Ovens
very rarely run away. It is far more likely the fuse will fail
or its socket will corrode (can't solder it in because the solder
would melt the fuse). If the oven does run away, the heater
transistors will open up and serve as fuses

Rick-

I've seen photos of these ovens on E-Bay, that had been stained by smoke coming
out of the adjustment hole. I'd rather have some kind of protection.

I believe the oven uses proportional control, so the transistors' maximum
dissipation would occur when the heating element is half on. In a "runaway"
mode, the transistors would be switched on with maximum current but nearly zero
voltage. Also, one transistor failure mode is a short-circuit.

With regard to John Miles' comment about the thermal fuse being to far from the
oven's heating element to be effective, perhaps that is true. However, the
earlier thermal fuse was rated at 108 degrees C, and it occasionally would open
in an oven that was apparently operating correctly in the range of 80 to 84
degrees C. The newer fuse is rated at 115 degrees C. I suspect the problem is
that it is opening due to a combination of time and temperature, not
temperature alone.

I've been running one of the new parts for about two years without a hitch.
The frequency has not been adjusted since about two years ago, and it still
takes 15 or 20 seconds to drift one Hz against a 10 MHz rubidium oscillator.

That HP 5334B is one nice counter!

73, Fred, K4DII