Using a Volt Ohm Meter
7/98 - Jens Moller - http://www.colomar.com/Shavano/vom.html
A very handy tool for trouble shooting problems is a VOM (Volt Ohm Meter) - also called a Multi-Meter. It can be used to test cables, AC power levels and Batteries. You'll often find yourself out on the road with problems that are causing you grief, but you aren't quite sure why.
This is usually called an Analog Volt Ohm Meter, Volt Ohm Meter, or
just referred to as a VOM. Up until the late 1980's this was the most
common type available (and by far the least expensive until recently).
It has a needle that moves across (left to right) a physical meter (called a 'movement'), some sort of rotary switch and a set of wire probes. The switch setting allows you to select which mode that it operates it. Because of the impedance of the physical meter, the accuracy of these for measuring certain types of signals or levels can be a problem, however, for the things discussed in this document, its not important at all.
This is usually called a Digital Multi Meter or DMM. It provides the same
functionality as any other VOM, and is often referred to as a VOM. These are
far more common than the Analog VOMs today, and the cost for an inexpensive
DMM is quite reasonable. The truth is, its easier to use than the Analog
variety, but, its no more accurate. I prefer the ones that allow me to select
the mode I'm trying to operate in (ie. having a rotary switch).
It has a digital display/read out and few moving parts, other than wire probes and possibly a rotary switch to select operational mode. The input stage normally has a very high impedance, which has less of an effect on some types of measurments.
My VOM travels in the cord case I use to carry my specialty and short cables in. This case also holds my spare fuses, soldering iron, solder, spare guitar strings and tape. The VOM is in a bag with a string pull to close it up with so I don't lose anything when its not being used. Mine was incredibly cheap when I bought it over 10 years ago and it still works fine. These don't need exceptional accuracy for the purposes I'll describe. I prefer the non-digital type because ones with a meter movement can still be used to measure voltages even if their on-board batteries go bad (which is a quite common problem) after a few years of disuse or forgetting to turn the VOM's Ohms mode off. I've crushed these in the past, and a cheap one is easy to replace. You can buy very expensive models that are used by technians who service gear - you may want to consider a higher end model if you are also going to do bench repairs, however, you would be surprised at how good even the cheapest models are for most of the functions that you'll ever need it for as a working musician.
A VOM will have 2 wires, called Probes attached to it, or separately as 2 independent electrical leads. Typically one is Red and the other is Black. Some measurements require that you observe the polarity of the source, others, it makes no difference. To keep things straight in your mind, always plug the Red lead into the plug marked + and the Black lead into the plug marked - and/or Ground.
NOTE: Always disconnect the cable completely before testing with the Ohm setting on your VOM - having the cable connected to something will invalidate your readings, or, could damage your VOM if power is hooked up to it
The Ohm setting on the meter is used to test the conductivity of something. This could be a wire, it could be a switch, it could be almost anything that is supposed to carry electricity from one point to another.
Cables commonly fail - this is a fact of life. They can fail in a number of ways. The most common failure mode is an open circuit; a total disconnect from one cable connector to another. The next most common is a dead short - often caused by mis-handling of cables, or something heavy falling/rolling over the cable. Less frequently, but still common, is a high resistance failure; this is usually related to damaged internal wires - often caused by stretching a cable and having the wire inside break.
The way to test for these problems is to test the Resistance of the wire. Resistance is measured in Ohms, and your wiring should have very little resistance in it. To test the resistance in your wires, you first need to verify the reading of a dead short on your meter. You do this by setting your VOM setting to the lowest resistance range (on some auto-ranging Digital VOMs, there is nothing to set), then touch the Red and Black probes to each other. The meter should read 0 Ohms. On a VOM with a meter movement, the needle will swing all the way to the right and there will be an adjustment (usually marked Zero or Ohms Adjustment) that you turn to get the meter to read zero (0) Ohms. If the VOM's battery is weak, this may not give you the results you want. Digital meters do not need to be manually zeroed out.
|Once the VOM is zeroed out, take your cable, and check the wire out, end to end. Your goal is to verify that each end of the cable is still correctly wired up. For example, a power cable will have at least 2 wires in it, sometimes 3. We want to verify that the wires are intact. If they are, reading from end to end will register a close to a dead short for each wire in the cable.|
If you have an open cable, one or more of the internal wires no longer connects all the way thru anymore. If in doubt, compare the cable to a known good cable. Some cables are open only if you wiggle the connector - make sure that you verify that the internal wires are truly connected.
If one of the wires measures substantially higher resistance than the others, you have internal damage to the cable, or a very poor solder connection somewhere.
To test for a dead short, at one end of the cable, connect the VOM probes across to of the cable connections - if the resistance reads 0 Ohms you have a dead short. If you have measurable resistance, there is likely damage in the cable. Some cables have some wires connected together - if in doubt, verify the cable against a known good one that is used for the same purpose.
Set aside any bad cables for later repair, unless you have the equipment, tools and time available to repair it now. I keep a Bad Cables bag (something very much like a pillow case) that helps me to locate cables that need repair later - anything in the bag gets fixed when I get home.
For exceptionally long runs of cable, there may be some resistance measured, but it will be slight. For 18 gauge wire and larger diameters, you will see 6/10th of an Ohm or less measured per 100 feet of cable. All wires in the cable should have equal measurements.
One of the many joys of performing that you may run across is unreliable AC power. Its typically more of an issue in older establishments, but it can happen anywhere, no matter how recently the wiring was put in.
I happen to live in a area where there are many small towns within a 150 mile radius that hire bands. Some are way up the in mountains, others are out on the plains (I live in Colorado Springs, which is at the foot of Pikes Peak). Outside of larger cities, the power may be a bit sloppier regulated than you hoped for. A common problem that causes strange behavior in my musical gear are Brown Outs. These are instances where the voltage drops down below normal for short, or extended amounts of time. Some types of equipment can be greatly affected by this. Anything mechanical (such as a Hammond B3 or M3 type of organ), or anything dependant on specific voltage levels (such as Tube amplifiers). Occasionally, I'll even have MIDI based synth gear reset itself as a result. Brown Outs are a bad thing. You can measure the voltage that powers your system with your trusty VOM.
|Slide the probes into an electrical outlet (one lead for each of the power leads), and be careful not to touch the metal part of any of the probe leads with any part of your body. The voltages here could kill you, please be careful. You may want to check a few other outlets in the room in case the voltage looks wrong to you. Test your power at home so you have a point of reference that you know is good.|
I have had instances where I had to leave the wires connected to a power outlet figure out when the Brown Outs occurred. Problems tend to be related to events that occur in the kitchen facilities, and its very hard for you to predict when you'll have a problem, other than things sounding quite bad for brief periods (or longer) of time. Many times the club owner isn't in the kitchen to cause the power problem and assumes that the band is to blame.
Some examples of problems that I've encountered and what I did to correct the situation (None of these occurred at the same establishment):
I bought a battery backup/power conditioner unit (these are available for Brown Outs on computer systems) for my Synth gear. This eliminated some unexplainable behavior at certain clubs. If you have synth gear, you may want to consider it.
NOTE: For low voltages, if you accidently touch the meter probes to the wrong battery contacts you won't hurt anything. The meter should be able to handle reversed polarity on the DC range with no problems.
I normally hate battery operated effects. Batteries go dead when you need them the most. Like any other electrical gear, battery powered gear starts acting peculiar when the voltage starts to drop. Unfortunately for me, some battery operated gear is quite indispensable - my Nady transmitter that allows me not to be connected to a guitar cable is one piece of gear I really like. I have an older rig that goes thru 9 Volt batteries pretty quickly. So, I switched to 9 Volt rechargable batteries. These NiCad batteries look just like the 9 Volt Alkaline batteries, but only put out 7.2 volts when fully charged. My Nady transmitter works fine with these, but I swap the batteries out with a recharged one every set (which means that I need 5 or 6 of these handy, as well as a charger that lets me get these powered backup again). When the Batteries start going, the sound quality goes down the toilet and as a result, I need to figure out which batteries are causing me a problem. NiCad batteries have a thing called a 'memory', in that if you always discharge them to a certain point (but not completely used up), they tend to start running out of power at that point. Once this happens you have to force them to discharge to get rid of the 'memory'. After 6 months of use, the batteries start having this memory problem and stop working right at the end of a set. Of course, the answer is to frequently fully discharge them and then re-charge them (this implies that I have time to do this, or will remember - I don't have the time and its a challenge just to remember to re-charge them). At the end of an evening, I check my NiCad batteries to see which ones are below 7.2 volts - and that are need the full discharge cycle. I have a number taped to each of these so I can figure out which battery is which. I toss questionable batteries into my 'Bad Cable' bag.
Most guitar effects use 9 volt batteries. So I tend to use battery eliminators when ever I can. Too bad that the cables that supply voltage to these fail frequently. With the VOM Probe you will need to check the connections and wiggle the cable at both the connector end, and also at the transformer end - the wires tend to break at either end (they use very cheap wire here and its not hard to damage). If you discover no voltage coming out of the battery eliminator, its usually a bad battery eliminator cable, the problem is usually in one of 2 places:
You can't always tell which end of the wire that the problem is at.
I won't cover the technique for this here, but if I have to open up a battery eliminator, I always replace the wire cable with 20 or 22 gauge copper wire - this will eliminate most future failures.
Note: Some battery eliminators provide AC voltages. If that is the case, it should say so on either the battery eliminator or the effect. Set your meter to the AC range and follow the same steps to troubleshoot the problem.
© 11998 - Shavano Music Online