I’ve been trying to track down the source of a loud buzz when operating my Leslie 145 in Fast mode. Slow mode sounds fine.
After a good cleaning of both the Leslie 147 amp and the Leslie Combo Preamp pedal, the noise was still there. I replaced the tubes in the Leslie and the noise still persisted.
I switched the AC plugs for the Fast and Slow motors, and found that the noise followed the circuit, and not the motors. After switching the AC plugs, the Leslie was noisy in Slow mode. With that knowledge, I switched the AC plugs back…and the Leslie was noisy in Fast mode again. So far so good. I figured it wasn’t the motors.
The relay and circuit for switching between Fast and Slow modes seemed to work fine. It was just very strange to me that the Speed circuit was affecting my Signal circuit.
I checked the connections on my 6-pin cable. During this, I had the leslie on and making noise in Fast mode, when I jiggled the 6-pin cable on the leslie. The noise went away! This is very strange. I investigate and find that the noise went away when pin 3 or 4 were not connected. This essentially was leaving the Speed circuit open, without having the Leslie Combo Preamp handle keeping the circuit open.
I thought something must be wrong with the Leslie Combo Preamp…at least the part that keeps the circuit open. But the wiring was so simple and everything looked fine. When the Speed circuit was closed, no noise. When it was open, there was noise.
In the open position, a small indicator light on that circuit lights up. In the closed position, it was not on. It was working properly, as far as indicating whether or not the Leslie was in Fast mode.
But I wondered, just out of curiosity, how it could be lighting up /and/ not completing the circuit. Lights, as far as I knew acted as a resistor in the circuit. I found that the light was a pilot light. I did some more research for “pilot light” on the web and found that, in some electronics, if a pilot light is not grounded, it can generate a 120Hz buzz.
A 120Hz buzz. That sounds like what I’m hearing. It is definately not a 60Hz hum! I removed the pilot light and the noise went away! Without the pilot light, the circuit was truly open. I will try grounding the pilot light at some point, but for now I can live without my speed indicator on the Leslie Combo Preamp.
Nice! Now I can cart this thing to the Scribblin’ gig this weekend! Yeeee Hawwwww!