[time-nuts] HP10811-60111 oven failure + repair

[Achim Vollhardt]

Fellow time-nuts,

my humble HP10811 + Shera controller combo signaled loss of lock since 
about a week after 5 years of non-stop operation. Frequency measurement 
showed it to be ca. 90 Hz low and the added oscillator enclosure (8mm 
aluminum case) barely exceeded room temperature. Measurement showed oven 
current to be zero and after some Google research, the thermal fuse for 
the oven controller became the prime suspect.

Opening the oscillator is rather easy due to screws, pulling the inner 
isolation lid as well. With help of the service manual, the fuse is 
quickly identified and proved indeed to be open.

I had a 24V, 1.5A polyfuse at hand which was soldered in parallel to the 
existing fuse. 1.5A seems a bit large, however I think some derating is 
advantageous due to the elevated temperatures. Closed everything and 
first connected the oven supply only:
initial 510mA, going down to 105mA after 10-ish minutes. Perfect.

Putting everything together again and the HP10811 is again within 0.1 Hz 
of 10 MHz without the control loop closed. Getting it back to its 
antenna later the day..

For some hi-res pictures of the procedure:

http://gulp.physik.uzh.ch/HP10811_oven-repair/

Hope this is of help to anybody with the same problem.

Regards,
Achim