[time-nuts] HP10811 losing EFC

Rick Karlquist richard at karlquist.com
Sat Jan 16 01:47:33 UTC 2010


Tom Holmes, N8ZM wrote:
> While on the subject of 10811's, I have one which appears to have a blown
> fuse. Bypassing same, it seems to work OK, so suspect that tiny filament
> or
> whatever simply succumbed to some shock or vibration, or maybe just old
> age.
> I don't recall the current draw now, but it seemed reasonable at the time,
> and it came up on frequency and appeared to be controlling the oven temp
> OK.
>
>
> Would like to replace the fuse with something more correct than a #22
> before
> I let it go online full time.
>
> Any ideas for where to get replacement, or a suitable substitute?
>
> Thanks. This is a fun list to monitor. I learn more in a day here than
> anything else I've ever done.
>
> Regards,
>
> Tom Holmes, N8ZM
> Tipp City, OH
> EM79xx

The fuse in an interesting topic.  It is a thermal fuse, not an electrical
one.  It deals with oven failure.  It does not prevent the oven from
failing, but rather limits the amount of damage and smoke if the
oven runs away.  The main purpose is to limit toxic outgassing,
rather than to protect the oscillator, although it may
accidentally do that.  I have never heard of a case
of an oven running away, although it is theoretically possible,
for example, if the thermistor is open or disconnected.
(I have never heard of a thermistor failing either for that matter.)
The fuse cannot be soldered in for the obvious reason that it
cannot tolerate solder temperatures.  It is instead inserted into
a non gold plated socket.  In 99+% of "fuse failures", the fuse
has not blown (as can be confirmed with an ohmmeter) but instead
is not making contact.  You might see if your fuse is still good.
In any event, I recommend bypassing the fuse with a jumper and
not worrying about it.  There is far more chance of the fuse
"failing" than the oven running away.

Rick Karlquist N6RK




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