[time-nuts] cesium clocks..

Chuck Harris cfharris at erols.com
Sat Jun 28 09:29:18 EDT 2008

This is a chicken vs egg sort of problem.  When I took apart the oven
on my 5065A rubidium, it looked to me as though the enamel insulated
nichrome wire developed a short about 50% into the coil, and wiped out
the transistor.  It also heated the rubidium lamp hot enough to reflow
the solder on its circuit board and thoroughly char the PCB's epoxy.

I am not sure how the quartz ovens are wound (I suspect they are the
same), but with the rubidium ovens, the nichrome wire is wound as a
bifilar loop.   This is done for two reasons, 1) to cancel the magnetic
fields, and 2) to make the start and finish wires of the winding happen
at the end of the oven where the terminals are.

-Chuck Harris

wje wrote:
> Yes, but my comment is rather specific... my oscillator failed because 
> the heater pass transistor shorted.
> This sent the oven heater into full-on. The overtemp sensor is far 
> removed from the heater. There's a ribbon cable between the driver board 
> and the temp bridge sensor board that runs directly over the heater, not 
> outside the oven insulation. Net result, transistor shorts, cable fries 
> before overtemp fuse opens. There's no reason the cable should be inside 
> the oven insulation.
> My rebuild fix - (after removing all the carbonized foam insulation) 
> refoam the oven, replacing the cable and transistor, and moving it 
> outside the foam!
> While I respect in general the brilliance of HP engineers, my classmate 
> was one, this isn't one of the more intelligent decisions.

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