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

[Don Robert House]

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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>>
>
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