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

[Jared Cabot]

Well, yay...

I put the OCXO back in the 5065A, and got a good 2nd Harmonic reading. So I started going through the alignment procedures and somehow I now have no 2nd harmonic again, and I can't get it back...

I'm starting to lose interest in this thing, seeing as I have the Leo Bodnar GPS standard anyway that I don't have to sit for hours twiddling dials and poking things just to get it maybe sort of working.

I might give this one more chance before I sell it off to someone with a little more patience, or who prefers fixing gear instead of actually using it. I think I've had this thing working for all of a couple days in the months I've owned it, whereas the GPS standard is the size of a pack of cards and has never failed in the same time.


Jared.



‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐

On Thursday, December 2nd, 2021 at 04:19, 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)
> > > > > 

> > > > > time-nuts mailing list -- time-nuts at lists.febo.com -- To unsubscribe
> > > > > 

> > > > > send an email to time-nuts-leave at lists.febo.com
> > > > > 

> > > > > To unsubscribe, go to and follow the instructions there.
> > > > 

> > > > time-nuts mailing list -- time-nuts at lists.febo.com -- To unsubscribe send
> > > > 

> > > > an email to time-nuts-leave at lists.febo.com
> > > > 

> > > > To unsubscribe, go to and follow the instructions there.
> > 

> > <P1010309_s.jpg><P1010305_s.jpg><P1010306_s.jpg>
> > 

> > > time-nuts mailing list -- time-nuts at lists.febo.com -- To unsubscribe send an email to time-nuts-leave at lists.febo.com
> > > 

> > > To unsubscribe, go to and follow the instructions there.
> > 

> > time-nuts mailing list -- time-nuts at lists.febo.com -- To unsubscribe send an email to time-nuts-leave at lists.febo.com
> > 

> > To unsubscribe, go to and follow the instructions there.
> 

> time-nuts mailing list -- time-nuts at lists.febo.com -- To unsubscribe send an email to time-nuts-leave at lists.febo.com
> 

> To unsubscribe, go to and follow the instructions there.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 855 bytes
Desc: not available
URL: <http://febo.com/pipermail/time-nuts_lists.febo.com/attachments/20211202/850b912d/attachment.sig>