The HP 3445A is a compact, rackable plug‑in digital voltmeter from Hewlett‑Packard’s classic 34xx family, offering fast autoranging DC and AC measurements, clear numeric readout, and system‑friendly outputs that made it a staple in production racks and vintage labs.

HP 3445A overview

The 3445A is a plug‑in digital voltmeter designed for HP’s modular instrument systems, delivering autoranging DC and AC voltage measurement with a clear numeric display and annunciators that simplify bench work and automated testing. It was built with system integration in mind, offering digital outputs and remote triggering options that let controllers poll or synchronize measurements in production environments.

Why it mattered

• Auto‑ranging capability: The 3445A provides automatic range selection for quick, hands‑off measurements, speeding up workflows compared with fixed‑range meters.
• Fast response and readable display: Designed for practical use, it settles quickly after step changes and holds readings clearly on the front panel, which helped technicians diagnose circuits without squinting at slow or flickering displays.
• Modular, serviceable design: Early HP plug‑ins like the 3445A used robust, repairable electronics and straightforward front‑panel adjustments, making them popular for restoration and long service life in lab racks.

The HP 3443A is a compact, high‑gain plug‑in digital voltmeter from Hewlett‑Packard’s 34xx family, prized for its autoranging capability, five DC voltage ranges, and system‑friendly design that made it a favorite in production racks and vintage labs.

HP 3443A overview

The HP 3443A is a plug‑in digital voltmeter module intended for HP’s modular instrument systems. It was built to deliver reliable, repeatable DC voltage measurements in a compact, rackable form factor that integrates easily into automated test setups. The 3443A emphasizes autoranging convenience and system compatibility, traits that kept it in use in production environments and repair benches.

Key characteristics

• Five DC voltage ranges for flexible measurement across small and moderate voltages, enabling the instrument to present the most useful reading without manual range switching.
• Autoranging operation that speeds up hands‑off testing and reduces operator intervention.
• Modular plug‑in form factor designed to work alongside other 34xx family modules to create compact multi‑function measurement systems.

These design choices reflect HP’s focus on instruments that were both serviceable and system‑friendly, allowing technicians to build automated racks with predictable behavior and straightforward maintenance.

Both the 3445A and 3443A were functional, but not calibrated. Both units were inspected and selected components were measured. Based on this, components were replaced (capacitors, aluminum electrolytic and tantalum) as needed. I used aluminum polymer capacitors to replace some of the larger value tantalum capacitors because they are less expensive and do not age like wet electrolyte aluminum capacitors.

It is not necessary to remove the bottom PCB in the 3443A to replace any of these components.

Using a properly calibrated 3440A main frame as a basis, the calibration of the 3445A and 3443A involves applying various AC and DC voltages up to 100vac at 100kHz and 1000vdc and adjusting the amplifiers and attenuators in the plug-ins. An oscilloscope is also required for some of the adjustments. It turns out that somewhat more modern equipment can be substituted for the gear called out in the HP calibration procedures: HP 3458A, Valhalla 2703, Systron Donner M107, LeCroy WavePro 960, HP 1253B and an HP 3245A.

As with the 3440A main frame calibration, thermal considerations were important. The system needed several hours to thermally stabilize, and “open cover” tweaking was kept to a minimum. Once the top cover was removed and replaced, several minutes were allowed for the system to re-stabilize before continuing. The result was a 3440A system that easily met it published specifications.