HP410B and RF Probe

2025/6/8

I picked up this HP410B RF voltmeter from a S.P.A.R.C sale. HP410B is considered a classic piece of test equipment in electronics. It has an input impedance of over 100 megaohms for DC, which is a great topic for every electronics hobbyist.

The HP 410B is widely recognized as one of HP’s most legendary laboratory-grade RF VTVMs. The RF response remains flat within ±1 dB up to 700 MHz, and it’s still usable up to 3 GHz. HP-410B cost $245 back in 1952(According to the HP catalog).

This 410B uses a special Eimac tube within the probe, the 2-01C diode. It operates at 5 volts AC. It's a miniature UHF diode, and it can operate up to 3 GHz. Its amazing diode, and its an active probe. The input capacitance is as low as 0.7pF, A stunning achievement for such a large probe.

There was only one small problem, it did not work at all. I stared at the glowing diode for a long time. It’s such a simple circuit — how could it not work?

I noticed it somehow works if I remove the probe cap and touch both the tip and somewhere on the probe body. I investiaged the probe schematic, and reallized that i couldn't find the R2, the 22 Meg resistor.  I located serveral photos of the probe owned by someone else, check out the R2 below.

I eventually got several construction details of the HP probe. I notice there also a resistor runs from diode top to the side clipper. see the photo below in Fig.3.3.

The key finding is that I noticed the diode tip has a spring contact, which is missing from mine. I then noticed more photos showing that the probes have the spring hat on them.

And I ultimately got a spare contactor hat, and this solved all problem, the HP410B function well then.

The last mystery—the missing 22 MΩ resistor—actually hides in the top half, as shown in the left photo below.