[sdiy] Tonewheel relics

[Rutger Vlek]

Dear all,

I have a decision to make on what to do with a 1970's tone wheel organ. I bought it cheap a few years ago. It's a Pari Attack, Italian Hammond clone. When in good shape, it sounds very close to the renowned Hammond Porta-B. It's completely valve-based, includes good percussion options and a built-in spring reverb. However, it has one major flaw. The tone wheel generator is based on 12 spinning drums (with all octaves per note on a single drum), and those drums are driven by gears from a main synchronous AC motor.

The original gears are made of an elastomer that reacts with the oil and gets crumbly/dry over the years. This was the reason I could buy it cheap, the tone wheel generator wasn't running because of it. I have gone through great lengths to create replacements for the gears. Drew them in 3D, contacted various suppliers (and had mostly negatives responses on "make"-ability of the gears), and finally had them made in china in a rapid proto house. The gears are in place now, and the tone wheel generator is running, but I don't feel it will last. The tolerances of the replacement gears aren't high enough, the don't run smooth enough, and the don't grab on to the shaft as well as the originals.

That's why I'm considering two options:
1) Selling the organ as it is
2) Designing a new drive system for each individual shaft (12 in total) based on 12 modern small-scale motors. Without the friction of the gears, very little force is needed to keep the shaft spinning. The crucial bit is to get to a stable and correct RPM for each shaft (the original gears have a ratio that make this happen, such that 12 identical drums result in a chromatically scaled tone-wheel generator output).

Can those of you experienced with modern motor control applications enlighten me on how feasible / costly the 12-individual motor solution would be? Ideally, I envision something like a single STM32 or AVR controller driving 12 motors, and providing master-tuning control, as well as individual motor speed adjustments. And depending on the RPM accuracy of modern motors a feedback loop that regulates their speed within n% accuracy to assure sufficient tuning between notes.

Any input is highly valued!

Regards,

Rutger