Hardware

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

The Signal Path
The Signal Path•May 23, 2026

Why It Matters

Repurposing the HP 5245L shows that legacy test equipment can deliver accurate DC measurements today, extending its utility and providing a cost‑effective alternative for hobbyist and educational labs.

Key Takeaways

  • •HP 5245L repurposed as DC voltmeter via specialized plug‑in module.
  • •The module implements an integrating ADC using a constant‑current ramp and comparator.
  • •Calibration relies on an 8 V reference and zero‑adjust potentiometers with planetary gear.
  • •Input impedance is achieved by injecting charge to cancel diode‑induced leakage.
  • •Manual range selection, no autoscaling, slows speed but keeps vintage accuracy.

Summary

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.

Original Description

In this episode Shahriar explores the capability of some frequency counters to measure DC voltage. While this may seem non-intuitive initially, the video shows how the architecture of the HP 5245L Nixie tube frequency counter can be used to realize an integrating ADC. A particular plug-in module, the 5265A, leverages the Start/Stop counter control to implement an ADC and display the equivalent DC voltage at the plug-in input. The theory of operation as well as the teardown of the plug-in in shown.
The plug-in module unit is zeroed & calibrated initially and then using a Fluke 5720A the performance of the entire system is evaluated. Despite being in storage for decades, the unit is incredibly accurate and precise.
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