[time-nuts] HP10811 losing EFC

[Magnus Danielson]

Rick Karlquist wrote:
> 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.

I have seen an OCXO have oven runaway due to failing thermistor. The 
vendor replaced the OCXO. So, they are not unheard off, but I haven't 
seen it in HP10811s, fortunatly.

I've also seen a pair of OCXOs with dead ovens.

Cheers,
Magnus