I finally cleared some space on my workbench last week for the hp 34401 a, and honestly, it's still one of the best pieces of gear I own. It's funny how in a world of high-resolution touchscreens and wireless everything, a piece of test equipment designed in the early 1990s can still hold its own. If you've spent any time in a professional lab or a well-stocked home workshop, you've definitely seen that familiar grey box with the bright orange bumper. It's the kind of tool that doesn't just sit there; it feels like the backbone of the entire bench.
There is something incredibly satisfying about the way the hp 34401 a operates. When you click that power button, you get a quick self-test, a couple of internal relay clicks, and then that beautiful green vacuum fluorescent display (VFD) springs to life. It doesn't need to boot up for thirty seconds like a modern Windows-based oscilloscope. It just gets to work. For a 6.5-digit multimeter, it's surprisingly snappy.
Why this specific model became a legend
You have to wonder why people are still paying hundreds of dollars for a used hp 34401 a on eBay when they could buy a brand-new, budget-friendly multimeter for half the price. The answer usually comes down to two things: reliability and trust. Back when Hewlett-Packard was still Hewlett-Packard (before the Agilent and Keysight splits), they built things to last for decades, not just until the next warranty cycle.
The hp 34401 a was a game-changer when it hit the market. It offered 6.5 digits of resolution at a price point that made it accessible to more than just the top-tier research facilities. It became the industry standard almost overnight. If a data sheet said a measurement was taken with a 34401A, nobody questioned it. That legacy carries a lot of weight even today. I've found that even if my unit hasn't been calibrated in a few years, it still tracks perfectly with much newer, more expensive equipment.
That iconic vacuum fluorescent display
Let's talk about the screen for a minute. Modern gear often uses LCD or OLED screens, which are great for graphics but can be hard to read from across the room or at weird angles. The VFD on the hp 34401 a is different. It's got that crisp, glowing green light that is visible from practically anywhere in the lab.
Now, if you're looking to buy one used, you do have to be careful about "burn-in." Since these units were often left on 24/7 for years at a time in manufacturing plants, the displays can get dim over time. But if you find one with a bright, clear screen, it's a joy to use. There's no glare, no backlight bleed—just the numbers you need to see, exactly when you need to see them.
Real-world performance on the bench
In terms of actual specs, the hp 34401 a covers all the basics and then some. You've got your standard DC and AC voltage, current, and resistance. But the real magic is in the 4-wire resistance measurement. If you're trying to measure low-ohm resistors or traces on a PCB, a standard 2-wire probe setup just won't cut it because the resistance of the leads themselves messes up the reading. Switching to 4-wire mode on the 34401A makes those errors disappear.
The accuracy is another story entirely. We're talking about 0.0035% basic DC voltage accuracy. For most hobbyist projects, that is absolute overkill, but it's nice to know that when you see 5.00001 volts on the screen, it's actually 5.00001 volts. It gives you a level of confidence in your troubleshooting that you just can't get with a handheld 3.5-digit meter.
Navigating the menus (or the lack thereof)
One thing I love about this era of HP design is that they didn't hide everything behind a dozen menus. Most of the primary functions have their own dedicated buttons. You want DC volts? Press the button. You want to change the range? Use the arrow keys.
If you want to get into the more "hidden" features—like Min/Max, Null, or the dBm calculations—you just use the "Shift" key. It's intuitive enough that you don't really need to keep the manual on your desk, though the original manual is actually a great read if you're into the theory of how dual-slope integration works.
Integration and the GPIB port
Even though the hp 34401 a feels like a "manual" instrument, it's surprisingly easy to automate. It comes with a GPIB (HPIB) port and an RS-232 serial port on the back. While most people today aren't using old-school GPIB controllers, you can easily get a USB-to-GPIB adapter or just use the serial port to log data to a PC.
I've used mine for long-term battery discharge tests where I have the meter send a reading to a Python script every ten seconds. It's rock-solid. It doesn't crash, it doesn't lose its connection, and the command set (SCPI) is so standard that almost any software library out there supports it. It's a reminder that good engineering is timeless.
What to look for if you're buying one now
If you're scouring the used market for an hp 34401 a, there are a few things to keep in mind. First off, check the branding. You'll see some labeled "HP," some "Agilent," and some "Keysight." They are essentially the same machine inside, but the HP-branded ones are the oldest. Some people prefer the HP logo for the nostalgia, but the Agilent-branded ones often have slightly newer internal components or firmware.
Another thing to check is the fan. There's a small fan in the back that keeps the internal reference stable. In older units, these fans can get noisy or seize up entirely. It's an easy fix, but it's something to listen for when you first power it up.
Also, look at the terminals. These units were often used with "shrouded" banana plugs, and sometimes the plastic around the terminals gets cracked if people were too rough with them. It doesn't affect the measurement, but it's a sign of how the meter was treated in its previous life.
Is it still relevant in a modern lab?
I get asked this a lot: "Why should I buy a 30-year-old hp 34401 a instead of a brand-new Rigol or Siglent?" It's a fair question. Modern meters have color screens, can graph data right on the display, and often have built-in web servers.
But there's something about the "feel" of the HP. It's like driving a vintage Mercedes versus a brand-new entry-level car. The new car might have more tech, but the old Mercedes is built out of solid materials and has a level of refinement that's hard to replicate. The 34401A is quiet, it's heavy enough that it doesn't slide around the bench when you push the buttons, and it has a pedigree that's unmatched.
Final thoughts on a bench staple
At the end of the day, the hp 34401 a is more than just a multimeter. It represents a specific era of electronics where things were designed to be repairable and documentation was king. If yours breaks, you can actually find a service manual with full schematics and a parts list. You can't say that about most modern gear.
Whether you're a professional engineer or someone who just likes tinkering with power supplies on the weekend, having one of these on your shelf is a bit of a rite of passage. It's reliable, it's accurate, and it just has that classic look that makes a workbench feel complete. It might be an old dog, but it still knows all the best tricks. If you ever have the chance to pick one up for a decent price, don't hesitate. You won't regret having this legend in your toolkit.