Some months ago I received a free Hammond organ, which has turned out to be as free as a free cat. Oh, I'm not complaining. Let me tell you. As with anything free, there was no assurance of perfection, and in this case, the old Hammond had a really loud hum, as in "Something is broke". I was sure it could be fixed.
There is precious little information available on old Hammond organs other than the classic Hammond B-2 series church organs. The home models like the 9822M just are not the sort of thing folks restore. I scrounged around and found a parts source, but they have one little rule; you have to buy a technical manual from them before they will sell you something. I plunked down, what $60? I think it was something like that. I got back a stack of papers, including some interesting fold out diagrams. The documentation was excellent, actually.
My "free" organ now in pieces, the serious diagnosis began. The footpedal alone turned out to be pretty interesting. One would expect a pedal connected to a potentiometer, as one might find any typical guitar pedal. But no! This pedal uses a light bulb and a light sensor, with a little tinted film that goes from light to dark, as you move the pedal up and down, that film slides between the light bulb and the sensor. Well, this thing wasn't working worth a thing, and the big loud hum seemed to live here. Turns out after some serious poking around, the hum had nothing to do with the pedal, but the pedal was truly broke. The pedal's little light bulb is powered by the power supply within the organ, and that power supply had a burnt out fuse internally. One fuse later, and one new 80-cent bulb later, and the pedal is good to go. Still, though, huge huge hum. Argh.
This here is Voicing & Preamp Board 124-000441. This guy is where the evil hum lived. Well, it lives there still, but not nearly as much. Okay, it doesn't look like much to you, but let me tell you what; half the parts you see on that circuit board have been changed out. I tested just about every resistor, and changed quite a few of them that were out of tolerance. I changed out every capacitor on that board (aka "recapping an amp"). I even changed a transistor. This was over a few days. Hum hum hum! Argh. Finally, I changed the last five capacitors on the board, they looked like little soda cans. I think those did the trick. Most of the hum is gone, but not all. What you don't see is the many many other circuit boards, bejeweled with hundreds of capacitors, ICs, resistors, diodes, and other parts I am sick of messin' with. The pre-amp is usually where trouble brews, and in my case, I think that was the case.
Well, I don't have an audio or video of the results, but it's pretty much buzz-free. With all those remaining parts on the board, my patience is shot. The organ is certainly better off than it was, and is playable. Not at all recording-quality stuff, but hey, it works. That's enough for now, so folks, that's the story. Get a free organ, pry a load of parts out of it, buy parts, read schematics, and well, end up with something I think I'll keep for a while.
