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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