Ok, so I carefully dismantled the OCXO and I may have found the culprit... The screws holding the two transistors to the metal body were both less than finger tight!
Don't know how that happened, so they were cinched up to 5in-lbs with the torque screwdriver as per the service manual, and I'll give the thing a test once it's all back together.
I'll have a dig around to see if I can find a small thermocouple or similar to see what the oven temperature is at now.

Also, I noticed that a 2.0uF, 20% film capacitor in there is looking like it's had a bit of heat-stroke.
It measures 2.119uF, 0.22ohm ESR, with a dissipation of 0.004 @ 40V, @ 1kHz, so it seems to test ok.
Does this look like it needs to be replaced (picture link below)? I can't find the same part on Digikey/Mouser, but it seems like Keysight have them as 'orderable' status.
Details are as follows: P/N: 0160-4947 Part Description: Capacitor-Fixed 2uF +-20PCT 50V polyester metallized THT

https://i.imgur.com/aZ0KUD8.jpg

Thanks!
Jared

‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐

On Monday, November 29th, 2021 at 06:30, cdelect@juno.com wrote:

Hi

Quick and dirty way to check the oven is to monitor current. It should start out
near the max rated power and drop back after a few minutes.

Bob

That film cap does look a little cooked. Since it is a 2uF 20% 50V part a more common 2.2uF 10% 63V part would likely be an acceptable substitute, and basically still within overall tolerance. Judging from the teflon solder cup used I suspect insulation resistance is a critical parameter. Be sure to clean away any flux remaining after soldering.

Bob L.

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

‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐

On Wednesday, December 1st, 2021 at 00:41, Robert LaJeunesse lajeunesse@mail.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:

...

Best wishes,

Larry McDavid W6FUB
Anaheim, California (SE of Los Angeles, near Disneyland)

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@lmceng.com wrote:

On 11/30/2021 7:19 AM, Bob kb8tq wrote:

And it's worth checking the thermal fuse, which I've had fail in a lot
of them.

Dan

Yes the fuses can just fail like any fuse. But with the oven it pays to
very carefully insure they didn't fail because of temp. and that they did
there job.
Regards
Paul
WB8TSL

On Tue, Nov 30, 2021 at 1:09 PM Dan Rae via time-nuts <
time-nuts@lists.febo.com> 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