Electronics      Reviews, Teardowns, & Guides

Introduction


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

It didn't work, so I tried repairing it;  then it sort of worked, then it didn't work...

This article will sort out some of the features and repair issues.  If'n yer gonna repair vintage cameras that have electronics, you might as well use a vintage multimeter.  And so, you might want to know how to keep a multimeter like this working.


A Quick Note

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In This Article

Meet the Micronta

Some Specs


Accuracy

Ammeter Function

Batteries

Diodes D3 and D4

Disassembly

Ohmmeter Function

Price / Value

Repair Attempt #1

Repair Attempt #2

Repair Issues, Common

Resistors

Rotary Switch

Similar Models

Voltmeter Function


Conclusion





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 in various models and permutations. 




Some Specs


AC Voltmeter Useful Range:  0-1000V AC
Casing Material:  Plastic
Connector Type:  Banana Plug
Connectors:  +DC10A, -COM, +V/Ohms/A, OUT
Decibel Range (Electrical):  -10 to +22 dB
Dimensions:  about 6.25" x 4.5" x 1.75"
Diode Tester?:  No
DC Voltmeter Useful Range:  0-1000V DC
Fuse:  Fast, 500mA 250V
Internal Resistance (Input Impedance):  30K Ohms / volt DC;  10K Ohms / volt AC
LED Indicator Lights?:  No
Made In:  South Korea
Meter Movement:  2-jewel mechanical
Ohmmeter Useful Range:  0-1 Megohm
Years Manufactured:  1982-83





Accuracy


On DC volts, I think it's 3% of full scale.

This is typical for semi-decent meters of this type.  Any worse than 3%, and they're not really worth using;  any better than 3%, and you'd be in Simpson territory (about 2% accuracy).

I'll have to check the manual for the other accuracy measurements, if they even provided them.  Because of the way these meters work, you can "tune" your meter to be really, really accurate on a specific measurement.  But you first need something more accurate to compare it to.  This is how people end up with ten multimeters.


Get yourself a Micronta here

Table of Contents




Batteries


1 x AA battery for most of the Ohm ranges.

1 x 9-volt battery for the RX10K range. 

You don't need the batteries to use the voltmeter or ammeter functions.  This is one of the advantages of analog multimeters (AMM's).  It's kind of like having an all-mechanical film camera.


Get yourself a Micronta here

Table of Contents




Diodes D3 and D4


These look like small, signal-type diodes, maybe 1N4148.  They are obscured by a bundle of wires over the main circuit board. 

At first I didn't know why they were there.  The schematic shows why:  they're in parallel with the meter movement.  So, if you get too much voltage applied to that, the diodes will conduct and form a dead-short.  This protects the meter mechanism. 

Before solid-state diodes, a lot of meters simply didn't have meter protection like that.  Too much voltage, and it would actually bend the needle or ruin the mechanism.

By the way, one of the diodes on mine was shorted.  I replaced both of them with those $2-per-hundred, "every kind of diode you want them to be" diodes.  You know the ones I mean;  they look like smaller versions of the 1N34A and are sometimes sold as such.  But they're definitely not 1N34A's.

There are two other diodes on the board, and one of them is in fact a germanium diode.  I have to check the schematic again to see what they do.


Get yourself a Micronta here

Table of Contents




Disassembly


The back is held on by three screws, recessed into the casing quite a ways.  These are fine-thread machine screws, but the ones that hold the circuit board in place are self-tapping.

Here's a quick photo of the circuit board, taken before the unit was left outside for a while.



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

Actually it's surprising how complicated a multimeter can be, without the schematic.  More about this later.


One of the circuit-board screws is obscured by the bundle of wires up near the top of the picture.

The smaller circuit board to the left is held on by one small screw that's difficult to reach, and one that's easier to reach but doesn't look like a screw (and it requires a flat-head screwdriver.) 


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Fuse


Fast, 500mA, 250V.

Just be aware of the possibility that some variants could lack fuse protection.  I don't know what the letter designations mean: 22-203A, 22-203C, 22-203U, etc.  So there could be some differences there.


Get yourself a Micronta here

Table of Contents




Ohmmeter Function


Batteries are required, as you would expect.  The voltage output may be enough to forward-bias semiconductor junctions. 

The "range" settings are actually multipliers.  If the meter says "100" but you have it set on "RX10K", that would be 1 megohm.  So this meter can measure resistors up to 1 megohm.  Actually there's a "500" marking on the dial, which at RX10K would be 5 megohms, but accuracy and precision really fall off by that point.

The zeroing dial works well.  Too bad my Micronta 22-203 still doesn't work in Ohms mode (yet).  But I finally repaired the Voltage modes!!


Get yourself a Micronta here

Table of Contents




Price / Value


These are not rare. You can often pick up a working one through this link for anywhere from $10 to $25, depending on condition.  On the upper end of that range, I'd make sure it's got the original box that's in good condition, the test leads are there, and everything works the way it should.

With battery corrosion or something, value might be $2.  You'd put quite a bit of labor into it just to fix that. 

You'll see the usual "wishful thinking" prices where people are asking way too much.  But don't pay a lot for this meter;  you can buy a brand-new analog multimeter, made in China, for anywhere from $10 to $30 through this link.  And it's probably as good as this one.  It uses basically the same technology.

Besides, if you were going to pay a lot for an analog multimeter, you might as well save up and get a brand-new Simpson.  Brand new, no issues, great meters.




Repair Attempt #1


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.

Wires corroded through, and what's this?  The rotary switch contact, a flat piece of metal, was corroded in half. 

Probably anyone else would have thrown it away.  Here was my initial hack-fix attempt:




When the switch is closed, the red wire connects to a black wire (which you can't see here).

At the moment, it's configured wrong.  That's because S3 should be open when testing voltage, not closed.

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.)


Get yourself a Micronta here

Table of Contents




Repair Attempt #2


The meter never seemed to work properly, so I was going to junk it.  Then it got rained on.

After it ended up back in the junk box, I was testing it and found "+V" was shorted to "-COM".  How did the meter work at all like this?  I'd noticed that a brand-new AA battery was dropping to 0.7 or 0.8 volts. 

The voltage was going through circuit paths that were designed to measure amps, not volts.  That means there were no resistors to drop the voltage before it reached the meter.  (Whoops.)  Good thing the protection diodes were there.

To fix this, I had to replace the other switch contact of S3 with something I made.  Now the switch would be open in voltmeter modes, and closed in the ammeter and ohmmeter modes.

So from here, it's wired correctly at least.  If the meter mechanism is still good-- which I'm not sure right now-- it should work.


Get yourself a Micronta here

Table of Contents




Repair Issues, Common


Battery corrosion.  You have to take apart the unit to get to the batteries.  So they tended to be left in there.  Battery corrosion in AMM's is problematic not only because it corrodes switches and wires, but also because bits of corrosion can get into the meter mechanism. 

Meter is damaged.  That happens to a lot of analog multimeters.  Could be from dropped meter, overvoltage, bits of corrosion, etc.  Not easily fixable, if at all.

The protection diodes could be shorted.  If the diodes short, the meter will temporarily not work.  If the diodes go open, the meter could permanently stop working.

Voltage on the Amps settings:  very common mistake.  High voltages will just blow the fuse, if it's the right type.  If someone put too slow a fuse in there, other stuff could get damaged as well.  As long as the meter mechanism still works OK, pretty much everything else can be repaired or replaced.


Get yourself a Micronta here

Table of Contents




Resistors


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.  (That, and they're not so accurate.)

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.


Get yourself a Micronta here

Table of Contents




Rotary Switch


The circuit board has the usual semi-circular tracks or circuit paths.  The rotary switch has metal contacts that correspond to these.

The rotary switch mechanism relies heavily on the plastic molding.  In other words, it's not like the rotary switches you'll see in some of the earlier vintage meters.


Get yourself a Micronta here

Table of Contents




Schematic


It's in the manual.

Many types of repairs can be done without a schematic.  But it's surprising how complicated a "simple" circuit board can be when you don't have the schematic.  One component directly next to another may have no connection to it whatsoever.  Some people are good at figuring out a schematic from a circuit board;  this is a skill I'm still working on.

Eventually I got this book and found it very helpful for the general troubleshooting approach.  It has a lot of specific examples (not the Micronta, though).  Great book.  I haven't tried this book yet, but it's also said to be essential. 

For full-on reverse engineering of circuit boards, 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.


Get yourself a Micronta here

Table of Contents




Similar Models


Micronta 22-201....  fewer ranges (18 instead of 27).  Lower internal resistance (20K Ohms per volts DC).  Also has diode protection but appears to have no internal fuse. 

Micronta 22-202....  fewer ranges (25 instead of 27).  Lower internal resistance (20K Ohms per volt DC).  Not sure if it's fused;  I don't have a schematic for it.

Micronta 22-204....  same accuracy, more ranges (43 instead of 27).  Higher internal resistance (50K Ohms per volt DC).  Also has diode protection but appears to have no internal fuse. 


Any of these are good meters if you're really careful to use the correct modes.  Lack of fusing on a couple of them, though, means that if you test mains voltage on the ampere modes, you'll probably destroy the meter.  So I would expect a lot of the "untested" meters to be no good.

Here again, if you like this general type of multimeter, just get one of these immediately.  More expensive than a Micronta by far, but totally worth it.


Get yourself a Micronta here

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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. 

As long as the mechanism is still good, most of the other stuff in this type of meter is fixable.  That's the way electronics should be.


If you like working on electronic projects, then by all means get yourself a vintage multimeter.  You might even find one that works perfectly.

If you want a multimeter that you can use right now, and you want accurate readings, get a good DMM, or a Simpson, or maybe one of these


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