Micronta 22-204C Range-Doubler VOM

I bought my first VOM from Radio Shack back when I was in college. 

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I couldn’t afford a Simpson nor a Triplett (and I disliked the styling of Tripletts at the time - the folly of youth!), but I did spring for a good-quality Micronta VOM.

22-204 catalog page 1974

It was next to the top of the line model in their 1974 catalog, touted as 50K ohms/volt with 43 ranges.

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It has a mirrored scale to eliminate parallax error when reading the meter, and a good set of ranges all the way down to 0.125 volts when using the “Range Doubler” feature that halves the full-scale reading on a voltage or current range. 

I use to think the "Range Doubler" was a gimmick. (It didn’t help that the range-doubler slide switch has a cheesy feel to it.)

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But you can only fit so many ranges on a rotary switch, unless you go to the extreme Simpson used in their 262 and 269 VOMs, with a chain-drive and a humongous switch turret as shown above. So I’ve grown to appreciate the range-doubler feature, especially as implemented on the Triplett 630-NA. (No cheesy feel to the Triplett switch!)

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The case and handle appear to be made from ABS plastic. It’s survived some rough use without any cracks and not too many scratches.

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The label says “Custom manufactured in Korea for Radio Shack a div. of Tandy Corp.”

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Here’s a view of the guts.

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It takes an AA cell and a 9 volt battery for the ohmmeter ranges. The resistors are mostly 1% metal film resistors (blue body) with some carbon film (brown body) and one wire-wound resistor.

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It has four calibration pots. I’ve never touched them.

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This side has the shunt for the high current range. It’s the 14 gauge bare wire visible beneath the circuit board, with a tap wire soldered to it. I wonder if the shunt is just copper or some fancy alloy like Constantin or Evanohm?

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Bottom view. The shunt is sleeved with fiberglass spaghetti on this section. Those aren’t cracks in the case, just some hair donated by Fang, our cat.

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Top view. The meter came with a spare fuse stored in a well just above the fuse holder, but I blew it once, probably measuring ohms on a hot circuit.

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Another view of the circuit boards and wiring.

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A side view.

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Top view. The handle has a rough texture to give a good grip on the slippery ABS.

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Bottom view.

I saved the text from Radio Shack’s web site before it went offline:

Digital Multitester  (220-0204)       Features        Faxback Doc. # 14075

This high-sensitivity Micronta Multitester is designed to measure AC and DC voltages, DC current and Resistance with accuracy and ease.  The large, single-knob Range/Function control is easy to use and read.  The Range Doubler switch (V-A/2 -- V-ohm-A) effectively doubles the number of AC and DC scales available.  This means you can obtain meter readings in the upper half of the scale, resulting in consistent accuracy.  An “ FF" position is provided to insure meter protection during transit.

The sensitive 15 microA meter movement with 4.25" (10.8 cm) face and mirrored scale makes accurate reading a simple matter.  The meter scales are in 3 colors for rapid identification.  The lowest DC ranges — 125mV and 25 microA -- are great for solidstate circuit work.  The handle can be flipped around to the back to support the meter at an easy-to-read angle.

These features combined with rugged design and quality parts insure you of may years of accurate measurements.

Digital Multitester  (220-0204)     Specifications    Faxback Doc. # 14077


Ranges: ............................................................... 43

  DC Voltage: ......................... 0-125-250mV -- 1.25-2.5-5-10-25-50

                                                       -125-500-1000 volts

  AC Voltage: ........................ 0-5-10-25-50-125-250-500-1000 volts

  DC Current: ................. 0-25-50 microA-2.5-5-25-50-250-500mA-5-10A

  Resistance: ................. 0-2K-20K-200K-2Meg-20Meg (Center scale 10)

  Decibels: ........................................-20 to +62 in 8 Ranges


Accuracy: ...................................... +/- 3% DC except as noted

                            +/- 4% AC, and 125mV to 2.5V, and 500-1000V DC

                                      +/- 3% of Scale Length on Resistance

Sensitivity:

  DC: ........................................ 50,000 ohms/volt (V-A/2) or

                                                25,000 ohms/volt (V-ohm-A)

  AC: ........................................ 10,000 ohms/volt (V-A/2) or

                                                 5,000 ohms/volt (V-ohm-A)

Meter Movement: ........................ 4.25" (10.8cm), 3-color, mirrored

                                               scale, 18 microA full scale

Batteries: ............................... Requires 1 AA penlight cell and

                                         1 V rectangular for Ohms function

Leads: .................................... 44" (110 cm) banana-plug style

Specifications are typical; individual units might vary.  Specifications are subject to change and improvement without notice.

Digital Multitester  (220-0204)        Operation      Faxback Doc. # 14076

WARNING:  To avoid electric shock, disconnect measuring terminals before

          removing battery.  Replace only with same type battery.  Open

          cabinet only when replacing battery.  Do not touch any inside

          area other than battery.

You'll need one 9-volt battery for 20 Megohm range, one "AA" penlight cell for other resistance ranges.  We recommend our 230-0583/0553 for 9V, and our 230-0582/0552 for "AA".  Remove the back case and install the batteries in the correct compartment.  Observe the proper polarity. Replace the back cover.  If you can't adjust the pointer for "0" on OHMS scale, it's time for a new battery.  Replace the relevant battery with a new one.  By the way, never leave weak or dead batteries in your unit. They may leak damaging chemicals (even "leak-proof" type). Also, if you are not going to use your unit for a week or more, remove the batteries.

For the most accurate readings, keep the meter lying flat on a non-metallic surface.  Also, use a Range and Range Doubler setting that results in a reading in the upper third of the meter scale.

Also, look at the scale from the point where the pointer and its reflection on the mirror come together; else a reading error due to parallax will result.

If the pointer does not normally rest exactly over "0" at the left side of the scale, adjust the plaster screw in the lower center of the meter face to bring the needle to "0".

Always observe correct test lead polarity when making DC measurements; Black into (-) COM and Red into the (+) V-ohm-A (or (+) DC 1000V or (+) DC 10A) jacks.

The maximum input limit for voltage and current measurement between (+) V-ohm-A and (-) COM is 1000V AC, 250V DC and 500 mA DC.

Exercise extreme caution when measuring voltages of 150 and above.

When not in use, always leave the Range switch in the "OFF" position.

Use the Range Doubler as follows:

  For Resistance readings, always use the V-ohm-A position.

  When using the V-A/2 position for all other functions, divide the Range switch setting by 2 and read on an appropriate scale.  For example, Range set to 250 AC V and V-A/2 -- the range is 125 volts (250 divided by 2) and you should read the red scale, following the 0 to 125 markings.  Another example:  lead in (-) COM and 10A with Range set to 10A and V-A/2 -- the range is 5 amperes (10 divided by 2) and you should read the black scale, following the 0-50 markings.

TEST LEADS

Use only the same type of test leads as are supplied with your unit. These test leads are rated for 1000 volts; replacements are available from your local Radio Shack store.

I believe there was an ARF thread on calibrating these, but I don’t have network service at the moment so I can’t look it up. I did save a scan of the manual for the 22-204A that includes a schematic. The link was posted in an ARF thread, but I don’t have the original URL any longer. I don’t think the 22-204C varies much from the A model. It looks like the C may have a couple of varistors that don’t appear on the A model schematic.

I sold my 22-204C VOM  at one of the NEARC meets several years ago. I sometimes wish I had kept it, though I have many better meters now.

Update: I found a scan of the 22-204C manual with schematic on archive.org

I also found the ARF thread on calibration. The calibration instructions are in this scan of the manual for the Archerkit version of the VOM.



© Steve Byan 2011-2016