Old Rotating Speaker circuit puzzle

Ivan Funk · From: Hamburg Pennsylvania, USA

I have this old oddball Oliver Orbital Power Projector. (sort of a Leslie upper horn)
I found the overall schematic but the schematic for the motor speed control module is nowhere to be found. The physical circuit board on the machine is buried in epoxy so I can’t see what’s going on.

The “Mode Ft. Sw.” is for switching between slow or fast speeds. (bottom left on schematic below)
The dumb part of the design is it put’s 120v AC through the foot switch.
The schematic isn’t that well laid out but if you stare at it a while it almost makes sense.
I’m thinking of adding a relay for the foot switch so there would be 12v on the foot switch instead of 120v.
If I had a schematic for the module I could see if there’s 12v somewhere I could use for a relay.

My question is: Do you think this is some sort of transistor based PWM speed control?
(I can see it does indeed pulse the motor for slower speeds)
If so can anyone lead me to a schematic (for the module) that might be similar?

Thanks,
-Ivan

George Biner · From: Los Angeles, CA

If you change the wall plug to a polarized one, then the bottom line will always be ground and the top one will always be 120V -- that might be an easy fix for the footswitch. Better yet, a 3-prong grounded plug and then ground the chassis for safety.

The motor control circuit is a puzzler. But PWM would need a low voltage power supply for the control components, which it appears are not there. If it was a DC motor, it could be controlled by an NPN and a resistor, but this motor is AC, with some kind of weird extra winding? Not sure what that symbol means.
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Bill A. Moore · From: Silver City, New Mexico, USA

The transistor is rated for high voltage, there may not be any low voltage availlable. They show no transformer.

Ivan Funk · From: Hamburg Pennsylvania, USA

Thanks George,
The extra symbol is the pilot lamp for "motor on".

The motor is a shaded pole AC motor.

Here's a picture showing the module buried in epoxy and a big transistor on a heat sink.
I did install a 3 prong power cord but there's no chassis to speak of. I could ground it to the motor housing?

The hot goes to the motor switch (middle line) so when the power is off off there's no hot anywhere.
But when the motor power is on and the "mode foot switch" is on fast speed it's putting 120v through the footswitch.
On low speed it breaks that connection and the module takes over and pulses the motor at a slower speed.

I can see a few diodes, capacitors, and smaller transistors half buried in the epoxy of the module. I don't know if it's a VFD or PWM.

It works fine as it is but I had to find a footswitch that could handle the power without burning.
Seems like a sketchy design.
I wish I knew what was going on in that circuit.
I'll test the small wires solder point's and see if there's DC anywhere.

I guess if all else fails I could run a separate 12v wall wart for a relay.

Ivan Funk · From: Hamburg Pennsylvania, USA

Thanks Bill,
I was posting at the same time.
Makes sense - no transformer.

Bill A. Moore · From: Silver City, New Mexico, USA

Back in the 80's I had a "Space Echo" in our PA rack, (chorus was in style then). I used a multi-pin Peavey footswitch to control effects from the stage, using a few Radio Shack 12V relays, powered by a wall wart.

Ivan Funk · From: Hamburg Pennsylvania, USA

Thanks Bill,
I had a Space Echo also. What a sound.

I'm leaning towards the 12v relay / wall wart solution.

Jeff Highland · From: New South Wales, Australia

A few years back. I built a pair of rotary speakers using Leslie drums and motors from scrapped organs. I used large mains lightswitches for the on off and speed footswitches Obviously these were fine with the 240V mains here.
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Ivan Funk · From: Hamburg Pennsylvania, USA

Thanks Jeff,

Update: I installed a small 12v power supply and a relay. Works good. No more 120v to the foot switch.
I still wish I knew what circuit is buried in the epoxy module but I guess it will remain a mystery forever...

Thanks everyone for your input.

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