On 9/14/2010 8:28 AM, Dan Rae wrote:
I hate to disagree with John who knows a heck of lot more than I ever
will, but in this case it will protect the oven from cooking up if the
control circuit fails with the heater full on, which can happen.
I did have a 5370B with a 10811 that had a bad thermistor in it as
well as an open fuse. I'd guess that's why -hp- fitted it.
But yes, the thermal fuses can and do fail open for no good reason,
and it sounds like this has happened here, but I would not recommend
shorting it out permanently.
Dan
On the fuse subject, here is a post from Rick Karlquist early this year.
Rick was an HP engineer who was part of the 10811 team.
-----Original Message-----
From: Rick Karlquist [mailto:[email protected]]
Sent: Friday, January 15, 2010 8:48 PM
To:[email protected] <mailto:[email protected]>; Discussion of precise time
and frequency
measurement
Subject: Re: [time-nuts] HP10811 losing EFC
The fuse in an interesting topic. It is a thermal fuse, not an electrical
one. It deals with oven failure. It does not prevent the oven from
failing, but rather limits the amount of damage and smoke if the
oven runs away. The main purpose is to limit toxic outgassing,
rather than to protect the oscillator, although it may
accidentally do that. I have never heard of a case
of an oven running away, although it is theoretically possible,
for example, if the thermistor is open or disconnected.
(I have never heard of a thermistor failing either for that matter.)
The fuse cannot be soldered in for the obvious reason that it
cannot tolerate solder temperatures. It is instead inserted into
a non gold plated socket. In 99+% of "fuse failures", the fuse
has not blown (as can be confirmed with an ohmmeter) but instead
is not making contact. You might see if your fuse is still good.
In any event, I recommend bypassing the fuse with a jumper and
not worrying about it. There is far more chance of the fuse
"failing" than the oven running away.
Rick Karlquist N6RK
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