Matador
Well-known member
I have a broken Mesa Boogie Revolver. It's essentially a Leslie rotary speaker cabinet, with a fast/slow switch and an adjustable speed control.
I can't make sense of the schematic as compared to the real thing. It looks like a pretty typical LM339 motor driver, non-inverting, with hysteresis. What I expect: in order to spin the motor, the gate of the MOSFET must be held between 2-5V (the VTH of the IRF510) to provide a ground return for the DC motor controls. The small DC motor attached to the center spinning assembly is used to provide a feedback voltage that is proportional to the rotational speed. When the speed hits a prescribed threshold, the gate of the IRF510 will be pulled low which stops the main motor from accelerating, until it's slowed enough past the hysterisis point that the comparator resets and re-energizes the motor again. The speed control essentially adjusts the pull down resistance, or the 'trip-point' of the comparator, making it stop at different speeds.
1) On the unit I have, there is no working speed control: it continues to accelerate until it shakes violently. Interestingly enough, the gate voltage of the IRF510 is always ground, so I don't know how the motor can be spinning. Pin 5 of the LM339 always reads 0V.
2) The LM339 is open-drain, so I don't know how the MOSFET can ever be turned on in reality, as I always thought a pull-up was needed. I don't see on on the schematic, nor on the board.
3) There is no 'accelerator' pedal jack on the physical unit.
4) There is a 'fast/slow' switch on the front panel that isn't on the schematic. It is a SPDT (not SPST like is shown in the footswitch), and has caps across each set of terminals. As near as I can tell, the wires from it go directly to the motor.
5) I can't figure out what relay 2 is controlling, or what the triac is for.
6) Node 'A' appears to always be pulled to 0V, meaning the relay is always energized.
7) I see how the foot-switch on/off appears to work: node B is pulled low, turning on Q2 and turning off Q3, meaning the pull-down for the 3K3 floats, putting both '-' terminals at 5V which turns off the IRF510 permanently.
If relay 2 is always on, then the trip point is roughly 200mV if the speed set to minimum. If the speed sensor is at 0V (motor stopped), then the + input is at 0V, and the - input is at 100mV, so the output (pin 2) is low. So how can the motor ever turn on?
I can't make sense of the schematic as compared to the real thing. It looks like a pretty typical LM339 motor driver, non-inverting, with hysteresis. What I expect: in order to spin the motor, the gate of the MOSFET must be held between 2-5V (the VTH of the IRF510) to provide a ground return for the DC motor controls. The small DC motor attached to the center spinning assembly is used to provide a feedback voltage that is proportional to the rotational speed. When the speed hits a prescribed threshold, the gate of the IRF510 will be pulled low which stops the main motor from accelerating, until it's slowed enough past the hysterisis point that the comparator resets and re-energizes the motor again. The speed control essentially adjusts the pull down resistance, or the 'trip-point' of the comparator, making it stop at different speeds.
1) On the unit I have, there is no working speed control: it continues to accelerate until it shakes violently. Interestingly enough, the gate voltage of the IRF510 is always ground, so I don't know how the motor can be spinning. Pin 5 of the LM339 always reads 0V.
2) The LM339 is open-drain, so I don't know how the MOSFET can ever be turned on in reality, as I always thought a pull-up was needed. I don't see on on the schematic, nor on the board.
3) There is no 'accelerator' pedal jack on the physical unit.
4) There is a 'fast/slow' switch on the front panel that isn't on the schematic. It is a SPDT (not SPST like is shown in the footswitch), and has caps across each set of terminals. As near as I can tell, the wires from it go directly to the motor.
5) I can't figure out what relay 2 is controlling, or what the triac is for.
6) Node 'A' appears to always be pulled to 0V, meaning the relay is always energized.
7) I see how the foot-switch on/off appears to work: node B is pulled low, turning on Q2 and turning off Q3, meaning the pull-down for the 3K3 floats, putting both '-' terminals at 5V which turns off the IRF510 permanently.
If relay 2 is always on, then the trip point is roughly 200mV if the speed set to minimum. If the speed sensor is at 0V (motor stopped), then the + input is at 0V, and the - input is at 100mV, so the output (pin 2) is low. So how can the motor ever turn on?
