[time-nuts] Fwd: Fwd: HP10811 Oscillator Thermal Fuse

Donald E. Pauly trojancowboy at gmail.com
Wed May 10 23:04:44 EDT 2017


In order to have a runaway trophy, the thermal fuse would have to have
been jumpered and then an oven control failure would have to have
occurred.  The rated temperature of 125° C is well above operating
temperature of 82°C.  The thermal fuse can easily be soldered in by
heat sinking the leads where they enter the fuse.  The leads are
plenty long enough. I have done this dozens of times on Amana
microwave ovens in the late 70s. The venerable HP105 oscillator
contains a thermal fuse in both the fast warmup heater and the
proportional heater.

"3-8 Each heater circuit contains a thermal fuse to prevent damage to
components within the oven due to overheating."

I hadn't thought about smoking the styrofoam which melts at 240° C.
https://en.wikipedia.org/wiki/Polystyrene#Extruded_polystyrene_foam I
calculate that the oven could reach at least 208° C at the rated 71° C
ambient.    Solder melts at 180° C.  Each power transistor has around
10 Volts across it and might even reach 300° C without destruction.
Many oscillators are operated above 20 Volts for the heater supply.
We have two reports of open thermistors in which disaster was avoided
by the thermal fuse blowing.


---------- Forwarded message ----------
From: Richard (Rick) Karlquist <richard at karlquist.com>
Date: Wed, May 10, 2017 at 5:34 PM
Subject: Re: [time-nuts] Fwd: HP10811 Oscillator Thermal Fuse
To: Discussion of precise time and frequency measurement <time-nuts at febo.com>

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

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