[time-nuts] Fwd: HP10811 Oscillator Thermal Fuse

Donald E. Pauly trojancowboy at gmail.com
Thu May 11 19:04:30 EDT 2017


This is the third report of an open thermistor which would have
resulted in a fire in an HP10811 oven.  There are a dozen other
problems that could cause such a fire.  See
http://gonascent.com/papers/hp/hp10811/thermal.jpg .  The leads are
long enough that they can be soldered quickly without a heat sink if
the socket is to be replaced for better reliability.  A hemostat can
also be used to heatsink the leads one at a time.  We have the 108° C
fuse which is prone to nuisance blows.  HP has superceded it with a
125° C version.  Panasonic seems to make a direct replacement.  It is
stocked by Digikey for under a dollar.  I can find no reports of a
nuisance blow of the 125° version of the thermal fuse in a HP10811.
All nuisance blows seem to be in the 108° version.

In the 1970s, many consumer electronic products with 60 cps power
transformers had a thermal fuse inside them.  I bought a Sony real to
real stereo tape deck which was not working with that problem.  It was
a nuisance blow since no problems existed in the tape deck.  I
installed a fine gauge piece of solder since the fuse was close to
180° C.  I used it for over two years with no problems.

Later model switching power supplies have windings of several volts
per turn.  Shorts in these transformers will blow the fuse or destroy
the switching transistors.  A 60 cps transformer operates at a small
fraction of a volt per turn. A few shorted turns in a will not draw an
excessive primary current but merely causes a hot spot.  The hot spot
will grow as additional turns short.  Line current will not be greatly
excessive even as the transformer heats up.  It can catch fire well
before the fuse blows.

I saw a living room that caught fire because of a 60 cps transformer
in a stereo receiver.  The line breaker never tripped.  But for the
grace of G-d, the house would have burnt down.  A thermal fuse in the
transformer would have prevented several thousand dollars of damage.
That would have paid for over a thousand thermal fuses.


-------- Forwarded message ----------
From: David G. McGaw <david.g.mcgaw at dartmouth.edu>
Date: Wed, May 10, 2017 at 8:20 PM
Subject: Re: [time-nuts] Fwd: HP10811 Oscillator Thermal Fuse
To: Discussion of precise time and frequency measurement <time-nuts at febo.com>

I too have had a fuse open up due to a failed thermistor in a HP10811.

David N1HAC

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