[GreenKeys] Associated Press box

[David I. Emery]

On Wed, Mar 18, 2020 at 12:45:58PM -0700, Harold Hallikainen wrote:
> I just received more photos. It definitely looks like a DC loop supply to
> me. When I worked in radio, there was a demodulator with built-in loop
> supply on the shelf under the model 15. Not sure where this would have
> been used.

	AFAIK the solid state VFT mux demod boxes used by AP and UPI
replaced a polar relay and DC 110 V 60 MA loop supply.    The polar
relay was driven by +-20 MA over a copper pair phone loop to the CO, and
in many places more than one customer was in series on that copper loop.
The polar relay switched a local 60 MA on off keying loop with the
selector magnets of the model 15 or 20 machine(s)  on it.

	The power supply in the photos looks like one of the loop power
supplies used for wire service machines in that era.

	IIRC the wire service VFT modem boxes (late 60's transistor and
IC construction) contained a solid state loop keyer and did not use the
original polar relay or the original loop power supply.   The polar
relays were a constant source of problems as they developed contact and
adjustment issues that caused increased bias distortion  which resulted
in  garble on the wire copy requiring replacement or repair and
readjustment. By the late 60s solid state loop keyers were available and
much more reliable.

	In the polar relay DC telegraph circuit era it was often true
that the actual wire service bit stream came via a huge rack mounted 
vacuum tube VFT FSK demodulator (wireline carrier tone telegraph
terminal) located at the serving telco central office that took a VFT
signal on an internal telco  voice circuit and decoded a particular FSK
tone pair with a particular AP or UPI wire and routed it out one or more
+-20 ma polar copper telegraph loops to the AP and UPI subscribers.   
This technology was more practical and reliable than long copper DC long
distance telegraph circuits and of course worked fine over most of the
various FDM-SSB carrier systems of the day - providing several wire
service telegraph circuits on one carrier voice channel instead using
multiple long distance copper pairs.

	The wire services eventually decided it would be cheaper and
more reliable to replace the leased local DC telegraph loops for each
wire with one or more of the VFT demod boxes... and lease audio circuits
that eventually had 22 or 24 different "wires" on one leased voicegrade
data circuit.   This allowed adding wires at a customer site (or
removing them) without requiring any telco changes or circuit
reconfiguration.

	And it got rid of a problem with the series polar loop with
multiple polar relays in series at different customers ... namely that
if any of the machines or one of the  polar relays was disconnected for
moving to a new location  or maintenance the whole loop could go open
causing all the machines  on the loop to stop printing... which of
course not just impacted YOUR station newsroom but all the other ones in
town - often resulting in angry calls from your neighbors on the circuit ... 


  

> 
> Harold
> 
> -- 
> FCC Rules Updated Daily at http://www.hallikainen.com
> Not sent from an iPhone.



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-- 
  Dave Emery N1PRE/AE, die at dieconsulting.com  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in 
celebration of what could have been, but wasn't and is not to be now either."