Ask Eddie: In Search of Semiconductors

Sep 1, 2011 9:00 AM, By Eddie Ciletti

THE QUEST FOR THE PERFECT DATASHEET

Polls


Mix Regional

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Many of the questions I receive on a regular basis involve the often-frustrating process of searching for replacement parts. This month’s reader question comes from someone who is also a friend. Darron Burke of Makeshift Studio teamed up with local Boston-area artist/technician Kevin Micka after a series of emails did not reveal the hidden secrets of a troubled unit.

Kevin’s Question: How do I find compatible replacement parts for my dbx 165 compressor/limiter?
I did my best to provide several options. But before getting specific on sleuthing for the dbx, a bit of background is in order. Back in the day, vacuum tube and semiconductor cross-reference books ranged in size from small “bibles” to big-city phone books, then evolved into software (on floppy) and eventually migrated to the Net. Google and other search engines are often a better resource than dedicated tools, provided you know what not to click. What you really want is a free “datasheet” from the device manufacturer that offers pin-out and operating parameters. Manufacturers include National Semiconductor, ON Semi (formerly Motorola), Fairchild and Texas Instruments (TI), to name a few.

Figure 1: This multimeter tests diode and static transistor hFE.

Figure 1: This multimeter tests diode and static transistor hFE.

Zillions of semiconductor devices have been made during the past 60 years. The country of origin is often identifiable by the prefix: 2N (U.S.), 2S (Japan) or BC (Europe). Transistors have three legs—Base, Emitter and Collector—and come in all shapes and sizes to accommodate a wide range of power and gain requirements. Old parts are discontinued and replaced by equal or better parts, which can be a challenge for search engines because the improved specs are not easily reconciled against the original. When in doubt, the original’s pin-out can often be determined by following circuit board traces back to the next component. Make a drawing and compare with the replacement part.

SEMICONDUCTOR TEST
Nearly all multimeters have a diode-junction test mode. (See Fig. 1.) A transistor is essentially two diode junctions linked at the Base. Some multimeters have a transistor socket that tests for static hFE. With the multimeter in Diode Test mode, connect the probes to the Base and Emitter of a known-good transistor. If there is no reading, reverse the probes for the transistor polarity (NPN or PNP). When correct, the junction voltage appears (0.3 volts for germanium, 0.6V for silicon). The voltage is temperature-sensitive and not exact. Now move the Emitter probe to the Collector and get a similar reading. There should be no reading when the probes are reversed. There should also be no reading between emitter and collector in either direction (polarity). When all else fails, read the multimeter manual.






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