[time-nuts] cesium clocks..

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Jun 27 19:49:40 EDT 2008

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.
> Bill Ezell

The usual reason for running temperature sensor leads over the oven 
inside the insulation is to thermally shunt them to the oven reducing 
heat transfer to the temperature sensor via the wiring.
Your modification may reduce the oven temperature stability significantly.


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