I wrote a long description regarding this phenomenon many moons ago. The thermal fuses in the HP ovens do fail, mostly just because of aging, and not because of any problems. The fuse holder contains a wax substance which is normally hard and keeps the contacts closed. If suddenly the contacts were to get too hot, the wax would melt, at a predetermined temperature which is a function of the wax, opening the fuse for good, since after the wax cools down to be hard again it is too late, the fuse is open. Aging can unfortunately have a similar effect where the springiness of the contacts eventually separate due to the breaking down of the wax substance. I have repaired many HP-105 oscillators as well as the 10811A series types by simply shorting out the fuse, which is actually typically socketed, with never any adverse results. In fact, one oscillator from a HP-105 that had this problem over 20 years ago is still working today with a short in the place of the fuse. Regards - Mike
Mike B. Feher, N4FS 89 Arnold Blvd. Howell, NJ, 07731 732-886-5960 -----Original Message----- From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On Behalf Of Dan Rae Sent: Tuesday, September 14, 2010 11:28 AM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] Questions about HP 5370B On 9/14/2010 1:23 AM, John Miles wrote: > >> As Bruce suggests, you'll want to peek inside to see that you >> really do have >> a 10811 oscillator. If so, then it sounds like the thermal fuse >> (F1, inside >> the 10811) might be open. > Just short it out, or if you like, put in an NTE part with a similar temp > rating. It serves little or no useful purpose. > > -- john, KE5FX > > > I hate to disagree with John who knows a heck of lot more than I ever will, but in this case it will protect the oven from cooking up if the control circuit fails with the heater full on, which can happen. I did have a 5370B with a 10811 that had a bad thermistor in it as well as an open fuse. I'd guess that's why -hp- fitted it. But yes, the thermal fuses can and do fail open for no good reason, and it sounds like this has happened here, but I would not recommend shorting it out permanently. Dan _______________________________________________ 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.