I buy and sell used fitness equipment as a hobby. I've learned to repair most issues I find except one, which happens to be the most common issue that goes wrong on ellipticals and exercise bikes, the electronic resistance control.
The electronic resistance control usually consists of a curved bracket lined with magnets that are positioned near a rotating steel flywheel with a spring to allow them to home to the flywheel when no tension is present on the cable.
The magnet assembly is on a hinge and connected via a small cable to a 6 volt servo motor that has the job of moving the magnet bracket to vary the resistance load for the user of the machine. Pulling the magnets closer to the flywheel increases pedal/stepper resistance and moving them away from the flywheel decreases the resistance.
The servo motor consists of a small electric motor that is geared to a circular pulley (to wind the resistance cable to vary the position of the magnets is wired to the upper console) and a B5K linear potentiometer that wipes from 10 oclock (max resistance with cable fully extended to allow the spring assisted magnets to be about 1/8th inch from the flywheel) clockwise to 4 o'clock position (fully wound around the servo wheel pulling the magnets as far away from the flywheel as possible to create the least resistance the machine allows - aka, resistance level 1)
The problem breaks down 50/50 between a worn out resistance motor in the best case (a $50 part) to an issue with the resistance circuit in the worst case scenario. In the later case, I usually have to part out the machine as I have not yet found a solution other than replacing the console (too expensive) or if I'm lucky and the machine has a lower motor control board separate from the console, the replacement of that is about $100. A replacement console runs $200 and up for used ebay stock.
What I'm trying to address is the latter case, when the motor works fine. By that I mean it can be controlled by a 6volt flashlight battery or even a single AA battery (with very slow turning of the servo but with still sufficient torque to move the magnets through the full range of motion). This is the first test I run to determine if the problem is with the motor or not.
Half the time the motor is fine and the problem is with the resistance circuit. In that case, I have to first check the wiring cable that runs from the console (or lower control board - LCB, if one is present) to the motor to insure continuity across the 5 wires that run the servo motor. There are two wires (yellow and blue) that send power to the motor (appx 4.8-5.0 volts) and there are 3 wires that control the B5K potentiometer that sends voltage back to the console or LCB to allow it to know where the relative position of the magnets are so that it can stop sending voltage at the appropriate time to fix the magnets in the correct position.
Of the 3 wires, the red and black wires are for power to the servo POT (appx 2.2 volts) and the white wire simply sends voltage back to the panel/LCB to allow it to know when to stop sending voltage.
I believe that the POT circuit is to blame for most of the non servo related issues. I'm not certain exactly what is involved at this point in the circuit and I have never seen a schematic of this system despite the fact that virtually every consumer elliptical and exercise bike has this exact system in place.
If anyone has any insights into what would be the most common components (capacitors, voltage regulators, ICs, microprocessors, diodes, transistors?) may be at play here that can potentially be replaced to fix this issue, please advise.
Here is an excellent video that illustrates everything I'm discussing above: