Usually, I don’t care much about high precision oscillator options being fitted to frequency counters, etc., because in the lab, any critical equipment is anyway connected to an external well-controlled 10 MHz reference, locked to DCF77. However, this time I need to install a OCXO (HP 10811) in a HP 5335A counter for service outside of the lab.

The only thing that needs to be done is to remove a jumper on the board of the 5335A (see red box in picture below), and mount the 10811 in the slot already prepared for the OCXO inside.

While such installation is fairly straightforward, it turned out to take more time than expected – simply because of the OCXO not showing any stable output signal.

After a few quick tests, the cuprit was found, a defective (open) thermal fuse. This is apparently a quite common issue for the 10811 oscillators, and you might get away with just putting in a wire jumper. However, I didn’t want to take any risk of overheating in case of a failure of the 25+ years old OCXO circuits. An exact match for the thermal fuse could not be found, so just soldered in (very carefully, cooling the case and leads!) a 10 Amp 109 degC fuse.

This is the OCXO with the new fuse installed.

This style of fuse as a non-insulated outer shell, so a shrink tubing sleve serves as insulation.

Finally, a note found in a datasheet of a common thermal fuse – it clearly states that lifetime will be limited when operating the fuse to close to the cut-off temperature. So clearly, thermal fuses are not the best protective mechanism for the OCXO case. Maybe better would be a bimetallic switch (self-resetting, but at least no subject to any significant aging), or some other device like a PTC.

Sure, we can slightly blame the HP engineers, because it is stated on most thermal fuse datasheets, like the one below, that the operation temp limit should be about 30 degC less than the cut-off, which is not quite the case for the 10811 OCXO. 80 to 84 deg C operation, 109 degC fuse cut-off.