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

[Bill Henry]

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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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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