Building a New AC Probe for a Vacuum Tube Voltmeter

Having previously restored a Hewlett-Packard 410C Vacuum Tube Voltmeter, now it’s time to try to replace the missing piece, the active alternating current (AC) probe that allows it to measure voltage at frequencies up to 700 MHz. Unfortunately, original Hewlett-Packard probes are either unavailable or only offered at absurd collectors’ prices, so perhaps a probe from another manufacturer’s more recent instrument can be adapted to work. Along the way we learn a great deal about parasitic capacitance and inductance in radio frequency circuits and the dreaded phenomenon of parasitic resonance they can create.