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

[Jared Cabot]

I was reading in a past thread on the time-nuts mailing list that the thermal fuses can occasionally spontaneously go open circuit with age, far more often than going open due to a real thermal fault.
(I wonder how many have been discarded over the years due to this issue?)
They appear to be constructed from two spring contacts held closed by wax. As the fuse heats up, the wax melts allowing the contacts to permanently open.
It would appear that age can cause the contacts to open too... A common fix is a wire jumper (The fuse is conveniently socketed, so a resistor leg offcut makes a nice jumper).

I plan to source a replacement thermal fuse, just to keep things a bit safer.

Also, I did get the pictures, thanks for that! I replied directly just now.


Jared.

‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐

On Wednesday, December 1st, 2021 at 02:05, 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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