[time-nuts] HP 5370B - result

Neville Michie namichie at gmail.com
Wed Sep 15 12:32:17 UTC 2010

The catch with the thermal fuse in an un-plated socket is that as it  
ages the socket oxidises and
the heater current generates enough ohmic heating through a high  
resistance socket
to heat the fuse through its leads. This opens the fuse the next time  
the oven is heated from cold.
I just repaired a 10811 today with the same problem.
Does anyone know the rated temperature for the fuse to drop out?
cheers, Neville Michie

On 15/09/2010, at 8:40 PM, Charles P. Steinmetz wrote:

> After cooling and restarting the 5370B and observing its behavior,  
> we judged that it was more likely it had an unheated 10811 than an  
> oscillator of lower stability, so we popped the lid (the  
> possibility that it didn't have a 10811/10544 was the reason for  
> leaving it intact so it could be returned).  Sure enough, we found  
> a cold 10811 and an open thermal fuse.  After replacing the thermal  
> fuse with a jumper, all appears normal.  (As others have noted, it  
> appears to me that if the heater control failed the crystal and  
> heater would suffer major trauma before the thermal fuse opened, so  
> we deemed it superfluous.)
> Thank you all for your comments and suggestions!
> Charles
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