标签: Hammond

It was an early autumn afternoon when I first saw her. It was my third year of engineering school, and what looked like the final year of her hell. She was propped up against some discarded trash, carelessly left in a mangled heap. Her innards were strewn about, hanging out a nasty opening in her back. I remember being dumbfounded that she was there at all... surely, someone would have noticed, called in about the mess. It seemed sick for someone to care so little about her... worse yet, I was practically salivating at this point... I had always wanted to play God. That night I gathered a trustworthy crew and an old Chevy, and we abducted her remains in the early hours of the morning. My crew helped me lug her carcass into the basement and promptly left me to my work. I grabbed my tools and carefully analysed her state of despair—tracing back her innards, determining if anything was missing. At one point in time, she was definitely a beautiful organ—a Hammond organ, or so the labels read. Now she was a hideous wreck, nothing but a scar on the face of our throwaway society. Some unskilled fool had clearly tried to service her back into health... her wires were everywhere (except where they should be, of course). Her cabinet speakers were dangling from their connecting wires. Sickening madness. Her 120V service cord had seen better days, her wiring mostly disconnected and hanging limp in her body cavity. I started my work by giving her a new power cord—ripping a replacement from a dead microwave. My beauty-to-be would be part organ and part microwave; this had me captivated. I carefully reconnected the entire wiring harness, remounted her speakers, double checked the connections, and as my heart pounded, I threw the switch. The lights came on. The hum of an open-input amplifier kicked in, gaining volume as the tubes warmed. My heart was racing as I reached down and touched middle-C, but not even a whisper. I meticulously fiddled with the tone controls, the volume pedal, anything I could to get a rise out of my monster. And yet, nothing. An empty shell of her former self. I switched her off, and returned to examining her carcass. Doubt started to set in. I checked the power connections for every sub-system: the power amplifier, the pre-amplifier, the DC power supply itself... nothing. They all seemed to work, throwing off heat and a warm orange glow from their dusty vacuum tubes. She was missing a voice—she just sat there, screaming nothing at the top of her lungs. Tracing the wires told me that the signals themselves came from a large nondescript metal box, the inside of which was absolutely packed with gears, actuators, and other mechanical-looking things. It was every bit as complex as the innards of an animal. All of this intricate metal—the bulk of my lady's weight—was ripe with grease and mechanical wear. She had certainly seen her share of players. I had her plugged in as I carefully looked over her corpse, and that's when I noticed that my girl was trying to speak to me... through all of her wreckage and despair, she had a little motor that was vibrating and throwing off heat, as it tried to start her mechanical being. To think that her little motorized heart had seized—the odds of me finding a compatible motor for my darling were slim to none. Doubt crept closer yet. While hopelessly trying to give the motor a manual start, a large metal can tucked in behind it caught my eye—could that be it? Her plagued little heart was an old AC motor, and all her suffering was at the hands of a dried-out start capacitor? I hurriedly tore a high-voltage capacitor out of that junked microwave, and crammed it in place of the old capacitor. I didn't even take the time to check values at this point—I quickly wired her up. I flicked the switch. The lights came on... my eyes widened. The hum of the amplifier began to rise over the sound of my own pounding heart. And the moment of truth I pushed my palm into her keys and out of her aching body roared the clamorous, atonal wreck of a mashed keyboard—I had done it! I had bent nature to my bidding, I had resurrected the dead... I don't know that I'd ever felt more accomplished. I excitedly poured over my new love, and we made haunting music well into the early hours of the morning. My townhouse neighbour would file many complaints. The lady and I would never care. Months had passed, perhaps a year, and my beloved and I had gotten well acquainted. We made music together in the darkness of my basement lair; I even dressed her up in modifications to hook into some guitar pedals. But, my precious was living on leased time. In fact, the lease to my building was coming to an end, and nobody in their right mind would help move my 300-pound lady up four flights of stairs to the new flat, myself included. She wasn't quite aware of it yet, but our parting was in the works. I put ads out to send her to a loving home, but nobody dared pull my beauty out of the basement I had her holed up in. The solution was clear in my mind. She had always been an entertainer—a final performance seemed appropriate. I sat and played her off into death, while friends and acquaintances destroyed her beautiful body with power tools and large, blunt objects... admittedly a savage death. We crushed my beloved into a heap of rubble, and I piled it into her dumpster grave that morning. Goodbye, my dear. You made for one hell of a party. Troy Denton is a computer engineering graduate from the University of Manitoba. When he's not working or sleeping, he's tinkering with embedded systems (and throwing the occasional workshop). He submitted this article as part of Frankenstein's Fix, a design contest hosted by EE Times (US).  

You may already know the Hammond B-3 organ—it was a fixture in rock bands in the 1960s and 1970s. You may not be as familiar with the Hammond A-100 , which is a console version of this classic organ . My mom has an early 1960s manufactured Hammond A-100 that was not working. She moved into a smaller house with no room for this spinet style organ, so we loaded it into the back of a truck (it's heavy!) and took it to our house. It sat untouched for almost three years, until I had a large chunk of time to take a look inside and diagnose the problem. This chunk of time came along in summer of 2004—the organ was about 40 years old at the time. We also had an old Wurlitzer organ I had repaired previously, and it was full of vacuum tubes, maybe 35 or 40, many for oscillators. When I opened the Hammond I was surprised to find only about a dozen tubes in an internal power amp and a big, long, heavy box suspended inside the organ enclosure. Simply as a matter of principle, I first replaced all the tubes in the amplifier and in the preamp of the built-in mechanical reverb. Afterward, the organ did in fact make tones, only not when the vibrato function was enabled. Before we get too deep into this story, some background on the Hammond "tone wheel" organs is needed. I've seen these organs described as "over-engineered," but my own description is "masterpiece." This machine is one of the most amazing electromechanical consumer products I've ever seen. The basic tones are generated by what's known as a tonewheel that starts with a synchronous electric motor whose rotational speed is set by the frequency of its AC power source, in this case, 60Hz. A shaft extends from the motor on one side, all the way across the width of the organ's internal cavity. A set of tone wheels are mounted on this shaft, which are not fully round, but instead, toothed wheels. As the shaft rotates, the teeth extend to almost touch a magnetic bar placed at the edge of the wheel. This whole motor and shaft assembly is mounted in that big, long, heavy, spring suspension shock isolated metal box. The magnets have a coil around their ends, and as a wheel's teeth pass by a magnet, they modulate the magnetic field, which in turn generates a current in the coil. The frequency of the AC current is set by the rotational speed of the wheel and the number of teeth on it. As an analogy, consider it as a guitar pickup that, instead of a vibrating steel string nearby, you have a rotating toothed wheel. The AC current of one or more of the 48 tone wheels is summed into an amplifier to make the organ sound, depending on which keys are pressed. Each key has a fundamental and eight harmonics. The standard signal path was working, it was the vibrato that was broken. I bought a copy of the service manual from Manual Manor and studied the schematic. There is also a service manual web page that discusses the operation of the vibrato. It describes a series of delays, via a cascade of low pass filters, that feeds into a rotating capacitive pickup. There is a cylindrical chamber driven from the same synchronous motor that drives the tone wheels, only on the other side of the shaft coupler. It connects taps from the delay line, in forward and reverse, to the amplifier's summing point by coupling them through a commutator inside the chamber via rotating air gap capacitors created by meshing parallel plates on a rotor with fixed plates in the chamber. After locating the signal path in and out of the assembly, an oscilloscope showed the signal out was shunted to ground when the vibrato was engaged. In other words, there are a set of static parallel plates through which a set of moving plates pass. The phase delayed signals are coupled into one of several sets of plates that create capacitors. The phase delayed signals are summed back into the primary tone to create a true frequency modulated vibrato (vs. tremolo which is amplitude modulation).   The chamber (upper left), the parallel plates, (lower left except two commutator plates centre right) and the phase delayed signal connectors (lower right). Also you can see the synchronous motor at centre left, which incidentally has a separate starter motor because the synchronous motor doesn't have enough torque to self-start. After a thorough examination of the disassembled contraption and more measurements, I figured out that 40 years of wicked (both wik-ed and wikd) oil had crystallized into a low impedance carbon coating inside the rotation chamber. Out it the garage, a long session with a spray can of Gumout and some rags eliminated the short circuit. Another four or five hours of careful reassembly, including re-stringing cotton thread through some tiny crevices, and IT'S ALIIIIIVVVVVE!! Brian Lowe submitted this article as part of Frankenstein's Fix, a design contest hosted by EE Times (US).