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

[Wilko Bulte]

Aha! That is very interesting background information! It nicely puts your comment in perspective to "bridge the fuse with a wire and be done with it". 

Big grin here when reading the "beer was involved" comment. I have used a similar approach on my business visits to HP CXO (well after T&M was spun off) ;-). A bit like a certain "beer ware license" (hi Poul).

Wilko

> On 1 Dec 2021, at 20:20, Bob kb8tq <kb8tq at n1k.org> wrote:
> 
> 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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