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Tuesday, July 14, 2015

HP 3478A Multimeter Modifications

    What kind of electronics engineer doesn't have a bench multimeter? Well, probably a lot, but since I didn't have any decent multimeter, aside from some very cheap UNI-T ones, I bought from eBay an old HP 3478A 5 1/2 digit multimeter.
It'd old, I know, but it was "really cheap" (about 120 Euros) and only had some minor faults. I mean, what's a stuck Power button and a missing AMPs range fuse, right?

So, anyhow, it arrived, I was thrilled when I got it and surprise surprise, it worked surprisingly well and gave very accurate readings (tested it against another, borrowed, Fluke  handheld multimeter and it was very accurate).

And, now what?  Well, the Power button was stuck in the "On" position, which is very convenient, so for now, until I can get a replacement switch, I'll just have to unplug it when its job is done. (the button is  all rusty and full of crud)

I proceeded to put it through its paces and measured....stuff.  I asked someone to lend me a Fluke DMM. Then, the rest was simple. For the  DC volts, a home made, dual channel power supply,  with 0-30 Volts on each channel proved very helpful and also a variable transformer for the AC volts helped, although, kind of tricky because the line voltage isn't really an ideal standard because of the constantly changing loading of the grid. The 2-wire and 4-wire Ohms also worked a treat.

As for the missing AMPS fuse....This thing has plugs in the back also, right? Which I'm not going to use, right? So, why not disconnect the wire from the front AMPS plug, run a fuse and a wire to some other pug to the rear terminals and that's it. So I did just that. I ordered a fuse-holder and some 3A 250V fast fuses, cut the original Amps range wire,  spliced it with the fuse holder and Voila! A fully functioning bench multimeter.

Also, if someone's curious, yes, I checked the battery for the ROM that holds the calibration data and it was either changed or still has some life in it, because it read about 3.6 Volts.

Future thoughts  - get a GPIB PCI card and start putting this little jewel to some good use in some automated tests for one of my projects I've been working on lately.


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