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Introduction

For a while, I've had one of those Radio Shack / Micronta multimeters from the 1980's.

I had meant to test it, but then I got sidetracked on other things.  So it sat in a box for a few years.

Recently I decided to test it.  

Let's see if it's fixable.  And while we're at it:  which is better, analog multimeters or digital?  (You knew I'd throw that question in there...)



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In This Article

Meet the Micronta

If It Ain't Broke...

More Fun

Diodes

The Extended Project




Meet The Micronta

It's a 27-range analog multimeter, made in 1982.  It has that very 1980's look, which is cool in a rather nerdy sort of way.  You will understand if you are into electronics.  

Anyway, Radio Shack sold probably millions of these Micronta multimeters in various models and permutations.  Indeed, the Micronta 22-203 is just one of who-knows-how-many.  I can't really check at the moment, because Radio Shack's website seems to be undergoing a revamp.   They have some meters listed in their support section, but no manuals or schematics there.




If It Ain't Broke... Well, It Prob'ly Is


Someone left the batteries in this thing since the Eighties, it looked like.

There was neon-blue corrosion everywhere.  It looked almost fake.  Must be some totally awesome 1980's alloy.

Two wires were corroded to useless.  

There was also a piece that looked important (and difficult to replace), but the corrosion broke it in half. 

That's the kind of repair that can leave something in limbo for months while you try to figure out how to get or make a new part.  And usually that doesn't happen, so the meter ends up in the junk box. 

Come on, it's a multimeter, they last forever... why can't it just work??    We're in the Twenty-First Century now, where everyone has flying cars and eats hamburger pills and lives in floating cities.   We should be able to look at this old tech and make it work.   

Nope... not when there are unique-to-the device mechanical parts.  Made of totally awesome 1980's custom alloys that you can't just make in your garage.

Good news, though.  After way too much effort, I actually fixed the original part.  Let's fast-forward to the "after" photo:




Fixed!

If I'd known ahead of time how much work it was to fix a broken piece of cheap alloy, I would have just bought a Fluke multimeter and never looked back. 
From that day on, I would never accept a free Micronta from anyone.  Ever.

(Ah, I probably still would anyway.)




Ok, by now you probably figured out that you're looking at a type of electrical switch.  When you turn that knob, it closes the switch.  It pushes that piece of metal against the other piece of metal. 

When the switch is closed, the red wire connects to a black wire (which you can't see here).  The connection is made by that piece of metal that has the v-shaped crook in the middle. 




More Fun


That switch-closing piece of metal had to retain some flexion, if it was to re-commence working in its intended function.  A big ol' lump of solder joining the two pieces would sort of alter that flexibility.  The good news:  the repair sits far enough behind the part that has to flex, so it works. 

Next!

Okay, now the Micronta would measure the DC voltage on a car battery.  That's useful.  I compared it to a DMM (digital multimeter), and it was pretty accurate.  Cheap DMM's are only 1/10th the price of the time that I wasted messing with this piece of vintage electronics.  But it measures DC voltage now!

Of course... you know you want to be able to use this for AC voltage, too.   The Micronta has a little switch to select -DC, +DC, or... AC. 

The real test of an old multimeter is whether it can test the mains voltage.  If anything is amiss in the circuitry, that's not going to go too well.

And it didn't.  (Try at your own risk!!  If there's a short, it could be dangerous.))

Sure enough, something blew out.  There was a spark at the end of the test lead, which told me that a lot of current was flowing very quickly.  And then, nothing.  Not even another spark when I tried again.

Thankfully, the internal fuse had blown.   What else might have gone open-circuit?  Or out of spec?   Time for the DMM.





Probably not too hard to figure out that circuit board, although reverse-engineering PCB's would still be a good skill to acquire.

Someone should make a book called "Reverse-Engineering Vintage Junque For No Good Reason".




Diodes


Just for kicks, I used another multimeter to test a pair of diodes on the circuit board. 

These were those red-and-clear, "might-be-a-zener-but-you're-not-sure" diodes.   They resemble those $2-per-hundred, made in China, "every kind of diode you want them to be" diodes. 

You know which ones I mean.  They are often sold as 1N34A's but most certainly are not germanium diodes.  (Real 1N34A's cost quite a bit more than $2 per 100).

Like the cheap fakes, the diodes in this Micronta are too small to have any markings, other than maybe a "K" for "cathode".   

These diodes are so small that I nearly overlooked them.  They are also obscured by a bundle of wires. 

I de-soldered one end of the diode pair.  Testing them, the meter indicated one of them was bad, conducting in both directions. 

The schematic would show the part number, right?  Not on this one.  There's no indication of what kind they are, so they're probably generic silicon diodes.

When my fake 1N34A's arrive, I'm going to try them in this circuit.  (Because they are probably generic silicon diodes.) 

I still haven't figured out why there are antiparallel diodes in this multimeter.  They aren't wired in parallel with anything else, but in series with resistors.   Weird.  I've seen this on some audio equipment.  Maybe a reader will know why they do this.

If I were smart, I'd probably get this book and this one.  If I were smarter, this book looks to be absolutely fantastic, although its application to a 1982 volt-ohmmeter would be kind of overkill.  But so what?  Totally rad.

A smart fellow (or lady) would be sure to have a handy array of common diodes, capacitors, and resistors before starting.   (That's a much better way to buy them than getting piles of generic fakes.)

It's true that you don't need an expensive multimeter to test diodes.  With that said, it helps to have a good one (such as a Fluke) if you want to be sure the voltage drops are being read accurately.  Cheap DMM's can give some sketchy readings.  I know this from experience.   Cheap DMM's also don't have auto-ranging, or capacitance metering (great feature!!).


Resistors


The resistors for the voltmeter ranges are easy.  There are no parallel bundles of resistors.  That means you can just test them in-circuit to make sure they're OK. 

The only problem is that cheap DMM's cannot measure past 2 megohms. 

Resistor R1 is 20.5 megohms.  I think R2 is 6.5 megohms.  My cheap DMM doesn't even know what to do with these.

Time to get the Fluke.





The Extended Project


These vintage electronic goodies are fun.  Repairing them can be an extended project.  The schematics are tough to find;  an Internet search for them tends to yield scores and scores of useless pages, generated from database programs that don't even have the actual schematics.

Or, you'll find sites with low-quality scans at $4.99 each.  

I'll fix that Micronta sooner or later.  Hopefully.

Even if and when the Micronta works again, it will still have its limitations.  I usually don't like digital devices quite as much, but this is one area where digital has clear advantages.  (Diode tests, to name one.)

If you actually need a multimeter that you can start using right now, just get this one, this one, or this one.   

(If you pulled one of these out of your tool kit, you would win every electronics-nerd contest that I could probably devise.  The only way I could hope to win such a contest would be to have this, which you'd better believe is on my long-term wish list.)

A good multimeter will last you for years and is very much worth it.   (That goes for o-scopes too).


              


Conclusion

Eighties tech is still useful when it works.  Multimeters are a special case, though, because they tended to sit on a shelf for years at a time.  That means battery corrosion is quite often a problem. 

There's also the fact that many of them have bad components, because someone used them to test too-high a voltage.

If you like messing around with electronic projects, then by all means get yourself a vintage multimeter

If you want a multimeter that you can use right now, and you want accurate readings, get a good DMM

Actually, you'll use the DMM quite a bit while fixing your vintage multimeter, or anything else electronic.


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