[GreenKeys] final question on ST-6...

[wa2hwj at att.net]

Don't forget the TTY carrier equipment (43A) that used vacuum
tubes. Talk about a heater! And, then there was the N-Carrier
systems. Whenever I went into a CO, I would just walk
down the hallway and feel the doors. When I felt a warm
door, I knew it was the N-Carrier room! Lots of 
vacuum tubes in that stuff, too. (1970's)

73,
Jack WA2HWJ

-------------- Original message from Don Robert House <drhouse at nadcomm.com>: -------------- 


> Bill, 
> 
> Very sound and very accurate advice. In the Bell System we used very 
> similar rheostats to adjust loop currents, both in the central office 
> and in the field. They were always attached to metal of one kind or 
> another. In the case of the Franklin Central Office in Chicago they 
> helped heat the building in both the winter and the summer ;-) 
> Some of the largest fans ran 24/7 trying the cool the area where all 
> of the telegraph circuits were "hubbed" 
> 
> I wish I had taken some photographs. I doubt many people would 
> believe what they were looking at. 
> 
> Best regards 
> and 
> 
> 73, 
> 
> Don 
> K9TTY 
> 
> 
> On 20 Jul 2005, at 10:15 AM, Bill Henry wrote: 
> 
> > Eugene, 
> > 
> > That is a 25 Watt, 2500 ohm rheostat on the back of your ST-6. It 
> > is supposed to get hot. The rear panel is a good heat sink, but 
> > don't touch the resistor! The loop supply of the ST-6 puts out 
> > about 150VDC @ 60 mA. With no machine in the loop, all of that 
> > power is dissipated in the loop pot. That rheostat normally 
> > dissipates about 9 Watts - and that is certainly enough to burn 
> > your finger. Plugging a machine into the loop will not reduce the 
> > heat dissipated by much as the machine's selector magnet DC 
> > resistance is only about 110 ohms, not much compared to the 2500 
> > ohm loop pot. But if you put several machines in series, the loop 
> > current will drop and you will then need to adjust the loop pot to 
> > get 60 mA. In practice, exact 60 mA is NOT critical - just try to 
> > get close (+/-10 mA). 
> > 
> > In regard to using a low voltage loop, that is not a good idea for 
> > older machines like the 15, 19, or most 28's. The selector magnet 
> > coils of a loop-driven TTY machine have a very high inductance - 
> > about 4 Hy as I recall. To assure that the magnet current rises 
> > quickly to a full 60 mA, you need to have a low L/R ratio to get a 
> > short time constant. That leads to a need for a high series 
> > resistance and that in turn to requiring a high voltage loop supply 
> > to get 60 mA. The ST-6 has been around since the early 1970's and 
> > many of them have run continuously over that time. It's a proven 
> > design and works well. All of the ST-6's have a 25 Watt loop 
> > resistor. Only the early HAL ST-6's have the loop pot. This is a 
> > feature many hams actively look for. Hook up your Model 15 and 
> > have a good time. 
> > 
> > Bill Henry 
> > 
> > 
> > At 01:10 AM 7/20/2005 +0000, Eugene Hertz wrote: 
> > 
> >> Ok... 
> >> 
> >> The rheostat on the back of the ST-6 that controls the loop 
> >> current... 
> >> 
> >> 1. It gets VERY hot. REALLY hot! Is this normal? Is there any way 
> >> I could reduce the heat? 
> >> 
> >> 2. Seems like I can get the loop current to 60ma at the lowest but 
> >> I can raise it to higher (not much) probably to compensate for 
> >> more devices on the loop..Question is, i would imagine my 28KSR 
> >> could probably work off lower voltage and/or current, no? 
> >> 
> >> Is there some way I could reduce the power consumed by the 
> >> rheostat, perhaps by reducing the current through it or voltage 
> >> across it? I recall some folks talking about running a loop on 80v 
> >> instead of 100+v. 
> >> thanks 
> >> Eugene 
> >> 
> >> 
> >> _______________________________________________ 
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> >> http://mailman.qth.net/mailman/listinfo/greenkeys 
> >> 
> > 
> > ---------------------------------------------------------------------- 
> > ---------------------- 
> > George W. (Bill) Henry Internet: ghenry at halcomm.com 
> > President WWW: halcomm.com 
> > HAL Communications Corp. Phone: 217-367-7373 
> > 1201 W. Kenyon Rd. FAX: 217-367-1701 
> > Urbana, Illinois 61801 
> > ---------------------------------------------------------------------- 
> > ---------------------- 
> > 
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> 
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