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two fails
Sunday, February 1 2015
Today I took another look inside that cheap MP3 player I managed to take apart yesterday. I wanted to see what integrated circuits it used so that I could determine where I might be able to intercept the unamplified audio before it made it to the speaker-driving amplifier chip. I had a good candidate for the amplifier: an eight-pin surface mount IC whose markings were unreadable even with an illuminated magnifier. I had to use a USB microscope to determine that it was an 8002b, an apparently only-in-China monaural audio amplifier (which makes sense; the MP3 player only has one speaker). After finding a PDF of its specs, I determined that its audio input was on pin 4. Tracing that back across the circuit board with a multimeter (checking for zero-ohm continuity with pin four at other places), I found it connected to first a capacitor and then a pair of resistors, exactly as it should have been. But when I managed to pop off one of the tiny surface-mount resistors, one that should have killed the audio, it didn't have the intended effect. In desperation, then, I tried an old trick from my ample experience with dual-inline-pin 0.1-inch-pitch ICs: bending pin 4 up from its solder pad to isolate it. But when I did this, the worst possible thing happened: the pin snapped off, along with some of the 8002b's black packaging. I tried globbing solder onto the chip's injury, and though I could occasionally get noisy audio out of it, it was clearly ruined. The lesson (and it was really only an $8 one) was as follows: do not attempt to bend pins of a surface mount device off the board they are soldered to, particularly when they are cheap and Chinese. In general, I try to avoid modifying surface-mount circuits, at least in the places where components are surface-mounted. But these days there are precious few circuits with the 0.1 inch through-hole pitch that I grew up with, so I have to adapt. My problem isn't just that components have gotten smaller; my vision has gradually deteriorated and my fingers are perhaps not as steady as they were when I was fifteen. Today I actually ordered a clip-on loupe glass to help me with such work in the future.
Usually engineering fails are infrequent for me, but I had another one later today when I tried to assemble my black-pipe indoor firewood rack. Our friend Michæl had used several union fittings to join the two halves of his rack together, thereby allowing him to get the parts of it screwed together tightly (otherwise, a pipe turned tighter in one fitting will loosen in the fitting at its other end). I thought I could get away with not using any union fittings at all and instead make the junctions tight by flowing solder into them. I've been able to solder steel (even galvanized steel) in the past. But no matter what I did, I could not get solder to flow into the galvanized threads of a fitting. I used plenty of flux, but perhaps I burned it all away with the amount of propane needed to get the fitting hot enough to solder.
It was Superbowl Sunday, but I would not have known that if Gretchen hadn't brought it up in jest. We made a Mexican meal together and watched a fair amount of television (starting with an old episode of Shark Tank, one of the few shows that I discovered that Gretchen started watching with me — the only other example I can think of is the Wire).
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