HP 00105 OCXO pre-heater

The 00105 OCXO seems to be HP’s first attempt at packaging an OCXO as a module, rather than as a piece of equipment.

The number indicates that it was built originally for the HP105 Quartz Oscillator, where the 00105 rightfully is assembly A1.

Apart from the “Theory of Operation” in the HP105 manual, we have very little documentation about the 00105.

One interesting design detail is that it has an extra heater so it can reach working temperature faster. Notice the “115VAC” at this bottom on this HP5065 block diagram:


The detailed schematic reveals a little bit more about this heater:


It is simply an extra resitive heater with a mechanical thermostatic switch.

If you wonder what role the A1 AC Controller plays in this, the detailed schematic has the scoop:


A 2000 {pF} “spark” capacitor across the mechanical thermostat.

In the HP5065 this heater is connected across one half of the power transformer primary, but since I’m running my HP5065 on 24 VDC, that doesn’t help me any.

The resistance of this heater is approximately 252 {Ohm}.

At 115 {VAC} that gives:

115 {V} * 115 {V} / 252 {Ohm} = 52.5 {W}

Interestingly, this is not part of the HP5065A specification, which only lists the 115VAC power required as 49 {W}.

There is little point in attaching this heater to the DC supply instead, at my battery backed supplys typical voltage it would heat only:

27 {V} * 27 {V} / 252 {Ohm} = 2.9 {W}

If the DC supply had been a -48V system, it might make sense:

54 {V} * 54 {V} / 252 {Ohm} = 11.57 {W}

I guess I’ll just have to wait…