[time-nuts] Fwd: HP10811 Oscillator Thermal Fuse

Richard (Rick) Karlquist richard at karlquist.com
Wed May 10 20:34:48 EDT 2017

The view from inside HP when I worked with the people
who designed and built the 10811 some 35-40 years ago
was that:

1.  10811 ovens rarely fail.

2.  When they do fail, it is rarely because the oven
runs away.  I know I have never encountered a runaway.
No one at HP had a "trophy" on their desk of a runaway
10811.  People tend to collect stuff like that.  One
engineer did have a 10811 with a 45 caliber bullet
fired through it (long story).

3.  From a business perspective, a failure is a failure
and so there is no business reason to have a fuse.

4.  Because the fuse could not be soldered in, it had
to be socketed, and the socket failures exceeded any
oven runaways by a good margin.  Therefore, it made
the "failure rate" worse.  That is all that matters to
the bean counters.

5.  The one and only reason it was in there at all
was the concern about toxic gases being released from
the foam.  Even without a runaway, foams tend to have
a "slow burn" and outgas "stuff" all the time.  Various
foams were evaluated to balance that issue with thermal
resistance and with the big issue with foam which is
mechanical fatigue.  This is similar to the wear out
of foam mattresses.

What should have been done with the thermal fuse would
have been to put crimp lugs on the leads and attach
the crimp lugs with screws.  However, there was no space
for all that stuff.

Rick N6RK

On 5/10/2017 4:43 PM, Dan Rae wrote:
> As I reported the last time this subject came up, or maybe the time
> before, the only time I had an open thermal fuse was in a 10811 that had
> an open thermistor.  I was able to replace that and get the OCXO
> working, but if the fuse had been replaced with a wire link I'm sure the
> entire oven would have been toast.
> There have been two types of fuse fitted, all I those have are the type
> that can be fitted with the plug in axial panasonic type, less than a
> dollar from Digikey.  The older ovens had a different type but I imagine
> it's similarly easy to replace.
> All my 10811s have the higher temperature Panasonic fuse fitted and I
> sleep well at night...
> Dan
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