Achim, Thanks for the great pictures. I have added them to my collection.
Several years ago, I had a similar issue with a 10811 (though I can't remember the variant, looked like yours though) that, indeed was F1, the thermal fuse. It is designed to open at something like 105 degrees C, IIRC, and prevent the unit from getting cooked by a runaway heater circuit. Amazingly, HP/Agilent still had the part in stock (and may still have) and I ordered a few as replacements. My recollection of the discussion is that the fuse tends to fail after so many years with no problem with the heater circuit. The troubleshooting guide basically guides you through what you found to prove the circuit is operating properly. However, the failed part in my unit did not look anything like the part in your unit. I did not recognize any of the markings on the part either. It was a 'plug in' part, like yours, though, not soldered. It was an axial part about the size of a 1/4 watt resistor. I am not familiar with 'polyfuse'. Are these thermal fuses? Your description suggests that they are current limited fuses rather than thermal limited fuses. While I suspect the original part has a 'current limit' protective feature, it is designed to be a temperature limit protection device. Joe -----Original Message----- From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On Behalf Of Achim Vollhardt Sent: Friday, October 21, 2011 4:47 AM To: time-nuts@febo.com Subject: [time-nuts] HP10811-60111 oven failure + repair 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 _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.