[time-nuts] Spectracom 8170 -> SWCC clock

Jim Hickstein jxh at jxh.com
Sun Jan 22 17:46:03 EST 2012

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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