Electronics     

Introduction


So in a previous article, I hoped that I would have the sense not to accept another Micronta multimeter.  And what should happen to end up on my desk, but... another Micronta multimeter.

This one, a benchtop model, was not working.

Micronta electronics can be good;  that's not the thing.  It's just that multimeters get zapped a lot from measuring too-high voltages (or being on the wrong setting).

We're going to see, first of all, if this one is repairable.  And while we're at it, let's talk about benchtop multimeters.



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In This Article

The 22-175A

If It Ain't Broke...

More Fun

Diodes

The Extended Project




The 22-175A


It's a digital multimeter.  I think the 22-175A was introduced sometime in the late 1980's. 

Micronta and Realistic were two of the brands that made Radio Shack great.  They were made for electronics enthusiasts.  (Today, try this link and you might find a used one.)

Nostalgia aside, the Micronta 22-175A looks to have a number of good features.  There are the usual voltage and current-testing functions, and the diode tester as you might expect.  But there's also a capacitance meter and a frequency meter as well. 

The plastic housing is kind of boring gray; it's typical of the stuff that emerged toward the end of the 1980's.  It just has a certain look to it.  Compared to something like a Simpson multimeter-- which still had that "battleship electronics" quality of the 1940's (and still does today)-- the Micronta looked cheap for its time.  Now, in 2018 I guess it's like, "Wow, cool: vintage!"  (Even I find myself saying this when looking at electronics that I thought were junk, back in the day.)

This is how we get repair projects that pile up on the workbench.  And this is how the Micronta somehow seemed like a "gotta-have it" acquisition.  Yep, I know I said I wasn't going to accept another Micronta for repair. 


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Another Broken Multimeter?


First attempt at power-on, and nothing. 

This time there was no battery corrosion;  perhaps something got fried from testing too much voltage.  But the fuses were still good.

Opening the plastic housing, I found the LCD screen, one circuit board, some wires, and a lot of empty space.  (Not a bad thing, necessarily.) 




There's one surface-mount IC chip that says "TCR6001", "119D", and "9325"; I couldn't find anything about it.  Around it are a few caps, resistors, and diodes. On the other side of the board there are the pushbutton switches for the meter's front panel buttons.

So, looking at the circuitry I didn't see anything scorched, melted, or bubbled;  that's a good starting point.


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Power On


Putting fresh batteries in it, this time the meter powered on.  The DC voltage seemed accurate. 

But the all-important Diode Tester kept showing "open circuit", no matter what I tried.  This meter shows a big "3", for "3 megohms or greater".  Hmmm, weird. 




After taking the thing apart again, testing a bunch of stuff, puzzling over what could be wrong... that's when I noticed it.

I had been using it with the positive test lead plugged into the wrong place. 

Wow, yeah.  Duh.

This is the kind of stuff that fries multimeters, if you're not careful.  But these days, I know enough not to use a multimeter like this on mains voltage until I'm good and sure that everything is A-OK in the circuitry, and the leads are plugged where they should be.  (Which, they weren't.)

But as for the circuitry, I still wasn't sure.


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Diodes


Using another meter, the diodes on the board were testing as dead-short, both directions.  Not even a voltage drop;  these things were acting as if they were solid wire traces.

Except that when I de-soldered one of the diodes, it tested "OK".

It seemed there had to be a zero-ohm path around them, which means there was no real purpose for the diodes to be there.  Either that, or there was some other component shorted, somewhere off in the... surface-mount IC, perhaps?  Without a schematic it's tough to find where.

De-soldering and re-soldering put way too much heat into that diode to be any good for it.  But after I put everything back together, the meter seemed to work well in every test that I tried.

Voltage... good.  Diode / Continuity test.... good.  Capacitance meter... good.  Ohms... good.  Just these four things are probably 90% of what I'd ever use a multimeter for. 

I didn't get to the Frequency test, the hFE test (transistors), or the Ammeter.  That last one, hmmm.... testing current seems to be how a lot of meters get ruined.  But sooner or later I'll try it.

Meantime, let's talk about the voltage.


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Voltage


A meter that's any good should give accurate voltage readings.  Obviously.  And if you're not sure the meter is 100% sound, you won't really want to test mains voltage.  (Seriously:  don't.)

So far, though, DC voltage reads fairly accurately-- not perfect, but good.  And it will read alkaline batteries accurately.

Many people don't realize that's not a given.  Some meters get this very wrong.

Sometimes by 50% or more.

My cheap GE-branded multimeter reads new AA batteries as "0.75 volts". Ridiculous.  So, too, my Micronta 22-203 reads them at about 0.75 volts.  If these were your only multimeters, you would throw away perfectly good batteries, waste many trips to the store, etc.  (Unless you knew enough to use the right type of resistor across the meter probes...)

My Amprobe DM-7C reads AA batteries accurately, right away.  And so does this Micronta 22-175A, within about 0.003 volt.


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Bench Meters


This project taught me something about benchtop multimeters. 

Why use a benchtop multimeter?  The display is at the perfect angle;  the meter is accurate and more precise than the typical handheld;  and it has more features.  If you have a lot of components or circuits to test, I could see a meter like this making the process more efficient.  More enjoyable, for that matter.

Handheld multimeters like this one might render benchtops somewhat redundant, but not entirely.  The interface, and the convenience of the benchtop form factor, is really nice to have.

Here's another thing I learned.  Although a used Micronta like this could be had for cheap, the fact is that good, brand-new benchtop units tend to be $300 and up.  There exist bench multimeters that are $1,200 or more, if you need that kind of precision and accuracy.

Something like this Rigol may seem kind of expensive compared to a $50 handheld DMM.  But I'm starting to think it might be worth it.  Get a semi-decent oscilloscope and a good bench multimeter like this, and that's most of what a test bench will need.


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More Repair Notes


Still no schematic for this Micronta 22-175A.  And I still don't know why the diodes test as dead-short within the circuit.

What I did find was that one of the wires had been corroded.  The solder flux, left in place for all those years, had just about corroded through all the wire strands.  So, that may have had something to do with why the meter didn't work initially.  Intermittent connections like that can be really difficult to pinpoint. 

If I do have to replace components-- even, perhaps, that surface-mount chip-- the repair process on this one should be doable.  This one looks a lot easier to fix than some old multimeters that I've seen.  The components are fairly easy to access, and there aren't too many of them.  You could almost replace every component on this board in one or two evenings, if you had all the parts. 

I'll probably replace the 1-microfarad, 50-volt caps on the board, simply because they're already almost 30 years old.  Whatever they do, there's obviously a reason why that capacitance and voltage were chosen.  What I want to know is:  how does capacitance affect the accuracy?  These things drift, and I'm sure that can mess with stuff.

Here again, when you have a vintage multimeter, it's always a good idea to have a known-good DMM you can test it against. 


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Conclusion


So far, this one seems OK.  Whatever was keeping it from working initially, it seems to be fixed now.  That one corroded wire may have had something to do with it.

I really like the benchtop form-factor.  And the 22-175A itself is not a bad little multimeter.  It's not top-of-the-line, but it does have capacitance and frequency metering. 

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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