IntroductionThere’s probably not a serv-ice engineer on the planetthat wouldn’t agree thatintermittent electrical faultsare the most difficult andfrustrating to trace andsolve. Although these mayseem like random disasterssent to test our patience,invariably they have a soundphysical cause. The trick, ofcourse, is to discover whatthis could be without spen-ding hours monitoring theoffending equipment untilthe elusive fault finally chooses to show itself. Thisapplication note, explainshow the service engineer’slife can be made infinitelyeasier with the powerful trouble-shooting features ofFluke’s ScopeMeter®

190 Series.

Application Note

Although usually highlycomplex, modern equipment isalso incredibly robust, withmasses of protection circuitry toguard against breakdowninduced by, for example, currentand voltage surges, mainsharmonics, EMI…you name it!This complexity, of course, alsohas its down side when itcomes to maintenance.Intermittent faults in particular,always the curse of the serviceengineer, can become a major

headache to trace in today’sultra-sophisticated systems withtheir fast and complex controlsignals. Fortunately the situation is farfrom hopeless since althoughintermittent faults share oneinfuriating characteristic, i.e.their apparent randomness,they all fall into several well-defined categories. This offersan important clue to tracing andsolving them.

Troubleshooting without tears withFluke’s 190 ScopeMeter Series

Troubleshooting with theFluke ScopeMeter® 190Any number of factors can leadto spurious signals that maycause equipment to suddenlydrop out for no apparent reason.Some of these are equipmentrelated, for example, defectivecircuitry, dry solder joints, dustor corrosion and overheating.Others such as main fluctuationsand voltage spikes due, forexample, to airco systemsswitching on, are related to themains supply. Identifying themost likely cause requiresexperience and intuition and, inmany instances, a lot of luck.Now, however, with the FlukeScopeMeter® 190 Series, luckdoesn’t have to enter theequation. This new handheldseries combines the functions ofdigital multimeter with digitalstorage oscilloscope inbandwidths up to 200 MHz andwith up to 2.5 GS/s real-timesampling per channel. The series also offers highlyinnovative troubleshootingfeatures found in the past onlyon very expensive desktoposcilloscopes. Available for thefirst time in a portableinstrument, these features makethe life of service engineermuch easier, enabling him orher to trace and solve allmanners of intermittent faults.

Tracking down faulty wiring Every service engineer knowsthat bad wiring is often one ofthe most frustrating intermittentfaults to trace. Touching thecircuit may trigger the fault buteven this is not predictable. And even if a scope isconnected to the suspect circuit,it may not be possible to keep a continuous watch on thescreen, or something may flash on screen and disappear

before its significance can bedecided.

Wouldn’t it be nice in thesesituations to be able to lookback say over the past 5 or 10seconds to get another chanceat spotting the glitch.Fortunately, the ScopeMeter®190’s automatic capture andreplay feature offers just thispossibility. The Scope continuously recordsthe last 100 screens in a FIFOmemory. As soon as you spot ananomaly on the scope orsuspect something has occurredthat you might have missed, theReplay button can be pressed tofreeze the last 100 screens andallow you to play them overagain. The feature, in fact,allows two sets of 100 screenswith individual time stamps tobe stored for later recall ordownload to a PC for moredetailed analysis.

Catching signal transientswith capture and replay Another potential source ofintermittent faults – mainstransients - can also be pickedup with the capture and replayfeature. In this case, however,the ScopeMeter needs to be setto trigger on the glitch yoususpect is occurring. Theinstrument then operates in‘baby-sit’ mode, capturing 100samples of the event on whichit has been set it to trigger.

If you’re looking for a voltagespike on the mains, for example,the instrument should be set totrigger on positive- or negative-going voltage pulses withamplitude slightly larger thanmain voltage. This method can’tbe used; however, to captureglitches that does not havevoltage levels higher than the

signal level. The solution then isto use pulse-width triggeringset to capture say negative-going pulses with duration lessthan a specified time. With the200 MHz version (ScopeMeter199), which has rise and falltimes of 1.7 ns, pulses asnarrow as 3 ns can easily becaptured using pulse-widthtriggering. Setting the trigger tocapture negative-going pulsesthat last longer than 20 ms canalso capture missing cycles onthe mains. See figure 1.

To see how often the eventoccurs, just leave theScopeMeter connected to thecircuit for aslong as you likethen pressing the Replay key tosee and analyse the capturedevents. You may, for instance,find that the glitch occurs oncea day at a particular timecorresponding to factorymachinery being switched on oroff, or weekly on a particularday. Whatever, you should beable to relate the apparentrandom fault on the equipmentto specific external eventswhich could be a big steptowards solving the problem.

Looking for sags and swellswith TrendPlot™Gradual deviations in sayvoltage or temperature canoften signal the start of

Figure 1 : Setting the ScopeMeter to trigger onnegative-going pulses longer than 20 ms willdetect missing cycles on the mains

2 Troubleshooting without tears with Fluke’s 190 ScopeMeter Series

minimum values andautomatically compressing thetime scale to show the completetrend from the start. See figure 2.

Analysing fast, complexelectrical processesWhilst capture and replay willregister fast events such asvoltage spikes and drop outs,more complex electricalprocesses often need somethingmore. A good example is theinvestigation of power cycles inan UPS (Uninterruptible PowerSupply) at switchover from mainsto inverter (and vice versa).

problems. A copying machine,for example, may operateperfectly for half a day afterwhich problems start. Ambienttemperature fluctuations overthe day may be the cause ofthis and it would be useful, inthis case, to monitor thetemperature over an extendedperiod.

The ScopeMeter 190’s TrendPlotfeature was introducedspecifically for occasions inwhich relatively long-termmonitoring is needed. Operatingin the ScopeMeter’s digitalmultimeter mode, TrendPlotoffers the ideal way to locatecauses of intermittent problemsthat occur perhaps only once anhour or once a week. Besidesambient temperaturefluctuations, voltage sags orswells on the mains or a failingpower supply are otherintermittent problems that canbe traced with TrendPlot. Inthis mode, the ScopeMeter actsin effect as a paperless recorder,plotting for up to 8 days theaverage, minimum andmaximum values of any selectedscope or meter measurement.Using this feature, long-termirregularities can easily bespotted. TrendPlot also offersthe important advantage ofaccurate time stamping with aresolution down to one minuteto show exactly when anirregularity occurs. This can bedisplayed as either time-of-dayor elapsed time, depending onthe nature of the change you’retrying to monitor. WhilstTrendPlot is running, theScopeMeter operatesunattended, continuouslyrecording the required datawhile dynamically setting thevertical amplitude scale todisplay the maximum and

Powerful troubleshooting features of theScopeMeter 190 Series

Automatic capture and replay of 100 screens. This feature allows thepast 100 screens to be reviewed to allow a second look at a one-timeevent. In normal use the feature continuously memorises the past 100sequential screens, allowing you to scroll through and freeze any-thing interesting for further analysis. It’s also possible to use theadvanced trigger capabilities of the ScopeMeter 190 to record 100specific events for future analysis.

TrendPlot™. For faults that may occur only once in a while over longperiods, this feature acts as a ‘paperless recorder’, plotting on screenthe minimum, and maximum and average values of a selected para-meter over a period up to 8 days with a resolution of 1 minute.

ScopeRecord™. This is a continuous sampling mode in which theScopeMeter stores points (consisting of minimum and maximum valu-es) continuously at the rate of 20 MS/s (mega samples/s). This enab-les it to capture events as short as 50 ns which can easily be viewedwith the mode’s 100x zoom feature. The scope has a 27500-pointmemory allowing continuous capture for up to 30 hours.

The quality of the switchover iscrucial to the operation of thesystem – any phase differencesinduced may cause the power tobe momentarily interrupted,which in computer systems ortelecommunications equipmentcould lead to catastrophic loss ofdata. The signal profile at switchover isunlikely to be known so insteadof spending a lot of timeattempting to set trigger signals,it would be far better to recordan entire sequence covering theevent and investigate itafterwards.

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Figure 2 : Plotting long-term irregularities with ScopeMeter’s TrendPlot feature

Troubleshooting without tears with Fluke’s 190 ScopeMeter Series

Here’s where the ScopeMeter190’s ScopeRecord mode comesin. This is a continuous-sampling mode in which thescope records points (consistingof minimum and maximumvalues) at a rate of 20 MS/s.With a 27500-point deepmemory, the ScopeMeter canrecord continuously in thismode for up to 30 hours,capturing glitches as short as 50ns. What’s more, a 100x zoomfunction makes the smallestdetails visible such as the shapeof an individual power cycle. Figure 3 shows what happenswhen an UPS switches overfrom the inverter to mainssupply. Although the switchoverwould not be visible with anormal display of say 200ms/div, captured withScopeRecord; the switchover isclearly visible using a 100xzoom factor. In this case, thedisplay clearly shows that nointerruption in supply hasoccurred, with the main voltageconnected and in phase withinjust a few milliseconds.

An ideal partnerWith the modern trend towardever faster, more complexelectronic control systems, theneed for sophisticated test toolscan only increase. Whilstadvanced bench-topinstruments may offer animpressive array of trouble-shooting features, their priceand lack of portability generallypreclude their use by mobileservice engineers. These needan instrument that they caneasily carry around and, sincethey never know what to expecton a call out, preferably onewith the widest range oftrouble-shooting features.Fluke’s ScopeMeter 190 Seriesfulfils these needs perfectly.Combining portability with arange of advanced features thatare both powerful and intuitiveto use, it’s the ideal partner fortoday’s service engineersworking in the field.

Fluke CorporationP.O. Box 9090, Everett, WA USA 98206

Fluke Europe B.V.P.O. Box 1186, 5602 BD EindhovenThe Netherlands

For more information call:In the U.S.A. (800) 443-5853 or Fax (425) 356-5116In Europe/M-East +31 (0)40 2 675 200 or Fax +31 (0)40 2 675 222In Canada (800) 36-FLUKE or Fax (905) 890-6866From other countries +1(425) 356-5500 or Fax +1(425) 356-5116Web access: http://www.fluke.com

© Copyright 2000, Fluke CorporationAll rights reserved.Data subject to alterations without noticeScopeMeter® is registered trademark of Fluke CorporationPrinted in the Netherlands 10/00Pub_ID 10297-eng

Figure 3 : ScopeRecord shows a voltage irre-gularity when switching off a UPS