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

J. L. Trantham jltran at att.net
Fri Oct 21 09:37:04 EDT 2011


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


-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Achim Vollhardt
Sent: Friday, October 21, 2011 4:47 AM
To: time-nuts at 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:


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


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