In article ,
Fred McKenzie wrote:

It turned out that there was an open thermal fuse in the supply line
inside the crystal oven. It was a plug-in device about the size of a
half watt resistor, maybe smaller. There were two wire-sockets that its
leads plugged into.

I dug out my notes to find out the specifics. My counter is an HP 5334B
with Option 1, the high stability oscillator. The oscillator is one of
the HP 10811 series. The original thermal fuse, F1, was rated for 108
degrees C. HP revised the part to one rated for 115 degrees C, part
number 2110-0617 (10811-80003).

The Radio Shack thermal fuse was 270-1322A, rated for 128 degrees C.
There is also an NTE Electronics NTE8115, "Thermal Cut-Off", rated for
117 degrees C. Either is larger than the HP part, and is a tight fit.
I wrapped them in electrical tape to protect against short circuits.
When soldering to the high-value resistors, I clamped the fuse leads in
a pair of pliers held with rubber bands.

Of course this may not apply to your counter if it doesn't have the
crystal oven!

Fred
K4DII