The HP 3440A is a plug‑in style digital voltmeter designed for HP modular systems and bench racks. It was offered as a versatile measurement unit with a focus on accurate DC voltage and resistance readings, remote triggering, and digital outputs for system automation. With the appropriate plug-in, it can also measure AC voltage, and DC current. The unit’s design emphasizes serviceability and predictable long‑term stability, making it a favorite in production test racks and vintage lab restorations.

Key Specifications

• Output format: The 3440A provides a 6‑column output including digits, polarity, and decimal annunciation; an alternate BCD output option is available for digital systems integration.
• Sample rate: Adjustable up to 5 readings per second from the front panel, with remote triggering capability for synchronized measurements in automated setups.
• Input characteristics: Floating inputs with high input impedance and designed common‑mode rejection to support accurate DC measurements across ranges. These attributes make the 3440A practical where predictable digital readout and system interfacing are required.

Notable Features

• System friendliness: The module was built to be remotely triggered and to provide digital outputs suitable for data collection or control systems, which simplifies integration into automated test sequences.
• Service documentation: Full operating and service manuals were published and remain available in archives; Keysight (HP’s successor) hosts legacy manuals for reference, useful when refurbishing or calibrating older units. Artek Manuals has the full set of professionally scanned manuals available for purchase at reasonable prices.
• Robust front panel and indicators: The 3440A includes clear annunciators and overrange indicators to help technicians spot measurement or wiring issues quickly.

I recently acquired an HP 3440A main frame and 3443A plug-in at a good price. The unit powered on, but behaved erratically, with the display blinking randomly and changing ranges. Beginning with checking the power supply, I measured several volts of AC ripple on the supply rails! Further checking revealed that the power supply filter capacitors were either open or had capacitance measuring a fraction of the marked value.

Replacing the power supply capacitors stabilized the voltage rails. I re-used the original can capacitor mounting plates for two of the capacitors, and used a new replacement for the other. Of course the new capacitors were smaller, radial type capacitors with higher working voltage and ripple current ratings than the originals.

The next problem was that a display module would only count to a certain number and then stop. This prevented the displays to the left of the defective module from counting properly as well. Swapping around the display modules confirmed the problem. Since four of the modules were interchangeable, the fastest way to find the problem was by comparing component values between the defective module and a good one. This lead to the replacement of a defective diode on the faulty module.

At this point the meter was functional, but required calibration. As this meter uses plug-in modules, the procedure is to calibrate the main frame first, and the module(s) next. This is the reason the procedure for the main frame uses a simple switching module, the HP 3441A Range Selector. However, this module is of limited usefulness, so I have never bothered to acquire one. Instead I used the 3443A High Gain Auto Range unit, restricted to the 10v, 100v and 1000v manual positions. This allowed me to calibrate the 3440A main frame successfully.

I found that the warmup time was important; it took about four hours for the unit to thermally stabilize with the covers on. When the top cover was removed to do an adjustment, it was important to replace the cover quickly and allow the unit to re-stabilize before proceeding.

The 3440A main frame calibration procedure calls for test equipment that was contemporary at the time it was manufactured. However, the calibration can be done with a good quality DVM, a good oscilloscope and a DC standard. I used a HP 3458A, a LeCroy WavePro 960, and a Systron-Donner DC Standard.

The HP 3440A main frame was successfully calibrated. However, the main frame alone is not useful. The next part will describe the repair and calibration of two plug-ins: 3445A AC/DC Auto Range and 3443A High Gain/Auto Range units.

This 3440A did not have the retaining bar that keeps the circuit cards in place in the chassis. So I fabricated one from aluminum bar stock: 30cm x 2.5cm x 3mm. I used a waterslide decal for the text that appears on the bar.

This completes the repair and restoration of this 3440A digital voltmeter.