April 29, 2013 by Kevin Keras
Well Equipped…Part 1
Not every instrument failure requires a call to the manufacturer (OEM) or an independent service organization (ISO). Some simple and common failures can be rectified by just about anyone with some common sense and common tools. Can’t help much with the common sense, but the tool part is a lot more straight forward.
***WARNING *** if you are not comfortable working with electricity please don’t mess around and call for help from you own facilities support folks or and ISO. If you kill yourself, don’t write me a nasty-gram from the afterlife.
The handheld DMM - Digital Multimeter (aka the voltmeter). The name voltmeter is used pretty loosely by a lot of tech’s and only describes one function of this device. Very capable DMM’s can be found at the local hardware store for under US$50. For a good tutorial click here.
Voltage - Most DMM’s can measure a wide range of AC or DC voltage. One of the most common problems when you fire up an instrument and get nothing is no AC power. Most US labs will operator on 110 or 200VAC. A zero volt reading means you probably have popped a circuit breaker. If the AC outlet you are plugged into had a ground fault button, try pressing the reset button and try again. If you get voltage at the outlet, but no action on the instrument, you may have blown a fuse. Not comfortable checking voltage? Try plugging the instrument into a known good working outlet instead. More knowledgeable techs can test DC voltages for printed circuit boards (PCB’s) inside the instrument. Most instrument power supplies will convert AC power into lower voltage DC power and distribute it throughout the instruments. Many PCB’s have incoming power marked at a connector coming from the main power supply.
Resistance - Resistance is a measure of a devices ability to restrict the flow of electrons in a circuit. If you crack open an instrument and see a charred component, it is likely a burned out resistor. If you can still see the value of that resistor (some have the value printed, others may use a series of colored bands), you can use the DMM to verify if it is blown (open circuit, infinite resistance). While you may be able to unsolder and replace this component, there is no guarantee that it will not blow again, as something else may have failed that caused too much current to flow thru it or too much voltage across it, causing it to cook. If you come across a cooked resistor (or any other component), better to have someone replace the entire module. Almost no FSE’s will spend time doing component level failure analysis as it is time consuming and ultimately more expensive.
Continuity – Some DMM’s allow you test for continuity (the closure of a circuit) that will result in a beeping signal. No beep, no continuity. A quick crossing of the probe leads will tell you what sound you are listening for. This is what you will use to check you fuses or diode. A diode allows current to flow in one direction only. Diodes can be checked by reversing the leads across the component. It should beep with the leads in one position, not beep in the other. Some instruments have a main fuse as part of the receptacle that the AC cord plugs into. MAKE SURE YOU UNPLUG THE INSTRUMENT BEFORE YOU DO THIS!!! You can pop this open and check if the fuse is good or not.
Current - Not really something I would advise a notice to attempt. While voltage is measured across a load, current is measured in series with a load. So, in order to check current, you need to break the circuit and use the DMM to measure current flowing through the meter as part of the circuit. Lots of potential to hurt yourself here…leave to a professional.
Temperature – One of the features of many digital mulitmeters versus their older analog counterparts is the inclusion of a thermometer probe. This can be very hand for diagnosing random failures that are related to run away heating problems - a common example might be an intermittent cooling fan failure. Try taking a cover off near the fan, tape the probe somwhere close and note the temperature during normal operation with fan running (and cover back on). Then open it up, and unplug the fan (replace the cover) and monitor the temperature increase. If you do this, be vigilant and don’t leave the instrument unattended. You are looking not only for a temperature spike but also abhorrent instrument behavior…so you want to be able to shut it down ASAP.
Okay, so there you have it. Some basic things you can do with a DMM. Just remember, when it comes to anything involving electricity, you should always consult with your facilities management. Never perform electrical testing alone and never in the presence of liquids (especially flammables). When in doubt…leave to someone in the know.