[time-nuts] HP 5370B low frequency modulation

Mike Feher mfeher at eozinc.com
Sun Jul 22 22:58:00 EDT 2007

Highly unlikely, but, possible, especially if it was in a corrosive
atmosphere. Of course then I would expect to see evidence of corrosion on
other components. When we do not know the answer we can come up with so many
possibilities and not have one of them correct. I have run across a lot of
those defective fuses over the years as HP has been using them forever.
There was no real evidence as to why they opened, and that is why before I
guessed at just plain ageing and the wax eventually getting soft. I just
checked the current flow within the oscillator where the fuse used to be
plugged in, which was at least nice of them, and always found it to be
normal. I just took a piece of buss wire and plugged it into the same two
pins were the fuse used to be and never had a problem again. The situation
Magnus describes of course is very unusual, and, in reality would only be
found by a time nut as it is so miniscule. There is also some periodicity to
it, which does suggest some control loop problem, but, I would hate to even
take a guess. - Mike 

Mike B. Feher, N4FS
89 Arnold Blvd.
Howell, NJ, 07731

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Dr Bruce Griffiths
Sent: Sunday, July 22, 2007 10:32 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] HP 5370B low frequency modulation

Mike Feher wrote:
> ); SAEximRunCond expanded to false
> Errors-To:
time-nuts-bounces+bruce.griffiths=xtra.co.nz+bruce.griffiths=xtra.co.nz at febo
> I believe it is almost impossible to have an intermittent thermal fuse due
> to their very clever design. Although over age they can open up. Once
> however I do not believe they can connect again. The fuse is simply to
> metallic contacts that are force to be in contact during assembly. Their
> normal desired resting state would be to be not connected. When closed,
> fuse cavity is filled with wax, which mostly determines when the fuse
> (temperature). With too much current through the fuse, the wax heats
> allowing the contact to spring apart to their normal resting position.
> with no more current flowing, the wax hardens almost immediately, never
> allowing the two contacts to meet again. - Mike 
> Mike B. Feher, N4FS
> 89 Arnold Blvd.
> Howell, NJ, 07731
> 732-886-5960

Whilst the fuse itself may not be the problem, it is a plugin component 
and the contacts may be problematic.


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