[time-nuts] Fwd: HP10811 Oscillator Thermal Fuse

Mike Feher mfeher at eozinc.com
Wed May 10 00:10:44 EDT 2017

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



-----Original Message-----
From: time-nuts [mailto:time-nuts-bounces at 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.





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

From: Bob kb8tq < <mailto:kb8tq at n1k.org> kb8tq at 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 at febo.com> time-nuts at febo.com>

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




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.




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