+++
date = "2017-12-01"
title = "Repair: Hewlet Packard 34401A Bench Multimeter"
tags = ["repair", "electronics", "equipment"]
categories = [ "Electronics Repair" ]
summary = "After watching The Signal Path for some I decided to treat myself to a classic 34401A 6½ digit multimeter."
aliases = [
  "/2017/12/01/repair-hewlet-packard-34401a-bench-multimeter/"
]
+++

![Featured](featured.jpg)

I've been watching [The Signal Path](http://thesignalpath.com/blogs/) 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. 

![The HP 34401A Bench multimeter](/img/hp-34401A-front.jpg)

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. 

![The insides of the HP 34401A Bench Multimeter](/img/hp-34401A-insides.jpg)

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?

![The bad fuse. 250V 3.0A](/img/hp-34401A-bad-fuse.jpg)

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. 

![The fuse is located on the back](/img/hp-34401A-fuse.jpg)

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.

![Testing my 5.0V Voltage Reference](/img/hp-34401A-test-5V.jpg)

For those interested, it's an Intersil [ISL21090BFB850Z](https://www.intersil.com/content/dam/Intersil/documents/isl2/isl21090.pdf)
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.