An Old School Digital Multimeter (DMM) - Updated

For those not aware, I'm getting up there in age.  But right now I'd like to talk about multimeters.  Specifically Digital Multimeters, sometimes abbreviated as DMMs.  And yes - my age and multimeters are related to one another.

If you are not up to speed on such things, a "Multimeter" is a handy device that can measure various electrical parameters.  These measurements typically include AC (Alternating Current) and DC (Direct Current), for both Current and Voltage.  Electrical Resistance, measured in Ohms, is also usually included - symbolized by the uppercase Greek letter Omega: Ω.

Below:  My very first multimeter was a cheap Micronta (Radio Shack house brand) analog meter, a model 22-204A.  The 204A wasn't very solid either mechanically or electrically, but it was sort of cute, in a boxy 1970's sort of way.  I don't remember what became of it, but it would be kinda cool to have another one - just because.  You can pick one of these up really cheap, because... quite frankly, they aren't all that great.

I've had another digital meter since my high school days, one that I've hung on to.  It was all I could afford at the time - a Micronta 22-185A, purchased at the Broadway Avenue Radio Shack in Boise.  It's a museum piece now, and most people probably wouldn't even admit to owning one of these.  However I am still very attached to it, because it helped me get so many sketchy cars running back in the day.  It's been reading about 25% too high on voltage lately though, so it's not OK to use any more.  I may try to find a replacement for it, not because it's awesome, but because it's been with me for so long.

I used both of the above meters to troubleshoot and fix various British cars I've owned (MG, Triumph, Triumph, Triumph), and also the KH 500 with its plethora of electrical issues.  With the ghastly Corvette, I never needed a meter, because 99% of its shittiness was mechanical.  When the alternator caught fire, it didn't require much electrical diagnosis to figure out where the problem was.  All that was needed was to rapidly get off the road and stop the car, then locate source of all the acrid smoke pouring out from under the hood.   I kept a fairly large fire extinguisher in that car at all times - mainly due to the Holley carbs. 

Back to meters!  I currently own several tiny and inexpensive multimeters.  I keep a cheapo Harbor Freight meter like the one below in each vehicle.  (click to enlarge any photo) 

 Harbor Freight gives these away for free when you make a purchase - if you have the correct coupon with you. 

 

I've not had occasion to use one of these things yet.  Nevertheless, I'm sure these are rugged little mass-manufactured items that will work OK, barring the usual manufacturing defects that we've all come to know and love about stuff from, ahem, overseas.

Recently, I bought a "more professional" hand-held multimeter, a Craftsman #82003.  This is in reality an (also Chinese-Made) Extech EX-505 with different color plastic molding and a different logo.  Electrically... no difference at all.  It's a pretty good meter, and I (mostly) like it.

I mentioned earlier that I'm getting up there in age, and this is where the multimeters finally tie in.  All these things have LCD displays.  I'm not very fond of LCD displays, even if they have back-lighting, as the Craftsman meter above does.  LCDs can't be read from much of a side angle.  Also if you work in a well-lit place, you frequently get enough glare to make the display unreadable.  NOT. A. FAN. OF. LCD. DISPLAYS.

I decided that I'd prefer to have a meter with a LED display.  With LEDs, you don't have to worry about the viewing angle, glare, or ambient light - None of that.  Just look at the display and see what you've got.  Guess what?  DMMs are no longer made with LED displays.

So off to Ebay I went, looking for an inexpensive meter that could manage to display bright numbers.  I found two for sale together, just $30 for the pair.

At the time I was thinking to myself, "I'll have a parts machine.  If one or both of them aren't working, I can rob parts off one to get the other working."  Nope!  They have completely different circuit boards and components.  Only the case and the front panel are the same!  I might have wasted my money...

For the record, the auction said these were used, not that they didn't work!  The adhesive from inventory and/or calibration stickers was hard as concrete, and embedded in the textured plastic.

Below: I have applied power to the one that has the early circuit board design. 

This early one can be used as a hand-held meter.  It came equipped with 4 NiCd "D" cell batteries, so it's capable of self-powering.  There is even a small incandescent bulb for short circuit protection mounted to the circuit board!  (EDIT: Nope, that's not what the light bulb does) The display is dim, and it pulsates.  The board power supply is low and swings a lot.  So something is shorting out and dragging the voltage low on the +5 volt supply.  I need to find that issue - but right now I can't use this meter at all.

Below:  The guts of the above Fluke 8000A meter, which looks nothing like the troubleshooting diagram for the Fluke 8000A, on page 37.  (EDIT: This version's diagram is a little further down on the .pdf file - I just hadn't scrolled down far enough).   Note the NiCd battery packs under the black plastic hold-downs to the right, and the automotive bulb at the center.

This is the newer meter, and as you can see the display is bright.  It's also a little dirty, but it mostly works.  On the right side of the image is my high school sweetheart multimeter.

This second meter has an issue reading AC voltage.  With the meter set to AC volts, and with the leads not touching anything, the displayed voltage might read as high as 60 volts.  Not useful.  The resistance and DC work well, though.  I think I can get both of these meters fixed.

In the meantime I have two other meters on order that appear to be in much better condition.  These are both GW Instek model GDM-8135.  These were made in the mid-to-late 90s, while the Fluke 8000a meters were made between 1972-1976.  The Fluke meters are showing their vintage :)  So am I, for that matter.

And yes, before you ask, I'm well aware that there are some modern meters with beautiful bright OLED displays.  This one, for example...

That baby costs $850, which is *way* too expensive for a hobby multimeter.  I'm shooting for something between Harbor Freight free stuff and a Keysight money burner.  However I still need a decently bright display.  Hopefully the compromise meters are in transit and work OK.  That would really make my day! 

Update:
My first impression as to the condition of the two meters was a little pessimistic.  I took a bit more time over the weekend to see what could be done with them.  The second meter just needed some work with the range and function push-buttons.  I think the contacts had become dirty over the decades.

The first meter clearly had an issue with the power supply, and it took me a while to figure out what that was.  After a while (old guy eyes), I realized that the filament in the light bulb was broken.  I wasn't exactly sure what the purpose of hiding an incandescent bulb away inside the guts of a digital multimeter was, but figured it was probably important for the filament to be intact.

Meanwhile I removed and tested the NiCd "D" cells.  They were dead of course, at 0.5 volts.  I've never been an admirer of NiCd batteries, so they will be off to disposal soon as I can find time for that.

The bulb is a #63 GE, a bayonet-mount, 6 Volt, 4 Watt bulb, rated at 0.63 amps of current.  I guess the current rating is where the #63 comes from.  This is interesting, because it turns out that the bulb is used for current limiting while charging the batteries.

If the rechargeable "D" cells are really dead, the power supply will cause too much current to flow into the batteries, generating heat.  That will be bad for the batteries, but also bad for the solid-state power supply.  Most solid state stuff isn't up to taking much heat.

Oddly enough, the light bulb is soldered directly onto the main circuit board.  You would think that something which is *guaranteed* to burn out at some point would be easily replaceable, like they are on a car's dome light.  But no... it's soldered in place.

I temporarily installed some alkaline "D" Cells just to see if the charging circuit would work.  I plugged it in, and charged them for a couple of minutes as a test.  Apparently it now works as intended, with the light bulb limiting the battery charge current.   Having the batteries in the circuit also eliminated the nasty voltage surge that was affecting the +5 volt bus.

Either the batteries are necessary as a dampener for the unregulated +5 volt power, or there is another issue in the circuit that the extra battery capacity is now masking.

Below, a new GE #63 bulb in place, limiting charging current to the non-rechargeable Duracells!  Note that I replaced the capacitor in front of the left Duracell battery with a mylar film cap.

Below:  Unplugged and running off the Duracells.  I like that this meter can be portable.  The other one must be plugged in to operate.  I have new Nickel-Metal Hydride cells on order for this one.  They self-discharge much faster than NiCd cells, but they last hundreds more cycles and cost less.

The LED displays are slightly different for each multimeter.  This older one has arrays of tiny individual points that light up in lines, similar to the chip below.

The later model has entire segments that light up, as seen below.  The newer display is noticeably brighter than the older meter's display.

Here I'm testing a few resistors of various values on the Fluke vs the Craftsman.  I could not take the resistance readings at the same time.   That's because the meters have different input impedance, and so the readings did not match up at all. 

Off by 1 KOhm

The voltages read about 10% low though, so I need to do some research to figure out what's going on there.  I bought a couple of power supply cords, since they didn't come with them.  The cases cleaned up very nicely.  I have discovered a secret method for removing hardened sticker adhesive and the grime that gets into the textured finish on these.  It works great.

The owner's manual warns against using hydrocarbon-based cleaning products on these cases, and a tiny experiment proved that these blue cases will melt if you apply anything like that to clean them.  Take my word, and don't do it.  Leave a comment and I will share my trick!

Getting there on the Electronics work bench:  Left to right, both Fluke 8000A DMMS, TekMotion soldering station, Offshore brand signal generator, and of course the mighty Tektronix 2465A Oscilloscope.  Got a couple of other *very* interesting bits that should be arriving soon.  Look for a new post shortly!

EDIT:  There is a follow-up post on the Battery-Powered Fluke 8000A.
 

Update: 11-6-2020

For some reason, I decided to revisit my trusty old Micronta 22-185A.  I replaced the four AA batteries again, turned it on, and tested it against my variable DC power supply.  It read perfectly.  It also read AC mains voltage at 122 VAC.  Until I'm proven wrong, I'll assume it's fine again.  Good to have my baby back again.  I cleaned the years of grime off, and put it away for safe-keeping.