Sometimes, a voltmeter is all you need
Brad Thompson, Contributing Technical Editor brad@tmworld.com -- Test & Measurement World, 2/1/2008
Two days before Christmas, our two-year-old Toshiba DVD player refused to play. Its 90-day warranty had long expired, and even if I could find a service shop, repairs would cost far more than a new player. Like most consumers, I immediately thought about purchasing a replacement. Venturing into holiday traffic and retail madness, however, didn’t appeal to me. Surely, my collection of state-of-the-art (circa 1994) test equipment could see me through the repair process?
On the minus side, I had no schematic diagram for the DVD player. And given manufacturers’ propensity to pack ever more functions into custom-designed, fine-pitch “jungle” ICs, a major component failure would be irreparable.
Gaining access to the DVD player’s innards proved surprisingly easy. I removed three screws and a thin sheet-metal cover, revealing three subassemblies—a single-sided power-supply board, a disc transport, and a double-sided multifunction board. I connected the player’s audio outputs to my workbench audio amplifier and loaded Miles Davis’ “Kind of Blue” CD. The disc spun and music played, albeit weakly and with great distortion—Miles sounded as if he were spitting in his horn.
Thinking that a solder joint had failed, I tapped and wiggled components on the multifunction board, with no effect. Then I recalled the First Axiom of Troubleshooting: Always check the power supply. Fortunately, the supply board’s silk-screened component legend specified the voltages conveyed via ribbon cable to the multifunction board.
Reaching for a voltmeter, I quickly discovered that the –12-V output read zero volts. Unsoldering a Zener diode didn’t restore the voltage, but unsoldering a 470-µF, 16-V electrolytic capacitor did. I found a replacement in my component collection, and minutes later Miles sounded like Miles should. Incidentally, the failed capacitor appeared perfectly normal, with no bulging ends or leaking electrolyte common to counterfeit capacitors.
So, even if your normal workday takes you far away from electronic hardware, don’t be afraid to tackle the next piece of malfunctioning consumer electronics in your household.
Sometimes, a voltmeter is all the instrumentation you’ll need.
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