[time-nuts] Spectracom 8170 -> SWCC clock

Bob Camp lists at rtty.us
Sun Jan 22 19:05:05 EST 2012


Hi

6K is about right. The resistor was typically a big ceramic wire wound 10K variable.

No guarantee it's right for a clock, just that it's right for a teletype.

Bob



On Jan 22, 2012, at 5:46 PM, Jim Hickstein <jxh at jxh.com> wrote:

> On 2012/01/22 15:29, Bob Camp wrote:
>> Hi
>> 
>> Teletype loop current as in 20 ma through the coil via a dropping resistor off of 125 vdc.
> 
> Value for the dropping resistor?  (I know, I'm an Extra, and I used to design digital circuits, so I should know this stuff.  But I've been in software for a long time.  Let's see.... E over I R.)
> 
> I measured the setting coil at the terminals: 11.5 ohms.  To limit 125 VDC to 20mA, this would need an additional 6.2 Kohms.  I suppose that represents the metallic circuit back to WU plus a bunch of other 11.5-ohm clocks on the same circuit, plus a compensating resistor back at the head end?
> 
> Locally, a D cell (or 3 in series, which I have), with about 200 more ohms, might do.  (Reaches into desk drawer.)  Let's see if I still have that bunch of 100-ohm resistors left over from making an ISDN terminator.  Why, yes! Quarter-watt.  P over I E.  90mW.  Eh, it probably won't blow up.
> 
> I tried measuring the winding coil, too: 0.1 ohms, but I'm not sure I was getting it in the right place.  And now I've put the face back on the clock. Otherwise I couldn't tell what time it was!  I am really trained to look at that spot on the wall for this information.  While the SWCC was in the hospital (for over a year) I had to buy another clock to put there.  The blank spot was driving me crazy.
> 
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