Add HP 34401A Repair blog

content/electronics/repair-hp-34401A-bench-multimeter.md

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+date = "2017-12-01"
+title = "Repair: Hewlet Packard 34401A Bench Multimeter"
+tags = ["repair", "electronics", "equipment"]
+categories = [ "Electronics Repair" ]
+description = "After watching The Signal Path for some I decided to treat myself to a classic 34401A 6½ digit multimeter."
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+
+![The HP 34401A Bench multimeter](/img/hp-34401A-front.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. 
+
+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.