HP 3468A Bench Multimeter Repair

HP 3468A Bench MultimeterHewlett-Packard has built a reputation for quality bench multimeters every bit as impressive as Fluke has for the handheld variety. Nowadays they’re called Agilent, but this unit has the HP nameplate on it, and date codes on the parts inside suggest that it was manufactured in early 1987.

The 3468A and it’s cousin the 3478A are 5½-digit 300,000-count multimeters.  Both have instrument interfaces, the former with HP-IL (serial) and the latter with HP-IB (parallel, aka GPIB.)  Basic 1-year DCV accuracy is 0.02% of reading + 2 counts for the 3V range.  The 3478A has ever so slightly better accuracy figures and adds a 30mV range.

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Beckman 3020 DMM Repair

B3020_S00_introBeckman 3020 DMM

Handheld digital multimeters were just beginning to gain popularity in the late 1970s.  Fluke was there of course, but so was Beckman.  This is a Beckman 3020 3½-digit 2000-count model that dates to 1979.  It has the classic Beckman ‘look’ and rotary switch design that persisted for many years, even after the Beckman Industrial division was absorbed by Wavetek circa 1992.

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DMM Basics – Dual-Slope ADC

DMM BasicsThis is the first in a series of posts looking at how a digital multimeter actually works.  First a little theory, then later we’ll look at the ubiquitous ICL7106 that basically started the ball rolling on handheld DMMs back in the late 1970s, and is still used today in your basic $5 hardware-store cheapie.

At the heart of practically every handheld digital multimeter is something called a dual-slope integrating analog-to-digital converter (ADC).  It isn’t like the ADCs used to encode our music, videos, and telephone conversations.  Those type of conversions require an ADC to produce a steady stream of digital data necessary to accurately reproduce a very rapidly changing analog signal.

That is not the case in a digital voltmeter.  Instead, we want to observe a voltage, that may or may not be changing, and distill it down to a single number that accurately and repeatably characterizes that input voltage.  There’s no requirement to do it any faster than the human eye and brain can perceive it.  About 2 to 4 times per second is enough.  And we’d like it to give accurate results even in the presence of electrical noise, like the ever-present mains hum.

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Nixie Glow

Nixie glowMy first ever digital multimeter was a Heathkit IM-1212.  I built it from a kit when I was a teenager (thanks, Dad.)  A whopping 2½-digit, 200-count unit with 1% ± 1 digit basic DC accuracy.  Barely a step up from a basic analog VOM, but at least you didn’t have to count divisions or figure out where the decimal point went.  The display used two Nixie tubes for the numbers.  Later on I converted it to an LED display, which pretty much destroyed its ‘coolness’ and ‘retro’ appearance.  Not very photogenic or blog-worthy nowadays…

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Fluke 83-III Rotary Switch

8x Function SwitchIt’s a fact that you can repair as many multimeters with a bottle of isopropyl alcohol and some Q-tips (cotton buds), as you can with a soldering iron.  In many cases, malfunctions can be fixed by careful disassembly and cleaning of affected parts.  Only simple tools and materials are required, but caution is recommended so as not to damage any of the components or plastic bits.

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Yokogawa 7534-03 Multimeter

Yokogawa 7534-03 MultimeterJapanese equipment maker Yokogawa made handheld DMMs for HP before HP became Agilent.  This 3200 count 3½ digit Yokogawa-branded model is very similar in appearance to the HP E2378A model.  Unlike the HP, this unit adds a high-impedance ‘ADP’ mode, presumably for current-clamp attachments and the like.  Otherwise, it has a very fairly mundane feature set.  It is also the only multimeter I’ve ever seen with a ‘Data Hold’ button on both the top and the side.

Opening the case reveals an obvious problem.  Something has gotten very hot and burned part of the circuit board.  There are burn marks on the inside of the case as well.  The resistor (R6) that died violently has already been removed.  There is a pencil-written note on the side of the unit that says “4.9 Ω 2 watt wirewound resistor”.  In retrospect, I realize this may have actually said “4.99 Ω”, but since it’s now been cleaned off, we’ll never know.  In any case, it was certainly helpful for someone to note what had been removed from the device.

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