Rewiring a Leslie Switch
The Hammond B3 organ plays sound through an external speaker, usually a Leslie rotary cabinet. Far from being a simple keyboard amp, a Leslie speaker consists of a compression driver at the top speaking into a rotating horn, and a bass speaker underneath speaking into a baffle that rotates in the opposite direction, creating a combined Doppler and tremolo effect. The rotating components can spin at a slow (40-50rpm) or fast (300-400rpm) speed, controlled by a half-moon switch mounted onto the organ console.
Modern reproductions of the original instrument usually have an internal Leslie emulation that can be controlled by a half-moon switch via a 6.35mm TRS connection. I acquired one of these a while ago and found that when I connected it into my keyboard, slow and fast speeds worked fine but the stop setting didn't. Rather than buying another one, I decided to see if I could mod it to work properly.
Connections
Upon opening the switch up, I found that it had eight pins:
Leslie switch connections
Several of these are connected together, presumably to make it easier to solder multiple connections, and only 1 and 8 are singular.
Investigating with a multimeter, I tested the connections between each pin when the switch was at different positions. First with the switch at Stop, pins 2-7 were all connected together:
Leslie switch connections - 2-3 -> 4-5 -> 6-7
When the switch was at slow, pins 2-5 and 8 were connected:
Leslie switch connections - 2-3 -> 4-5 -> 8
Finally, when the switch was at fast, pins 1 and 4-7 were connected:
Leslie switch connections - 1 -> 4-5 -> 6-7
Next, I connected an old TRS cable into a Neo Ventilator II and made connections between tip, ring and sleeve in an attempt to figure out which ones gave me which Leslie speed. With the Ventilator set to CU-1 mode, I found:
- Connecting tip to sleeve sets the speed to slow
- Connecting ring to sleeve sets the speed to fast
- When none are connected, the speed will be set to stop
Equipped with this knowledge, I was able to get the soldering iron out and re-wire it as such:
Tip in this case is on pin 8, ring is pin 1 and sleeve is pin 4 or 5.
I finally reassembled it and connected it up, and slow, stop and fast all worked beautifully!
Footnote
Although disassembling things is all very well, be careful with components inside that help the thing roll, rotate or click. You may accidentally remove a cover that you probably shouldn't and end up chasing ball bearings and springs across the floor. Don't ask me how I know this.
External links
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Neo Ventilator II: https://neo-instruments.com/ventilator-2
Information on the Ventilator II. I got this one over the Mini-Vent for the ability to connect a half-moon switch.
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Hammond CU-1 switch: https://hammondorganco.com/cu1-leslie-switch
The Hammond CU-1 Leslie switch, probably the closest thing to a standard in this case.