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
WB0KVV

https://www.febo.com/pipermail/time-nuts/2017-May/date.html

I disagree about permanently jumpering the thermal fuse.  The
oscillator is rated down to -55° C and the oven can maintain 82° C at
that temperature.  This is a 137° C rise.  It is likely capable of
more heat than that.  At 25° C, it would therefore be able to reach at
least 25+137=162° C.  This is near solder melting temperature and
would quickly cook the components.  It is rated to work at 71° which
would give a temperature of 208° C and do serious damage.  That
thermal fuse is very important.  There are a dozen failures which
could result in a thermal runaway.

This web page discusses the fact that a thermal fuse should be 30° C
above normal operating temperature to prevent nuisance blows.
http://www.simonsdialogs.com/2016/09/a-thermal-fuse-and-hp-10811-60111-repair/
HP originally used a 26° C margin and later changed it to a 33°
margin.  This explains the frequent failures.  This poster used the
wrong type of fuse but the correct type is commercially available.

πθ°μΩω±√·Γλ
WB0KVV

---------- Forwarded message ----------
From: Bob kb8tq kb8tq@n1k.org
Date: Mon, May 8, 2017 at 6:07 PM
Subject: Re: [time-nuts] HP10811 Oscillator Thermal Fuse
To: Discussion of precise time and frequency measurement time-nuts@febo.com
Cc: "Donald E. Pauly" trojancowboy@gmail.com

Hi

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.

Bob

Respectfully I disagree. I have totally shorted out numerous oscillator’s fuses with no problems. The design of those thermal fuses makes them vulnerable to opening just due to years of use without problem, in any of the electronics. My shorted out oscillators still work. Ideally, if handy, it is easy enough to replace them if one is available. They just plug in. If there is a thermal runaway, that caused the opening, to me that implies that there are other issues. Regards – Mike

Mike B. Feher, N4FS

89 Arnold Blvd.

Howell, NJ, 07731

848-245-9115

-----Original Message-----
From: time-nuts [mailto:time-nuts-bounces@febo.com] On Behalf Of Donald E. Pauly
Sent: Tuesday, May 09, 2017 9:49 PM
To: time-nuts; Donald E. Pauly
Subject: [time-nuts] Fwd: HP10811 Oscillator Thermal Fuse

https://www.febo.com/pipermail/time-nuts/2017-May/date.html https://www.febo.com/pipermail/time-nuts/2017-May/date.html

I disagree about permanently jumpering the thermal fuse.  The oscillator is rated down to -55° C and the oven can maintain 82° C at that temperature.  This is a 137° C rise.  It is likely capable of more heat than that.  At 25° C, it would therefore be able to reach at least 25+137=162° C.  This is near solder melting temperature and would quickly cook the components.  It is rated to work at 71° which would give a temperature of 208° C and do serious damage.  That thermal fuse is very important.  There are a dozen failures which could result in a thermal runaway.

This web page discusses the fact that a thermal fuse should be 30° C above normal operating temperature to prevent nuisance blows.

http://www.simonsdialogs.com/2016/09/a-thermal-fuse-and-hp-10811-60111-repair/ http://www.simonsdialogs.com/2016/09/a-thermal-fuse-and-hp-10811-60111-repair/

HP originally used a 26° C margin and later changed it to a 33° margin.  This explains the frequent failures.  This poster used the wrong type of fuse but the correct type is commercially available.

πθ°μΩω±√·Γλ

WB0KVV

---------- Forwarded message ----------

From: Bob kb8tq < mailto:kb8tq@n1k.org kb8tq@n1k.org>

Date: Mon, May 8, 2017 at 6:07 PM

Subject: Re: [time-nuts] HP10811 Oscillator Thermal Fuse

To: Discussion of precise time and frequency measurement < mailto:time-nuts@febo.com time-nuts@febo.com>

Cc: "Donald E. Pauly" < mailto:trojancowboy@gmail.com trojancowboy@gmail.com>

Hi

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.

Bob

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https://www.febo.com/pipermail/time-nuts/2017-May/date.html

The poster at http://www.simonsdialogs.com/2016/09/a-thermal-fuse-and-hp-10811-60111-repair/
reported that the thermal fuse saved an oscillator from destruction
when a thermistor opened.  HP made thousands of these which sold for
$800.  If a large portion failed it would have been addressed.  They
raised the temperature of the thermal fuse to reduce nuisance blows.
I say that nuisance blows equal to real blows are worth the trouble.
You may be getting oscillators on the surplus market or eBay which
have a higher proportion of nuisance blows.

πθ°μΩω±√·Γλ
WB0KVV

From: Mike Feher mfeher@eozinc.com
Date: Tue, May 9, 2017 at 9:10 PM
Subject: Re: [time-nuts] Fwd:  HP10811 Oscillator Thermal Fuse
To: Discussion of precise time and frequency measurement time-nuts@febo.com

Respectfully I disagree. I have totally shorted out numerous
oscillator’s fuses with no problems. The design of those thermal fuses
makes them vulnerable to opening just due to years of use without
problem, in any of the electronics. My shorted out oscillators still
work. Ideally, if handy, it is easy enough to replace them if one is
available. They just plug in. If there is a thermal runaway, that
caused the opening, to me that implies that there are other issues.
Regards – Mike

Mike B. Feher, N4FS

89 Arnold Blvd.

Howell, NJ, 07731

848-245-9115

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

https://www.febo.com/pipermail/time-nuts/2017-May/date.html

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.

πθ°μΩω±√·Γλ
WB0KVV

---------- Forwarded message ----------
From: Richard (Rick) Karlquist richard@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@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

3. People tend to collect stuff like that.  One
   engineer did have a 10811 with a 45 caliber bullet
   fired through it (long story).

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

5. 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.

6. 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