https://www.febo.com/pipermail/time-nuts/2017-May/date.html
In order to have a runaway trophy, the thermal fuse would have to have been jumpered and then an oven control failure would have to have occurred. The rated temperature of 125° C is well above operating temperature of 82°C. The thermal fuse can easily be soldered in by heat sinking the leads where they enter the fuse. The leads are plenty long enough. I have done this dozens of times on Amana microwave ovens in the late 70s. The venerable HP105 oscillator contains a thermal fuse in both the fast warmup heater and the proportional heater. "3-8 Each heater circuit contains a thermal fuse to prevent damage to components within the oven due to overheating." I hadn't thought about smoking the styrofoam which melts at 240° C. https://en.wikipedia.org/wiki/Polystyrene#Extruded_polystyrene_foam I calculate that the oven could reach at least 208° C at the rated 71° C ambient. Solder melts at 180° C. Each power transistor has around 10 Volts across it and might even reach 300° C without destruction. Many oscillators are operated above 20 Volts for the heater supply. We have two reports of open thermistors in which disaster was avoided by the thermal fuse blowing. πθ°μΩω±√·Γλ WB0KVV ---------- Forwarded message ---------- From: Richard (Rick) Karlquist <[email protected]> Date: Wed, May 10, 2017 at 5:34 PM Subject: Re: [time-nuts] Fwd: HP10811 Oscillator Thermal Fuse To: Discussion of precise time and frequency measurement <[email protected]> The view from inside HP when I worked with the people who designed and built the 10811 some 35-40 years ago was that: 1. 10811 ovens rarely fail. 2. When they do fail, it is rarely because the oven runs away. I know I have never encountered a runaway. No one at HP had a "trophy" on their desk of a runaway 10811. People tend to collect stuff like that. One engineer did have a 10811 with a 45 caliber bullet fired through it (long story). 3. From a business perspective, a failure is a failure and so there is no business reason to have a fuse. 4. Because the fuse could not be soldered in, it had to be socketed, and the socket failures exceeded any oven runaways by a good margin. Therefore, it made the "failure rate" worse. That is all that matters to the bean counters. 5. The one and only reason it was in there at all was the concern about toxic gases being released from the foam. Even without a runaway, foams tend to have a "slow burn" and outgas "stuff" all the time. Various foams were evaluated to balance that issue with thermal resistance and with the big issue with foam which is mechanical fatigue. This is similar to the wear out of foam mattresses. What should have been done with the thermal fuse would have been to put crimp lugs on the leads and attach the crimp lugs with screws. However, there was no space for all that stuff. Rick N6RK _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
