[time-nuts] Re: HP 5065A, no 2nd harmonic.

[Bob kb8tq]

Hi

The reason for the thermal fuse (which is *not* a feature of other OCXO’s) 
is the 10811’s deployment in a wide range of devices. There are a few that
do have restrictions on them. The 5065 is not one of those devices. 

The protection provided is minimal. The pass transistor will fail in a runaway
situation. Unless you happen to be on a submarine or something similar
you are not creating a significant problem when it does. 

The thermal fuse significantly degrades the reliability of the device. It is by
far the most likely part in the entire OCXO to fail. Putting up with this failure
process (and the cost of the fuses) is not reasonable if the benefit is quite
small. 

Indeed there was some beer involved in the discussion, but HP OCXO 
engineering was onboard with the above information last time we chatted 
about it. That would have been in the early 1990’s …. These failures have
been going on for a *long* time. 

Bob

> On Dec 1, 2021, at 7:41 AM, Wilko Bulte <wkb at xs4all.nl> wrote:
> 
> In general it is a very bad idea to defeat safety provisions that a manufacturer put in their product. They are there for a reason, any manufacturer would omit them, if only to save on cost. 
> 
> I would think no-one on this list would replace a blown fuse in their house fuse box with a nail. Or?
> 
> Wilko
> 
>> On 1 Dec 2021, at 12:13, Askild <megafluffy at gmail.com> wrote:
>> 
>> Hi,
>> 
>> Just a little warning.
>> Included are some pictures of a 10811 where thermal fuse protection have
>> not functioned.
>> I do not know why, if the fuse did not work, or somebody replaced it with a
>> wire, as this was in a HP 8663A that I found in a recycling bin, and I do
>> not know the history.
>> 
>> 
>> Regards,
>> Askild
>> 
>> 
>> 
>>> On Tue, Nov 30, 2021 at 8:34 PM Bob kb8tq <kb8tq at n1k.org> wrote:
>>> 
>>> Hi
>>> 
>>> The thermal fuses die on their own far more often than they die protecting
>>> anything. The fuse is there to provide safety is certain odd situations.
>>> The
>>> simple answer these days is to replace it with a piece of wire.
>>> 
>>> Bob
>>> 
>>>>> On Nov 30, 2021, at 12:05 PM, Larry McDavid <lmcdavid at lmceng.com> wrote:
>>>> 
>>>> Do these thermal fuses fail spontaneously or is there a common cause?
>>>> 
>>>> If the "heater" transistor mounting screws were quite loose, those
>>> transistors themselves might have overheated and failed. But, a thermal
>>> fuse should fail by high temperature...
>>>> 
>>>> Sounds like progress, though. Cold OCXO means low frequency.
>>>> 
>>>> Jared, I sent you a private email with some pictures; did you get that?
>>>> 
>>>> Larry
>>>> 
>>>> 
>>>> On 11/30/2021 8:03 AM, Jared Cabot via time-nuts wrote:
>>>>> Well, I found it, the thermal fuse is open circuit.
>>>>> If I briefly bypass it with a jumper wire, the oven pulls the expected
>>> current.
>>>>> Looks like I need something that will trip at roughly around 115C,
>>> rated to 1.0A
>>>>> (HP Part number 10811-80008)
>>>>> Hopefully I can find something in Akihabara to make it easier to get my
>>> hands on ASAP...
>>>>> Jared
>>>> ...
>>>> --
>>>> Best wishes,
>>>> 
>>>> Larry McDavid W6FUB
>>>> Anaheim, California (SE of Los Angeles, near Disneyland)
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> <P1010309_s.jpg><P1010305_s.jpg><P1010306_s.jpg>
>> 
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