[time-nuts] Questions about HP 5370B

Rex rexa at sonic.net
Tue Sep 14 19:54:53 UTC 2010


  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:rich... at karlquist.com]
>  Sent: Friday, January 15, 2010 8:48 PM
>  To:thol... at woh.rr.com  <mailto:thol... at woh.rr.com>; 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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