[time-nuts] HP10811 Oscillator Thermal Fuse

Donald E. Pauly trojancowboy at gmail.com
Mon May 8 19:25:40 EDT 2017


We got a new beam tube for our #2 HP5061B cesium clock.  When the tube
was changed, it momentary locked for a couple of minutes but then
unlocked.  The oscillator had been set on frequency with our #1
HP5061B a while back but it was suddenly 100 cycles low at 5 mc.
Oscillator oven current was normal at 38.  This oscillator is used on
many other HP instruments such as counters.

KB7APQ tore apart the HP10811 oscillator and found that the thermal
fuse was open in the oven heater.  We think that it was a nuisance
blow.  Does anyone have any experience on this thermal fuse?  Digikey
seems to have
one that will work for under a dollar at
.  We temporarily jumpered the fuse, the oscillator came right up on
frequency and we got our first lock.  We watched the actual oven
current go down when it reached operating temperature.  The oscillator
oven current indication is useless on the HP meter since it is not
actual oven current.

I found this post:


I dug out my notes to find out the specifics.  My counter is an HP 5334B
with Option 1, the high stability oscillator.  The oscillator is one of
the HP 10811 series.  The original thermal fuse, F1, was rated for 108
degrees C.  HP revised the part to one rated for 115 degrees C, part
number 2110-0617 (10811-80003).

The Radio Shack thermal fuse was 270-1322A, rated for 128 degrees C.
There is also an NTE Electronics NTE8115, "Thermal Cut-Off", rated for
117 degrees C.  Either is larger than the HP part, and is a tight fit.
I wrapped them in electrical tape to protect against short circuits.
When soldering to the high-value resistors, I clamped the fuse leads in
a pair of pliers held with rubber bands.

Of course this may not apply to your counter if it doesn't have the
crystal oven!


Does anyone have any experience on this thermal fuse or words of wisdom?


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