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

[Bill Henry]

Hi Don,

Yes, heat WAS normal.  Lack of heat indicated a problem - blown fuse, power 
supply, tube - whatever.  Times do change don't they?

Bill


At 06:51 PM 7/20/2005 -0500, Don Robert House wrote:
>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
>>>
>>>
>>>_______________________________________________
>>>GreenKeys mailing list
>>>GreenKeys at mailman.qth.net
>>>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
>>---------------------------------------------------------------------- 
>>----------------------
>>
>>_______________________________________________
>>GreenKeys mailing list
>>GreenKeys at mailman.qth.net
>>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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