Note: This is not about Star Trek, or Google, or Lady Gaga. It’s about fixing stuff.“Dad, the piano’s broken!” I gently removed my daughter from her keyboard, wiggled some wires, turned things off and on, and agreed, “Yup.” The notes sounded right but were
very faint. I changed the speakers, checked the power cube’s voltage, and poked at the output jack; nothing helped. Dang. It needed actual repair.
This was another Craigslist find, an electronic stage piano built a decade ago. We couldn’t afford to have it in the shop for weeks because my daughter needed to practice, and I didn’t want to risk it coming back with a large bill for shoddy work only to break again.
Oh, all right. I’d try to fix it myself.
This piano is old-school sturdy: 40 lbs of steel and MDF (medium density fiberboard; if wood were beef steak, MDF would be a
very hearty meatloaf.) A web search found praise for its reliability but no service manual or schematic. The manufacturer didn’t even list it as one of their own.
I flipped it belly-up onto a sofa. The bottom had two dozen benign screws around the edges and ten wicked-looking ones in the middle (probably holding the keys onto the wood; I’d
not touch them unless things got
really bad). I removed the less-scary screws and the piano fell into top and bottom pieces connected by short bundles of orange wires.
Two circuit boards were bolted to the top shell, one with buttons and lights and one with all the jacks. They could unplug from the orange bundles, but themselves were made inseparable by a stiff wire bridge into a pair of flat green nunchucks.
My diagnostic tools had dwindled to a cheap Radio Shack meter (having sold my oscilloscope to a grad student who drove from NYC with his girlfriend to fetch it.) So with the tool I had, I measured what I could -- DC voltage and resistance. After a few fruitless checks I knew: Without a diagram, I had no plan.
Oh, all right. I had to draw a schematic.
The process is not unlike dissecting a fish to make an anatomical drawing, only less messy and you end up with symbols instead of literal pictures. I removed the circuit boards and sat at the dining room table with a big sheet of paper. I turned the boards over and over, tracing the path of each lead from each component along its track underneath until it connected to another component.
The parts were all comfortingly familiar, and I found that I could still
read them; I remembered how to find part numbers and values and knew exactly what each one
does. I patiently read the physical reality of the dozens of components and translated them to arcane schematic symbols. (Every business -- horses, newspapers, sailing -- has its private language and signs like this; they’re efficient shorthand for insiders, effectively mystify outsiders, and provide a very satisfying accomplishment when an apprentice, who was an outsider, finally becomes an insider.)
Once on paper I could
see what the circuit was supposed to do, as though the designer, half a world and ten years away, were next to me to explaining what he did and why. It was
obvious what parts were used as power supplies, as amplifiers, as muting circuits. It was all well marked and well done.
Now it was obvious what to check. The amplifiers, which should float halfway between their supply voltages, were stuck high, while their shared halfway-voltage reference was stuck low. Even after disconnecting the reference it was
still stuck. As the reference only had three components, one of them
had to be bad! I tested them in happy anticipation. One of them was dead! But instead of being an electrolytic capacitor (which, like
that kid in school, is always your best first suspect) it turned out to be an open-circuited 10k resistor, worth about two cents and abundant in my junk bin. I found one and put it in. And the piano played.
Oh, all right. I guess I’m stuck being a tinker after all.
--WF
