8/B)
>>>                     (or for a negative loop switches J8/C to J8/B)
>>>
>>> FSK TU receive/transmit (no internal loop supply)
>>> AN/SGC-1 -
>>>     SEND/RCV loop isolated (TTY+, TTY-)
>>> AN/URA-17E (CV-3510A)
>>>     SEND loop isolated (+loop J5/A, -loop J5/B)
>>>     RCV loop isolated (+loop J8/A, -loop J8/B)
>>> AN/URA-17F (CV-3510B)
>>>     SEND loop isolated (+loop J9/A, -loop J9/B)
>>>     RCV loop isolated (+loop J6/A, -loop J6/B)
>>> CV-2460
>>>     SEND loop isolated (+loop TB1/6, -loop TB1/7)
>>>     RCV loop isolated (+loop TB2/5, -loop TB2/6)
>>>
>>> AN/URC-32 transmit keyer (+loop TBD/1, ground TBD/16
>>> AN/URT-23 transmit keyer isolated (+loop J7/B, -loop J7/C)
>>>
>>> Note - I believe that all low level devices have one side grounded and
>>> the
>>> other is signal with +/-6v signaling.
>>>
>>> Nick England K4NYW
>>> www.navy-radio.com
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