Sometimes, you’ve got to think simple first
By Bill Schweber, Technical Editor -- EDN, January 7, 1999
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 My plain-vanilla, 49-MHz analog cordless phone started acting weirdly recently. Among its intermittent symptoms, it wouldn’t pick up properly when a call came in, it would suddenly spew out overwhelming noise during an ongoing call, and it wouldn’t "flash" properly to access call waiting. I was torn. Should I just trash it and spend $50 to $100 for a replacement or should I consider getting it repaired—which would probably cost more than the replacement? But first, being the electronics person and tinkerer that I am, I figured I would open the phone and perhaps repair it myself—an idea that would work only if the fault was something visible or an intermittent connection.
As I struggled to open the phone, I thought of all the technical possibilities that might account for the malfunction and tried to deduce their common thread. Was it a problem in the RF section or external interference from a nearby transmitter? Maybe it was the IF-to-baseband area that needed a jiggle? Should I give it a blast from my hot-air gun, followed by freezing spray to stress the circuitry and root out the intermittent component? I measured the battery voltage, which checked out OK. So...on to the surgery! In the end, though, dumb luck shined on me. I remembered that I had another phone with the same type of rechargeable battery, so I said, "What the heck?" and swapped battery packs. After an eight-hour recharge, I gave the malfunctioning phone a try. It worked fine...and it has continued to do so ever since. There’s a valuable lesson here: Sometimes, the common factor that links disparate symptoms is nothing fancy or sophisticated but just a taken-for-granted function, such as battery pack that apparently couldn’t properly hold a charge under load. Though I tried to connect the dots of the symptoms I observed to draw a complex picture, I neglected to look at the unpredictable, dynamic effects of a dc source that was marginal. Maybe if I understood less about cordless phones, I would have stopped thinking at such a sophisticated level and instead admitted the obvious to myself: "I might as well swap battery packs, because that’s the only internal part I can really replace." Sometimes, a little knowledge can falsely empower you (to use today’s stylish phrase) and lead you down some unnecessary paths. Be sure to stop and think simple first before constructing complex theories to account for what you see. |
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 Bill Schweber, Technical EditorYou can reach Technical Editor Bill Schweber at 1-617-558-4484, fax 1-617-558-4470, or bill.schweber@cahners.com. |
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