The Signal Path

TSP #337 - How Can HP 5245L Nixie Tube Frequency Counter Measure DC Voltage? Theory, Teardown & Demo

The video explores converting the classic HP 5245L Nixie‑tube frequency counter into a DC voltmeter by installing a dedicated plug‑in module. After a brief recap of previous teardown work, the host explains that the instrument’s internal counter can be harnessed for voltage measurement rather than just frequency. The core technique is an integrating analog‑to‑digital converter: a constant‑current source charges a precision capacitor, creating a linear voltage ramp. A comparator watches the ramp until it reaches the input voltage, generating a stop pulse for the counter. Knowing the current, capacitance, and clock frequency yields the measured voltage. Calibration uses an 8 V reference and a zero‑adjust potentiometer, the latter featuring a planetary‑gear reduction for fine tuning. Notable details include the clever charge‑injection scheme that compensates for diode leakage, effectively achieving ultra‑high input impedance with 1950s components. The module’s front panel offers manual range selection via voltage dividers and switches, and the video highlights the intricate gear‑reduced potentiometer and the module’s block diagram under a Tago T50 microscope. The demonstration underscores how vintage test gear can be repurposed for modern measurement tasks, offering hobbyists a low‑cost, high‑precision solution while preserving the nostalgic aesthetic of the original instrument.