Repair: Hewlet Packard 34401A Bench Multimeter

I’ve been watching The Signal Path for quite a while now and I really like the repair videos. After doing a lot of audio repairs I wanted to give electronics test equipment a try.

The Agilent/HP/Keysight 34401A is a very popular bench multimeter with decent specs:

• 6½ digit resolution
• 10 measurement functions: DC/AC voltage, DC/AC current, 2- and 4-wire resistance, diode, continuity, frequency, period
• Basic accuracy: 0.0035% DC, 0.06% AC
• 1000 V max voltage input, 3 A max current input

After scouring the internets for a few weeks I finally found an auction for a defective unit and won. The description simply stated DC current measurement did not work. No word on the other fuctions whatsoever.

When I got the unit in the mail I prepared to make a Youtube video of this repair. Well, the repair turned out to be so easy it’s barely worth this blogpost, let alone a whole Youtube video.

First things first, I took the cover off and took a peek inside. Very clean and nothing out of the ordinary there.

Next I powered the unit on and ran a self test (hold the shift button when powering the unit on). It passed.

Next I started with some measurements: DC voltage, resistance, frequency, continuity. All good. Then I was time to check DC current. Nothing. Open circuit. Well, guess what’s the first thing to check when a fused circuit is open?

Yeah. The fuse. It’s a 250V 3.0A fuse in a holder located on the back of the unit. It was blown. There’s also a 7.5A fuse inside of the unit for extra protection, but this fuse turned out to be fine.

Well, I put in a new fuse and voila, DC current measurement is working again. Here’s a picture of the unit testing a 5.0V reference IC.

For those interested, it’s an Intersil ISL21090BFB850Z which I plan to put in a box. The IC generates 5.0V ±0.025% whereas the HP 34401 is specified with a basic 0.0035% accuracy.

So the 5.0016V reading is well within spec. Nice!

Next question: keep it or sell it? I haven’t decided yet.