HP 456A Current probe - restoration

This current probe is a real beauty. Released as a new product by the Hewlett-Packard Company in 1960, the 456A was HP’s first solid-state, stand-alone, clip-on current probe. Its elegantly designed amplifier uses two— then “state-of-the art”—PNP germanium transistors. The Original Probe In 1960, The Hewlett-Packard Journal (July-August, Vol. 11) proudly announced: “This new probe measures current over the full range of the frequencies most commonly used in typical work—25~ to 20 megacycles—and over an amplitude range from below 0.5 mA to 1 A rms.

HP456A current probe

Below you'll find the circuit diagram of the HP-456A probe.

Maybe the concept of the circuit is unclear at first but actually the idea behind it is really simple and effective. (Especially considering the fact that this performance was achieved, with what would be considered nowadays, mediocre components at best.)
Q1 functions as the input transistor in common base configuration. As you will know in a CB setup, the base is hold at a fixed voltage (0V in this case), the emitter is used as the input and the collector is used as the output. Main characteristics (and in this advantages) are the low input resistance and the high frequency. The latter is caused by the fact that the base is at a fixed voltage effect and the Miller effect is eliminated. Also because the emitter current equals the collector current, there is no current amplification.
Q2 is the output amplifier and is set up in a common emitter configuration (emitter hold at a fixed voltage).
The combination of the two transistors works as opamp in current to voltage topology (eq. transimpedance configuration.) See below

Rf is implemented as R9 parallel tot R10. Beautiful isn't it?

The reason the setup didn't work in the one I bought is that the cable actual current probe was broken on were it connected to the PCB in the amplifier case. (in the circuit near C2 and C3). After fixing this it works like a charm. Not bad for a 25+ years old device!

I you have any questions let me know!

73 PA3COR