Help! Exercise Bike & Elliptical Magnetic Pedal Resistance Controller Circuit Explained

[vestaviascott]

Clean the potentiometer track (squirt contact cleaner in or use a syringe and get some petroleum jelly in there).

If the system has a servo motor that isn't 'tracking' properly then you either have a bad drive to the motor or a bad feedback to the controller. Since the motor actually moves I'd go for a 'missing' (or intermittent) feedback signal.

It's also possible that the gearing between motor and feedback pot is missing tooth or two - this will also result in 'hunting'.

Hi, thanks for sticking with me on this. I appreciate your feedback. However, I've swapped the display console out with a known good one and the entire system works as expected.

Because of that, I have eliminated the servo motor and potentiometer hardware as a cause of the fault and I'm isolating on the display console to trace the fault. I'm currently focusing on the motor driver IC chip and the feedback circuit. Your suggestion was to trace the feedback circuit from the motor to the microprocessor. That's where I'm at.

I'm trying to determine if the chip alone is the cause of the fault or if there are additional components that may be to blame. My thoughts are just to replace the chip and see what happens. The issue with that is the chip is a WT7901 and I can't find one. However, I have other consoles that use a similar chip BD6210 and I'm considering if I can substitute that chip and I'm looking for a pinout diagram and spec sheet to compare the two chips and see if they may be interchangeable.

 

[vestaviascott]

If the system has a servo motor that isn't 'tracking' properly then you either have a bad drive to the motor or a bad feedback to the controller

I suspect bad feedback somewhere inside the display console. The blue wire is attached to the potentiometer wiper/center pin (the same wire is white in the video). The video indicates that this wire sends voltage back to the console to allow it to determine how far the motor has turned.

That wire does not have continuity, to the IC chip as best I can tell (i'm testing the faulty console here, not the known good console). However, the gold wire next to it does along with the yellow wire and the purple wire. They all have continuity to the IN3 pin of the IC (WP7901):

That chip is U5 in the pic below just to the right of the 12 pin connector:

 

[vestaviascott]

Just to keep this thread moving and since there's not many pics here (lol, joking), here's the exercise bike with the known good console in place. I borrowed the display console from a slightly older model but it has the same functions as the display with the faulty circuit/chip. With this display console, the machine operates 100% functional as expected.

And here is the faulty console on the "test bench":

 

[kellys_eye]

How do we determine the connections AT THE BOARD? There is nothing to show which colour wire goes to which pin on the board connector (since that ribbon cable has wires all the same colour - except for the 'key' marked one i.e. red stripe).

Measure the voltage across pins 2 and 8 of the IC. This will (should) vary as the motor moves, changing polarity as it reverses movement (you can also measure this at the motor itself). The output should be 'zero' when the motor/feedback circuit is in balance.

 

[vestaviascott]

Thanks for the tip on the motor test voltages, here is a pic to better reference the harness wires at the board:

From left to right:
Red, Black (go to the 6v DC motor)
Yellow 1, Blue, Green (go to the 6v DC motor potentiometer - the blue wire is connected to the wiper and should be the feedback voltage to the uP for position sensing)
Yellow 2, Gray (go to the 6v dc motor potentiometer's fine tune adjustment screw)
White, Brown (Go the the hall effect sensor that measures pedal crank rotation for RPMs)
Pink, LightBlue, Purple (go to the DC power adapter socket at the front of the bike)

 

[vestaviascott]

While I have the board in front of me I did a continuity test between the blue wire and the uP chip labeled IC1 on the board. The BLUE wire is connected to the wiper of the potentiometer and according to the video is responsible for feeding back voltage to the uP that the uP uses to access the position of the motor.

I got continuity between the BLUE wiper wire and a single pin on the uP. Its pin 2 if measured from the top left corner of the chip as in the pic below: