[time-nuts] HP10811 Oscillator Thermal Fuse

Bob kb8tq kb8tq at n1k.org
Mon May 8 21:07:45 EDT 2017


You will get a lot of diversity of opinion on the topic of the thermal fuse on the 10811. 
My feeling is that they are a nuisance and contribute very little to the design. I’d just
short it out and move on. In the era of failure prone heater transistors or faulty thermistors,
the fuse may have made sense. That era ended before the 10811 went into production.


> On May 8, 2017, at 7:25 PM, Donald E. Pauly <trojancowboy at gmail.com> wrote:
> https://www.febo.com/pipermail/time-nuts/2017-May/date.html
> 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
> https://www.digikey.com/product-detail/en/panasonic-electronic-components/EYP-05BE115/P10907-ND/295803
> .  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:
> [https://groups.google.com/forum/#!msg/rec.radio.amateur.homebrew/g3Up39Wljak/DGr8OeLdz2gJ
> 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!
> Fred
> K4DII]
> Does anyone have any experience on this thermal fuse or words of wisdom?
> πθ°μΩω±√·Γλ
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