VIEW FROM THE FLOOR (May 01, 2000)
Can you think like an electron? Believe it or not, if you're stuck with one of those migraine-inducing intermittent electrical faults, it can make the difference between minutes and hours. Why think l...
Can you think like an electron? Believe it or not, if you’re stuck with one of those migraine-inducing intermittent electrical faults, it can make the difference between minutes and hours. Why think like an electron? Because electrons, like gremlins, seem to have a mind of their own, and can surface just about anywhere in a suspect circuit. Except, of course, at the moment when the technician stands over the open panel, multimeter in hand. Tracing this dreaded “intermittent” requires a stealth and instinct. And also a few basic tools.
One is a high impedance multimeter. I prefer analog meters, but digital is more rugged and far more common. Why analog? Because intermittents which present as voltage spikes make the analog needle “twitch”, a tell-tale sign which you can catch in the corner of your eye. Flipping digits are simply harder to detect, since many autoranging meters respond so sensitively to “normal” voltage fluctuations that the numbers always bounce anyway. The other indispensable tool in my belt is a continuity tester. Continuity testers buzz or light up to confirm a good wire or circuit path. My favourite is the double D-cell flashlight variety. They’re rugged, and bright enough to show continuity by illuminating a wide area, perfect for the times when the tech’s head is jammed in some dark, inaccessible hole.
Supporting actors include insulated lineman’s pliers (the more expensive the better), insulated screwdrivers, jumpers with covered, color-coded alligator clips, and a couple of those long handled surgical instruments which my local supplier calls “hemostats”, and I call “clamps”. Get the stainless steel models with a long reach.
Now that you’re properly armed, where do intermittents hide? The simple answer is usually “anywhere”, but a little instinct (and luck) can go a long way. The first step is to resist the temptation to blame the computer. Contrary to popular belief, PLC’s and PC-based controllers are not often the cause of intermittents. If the suspect black box has a self-diagnostic program, run it to convince yourself, then move on the less glamorous, but more likely causes. Check connectors and card edge slots. It sounds simple, but you’d be surprised how many intermittents are caused by improperly seated boards, relays and connectors. Sometimes a wiggle is enough to spot the problem. If a little movement cures the fault, shut down, remove the connector and inspect for wear or damage. Damage can be almost microscopic, so good lighting and even a magnifying glass is useful. For miniature and submini connectors, a 2-D flashlight magnifier is a technician’s best friend. Replace damaged connectors as soon as possible, but in an emergency, consider soldering the component directly into place. Soldering on a wave-soldered or surface mount printed circuit board is not for the faint of heart or unsteady of hand. I use an iron, rather than a gun, and use the lowest wattage possible. I also use heat sinks everywhere. Remember those “hemostats”? They make excellent heat sinks, and solder won’t stick to them.
“Cold joints” are another possible cause of intermittents, and often respond by simply touching a hot iron to the suspect joint. And speaking of heat, overheated semiconductors often act up when overheating, so a shot of cooling spray (it’s made specifically for cooling electronic components) on a running board can pinpoint a bad component. There are more ways to troubleshoot wayward electrons than are words to describe them, and every technician has a unique set. Yours will be different. Not better or worse, just different.