Repairing a fitnessquest edge 491 exercise bike computer

I’ve had a FitnessQuest Edge 491 Recumbent Bike sitting in pieces in my home gym for over 2 years. That’s the bad part about being motivated to fix things, you tend to keep some broken stuff that maybe one day you’ll figure out =) This particular bike worked fine for a few years and out of the blue the computer started messing up.. basically you would get 5 or 7 minutes into a workout and it would seem to end the workout and reset the machine. Usually it’s almost as expensive to have the manufacturer send you a new computer circuit board as buying a new bike. I figured I’d keep the bike around cause A) I could still set resistance on it and then pull the batteries after resistance was set and B) I might be able to hook it up to a bike fitness program using the CSAFE protocol.

I spent probably 20 or more hours a few years ago looking at the bike computer‘s circuit board, taking pictures of the front and back and tracing out some of the components. There was no schematic for the board, so I was trying to identify the inputs and outputs — cpu chip, speed circuitry, power supply, lcd pinouts. I thought maybe the "speed" sensor wasn’t working right and the bike was turning off because it wasn’t sensing any movement. My first thoughts were the cable from the speed sensor switch on the wheel.

The bike has a magnetic switch on its wheel. It’s pretty cool.. every revolution the magnet on the wheel passes by a stationary magnet on the bike’s frame and it makes a connection. I checked that this switch was working properly by using the continuity test on the multimeter. All seemed fine. I also checked the wire harness & connection to the bike’s computer circuit board. Again, all seemed good.

While trying to figure out if the issue was related to the speed input to the CPU chip, somehow I hosed things further. Now when you rode the bike, it wouldn’t register any speed. This of course affected the distance, calories, etc too — since that is all based off of your speed & resistance. So I looked at the board for a few more days and finally gave up and tabled the project.

Last week someone gave me a free recumbent bike. The bike’s computer wasn’t working and they said the display never came on at all. When we loaded it to take it home, the computer made a few beeps. I played with the batteries some but couldn’t get it to beep any more. After taking it home and exposing the circuit board / battery holder I determined the batteries weren’t making a good connection. There was no voltage at the battery tab, but the batteries themselves had about 5.5v.

Now I started thinking more about the broken FitnessQuest bike. I knew that the computer had 2 voltages going to it — +5v and +3v. It does this via 3 wires to the battery compartment, red wire (power for 5v) at the 1st battery, ground at the 4th battery, and a yellow wire (power for 3v) at the 3rd battery (essentially a wire between half of the batteries so instead of 6v you get 3v). I hooked some alligator clips to the battery compartment tabs and determined that the CPU chip and other functionality was running off of the 3v. The motor that changed the resistance needs the 5v however.

During these little experiments I also noticed the CPU wasn’t acting flaky and shutting off after a few minutes. A-ha!!! So that was the problem after-all! The battery tabs over time had been bent enough from putting batteries in and out that they were no longer making a good connection. I bent the battery tabs out with good hope it would fix the issue.

Then I saw it! A lifted solder pad near the CPU blob on the circuit board, possibly from a component I had unsoldered at one time. The trace from the solder pad ran toward another solder pad around the transistors involved in measuring speed. I checked continuity between both solder pads and…. BINGO! No connection. I then tried testing continuity just before the lifted solder pad and continuity was there. So, I soldered a jumper wire between both solder pads.

Often the problem is simpler than it may seem. In the case of the FitnessQuest bike, the battery tabs for the +3v were not making a proper connection sometimes. When riding the bike, as the bike shook slightly or you bumped the handle bar, it would cause the voltage to be lost for a split second, resetting the computer which also down-cycled the magnetic resistance on the bike. To the user (me) it appeared that the program had ended prematurely and the computer was "on the fritz". But it was all due to the power supply (batteries & battery tabs in this case). It’s easy for most people to look over these things and just suspect the entire computer died cause everything "looks okay" and it used to work & it now doesn’t. So I have a fully functional exercise bike back!

The bike stopped reading speed again. I checked my wire jumper I had added for the lifted solder pad and connections seemed fine. At the bench I kept probing different points with the meter (black lead on ground, red lead probing for DC voltage). Somewhere along the line I realized probing the crystal resulted in a beep from the computer. On a hunch I finally decided to replace the 32.768KHz "watch crystal" which looked like it was involved in the timing of the speed calculations. I bought a huge number of these on eBay for a cheap price.. after replacing the crystal the bike started reading speed again & has been working great all of 2012! I guess after all the mis-handling of the circuit board as I was diagnosing it, I shortened the lift of the crystal. It took a long time to get everything worked out with this bike but I saved it from the landfill and really is a good feeling to finally have it fixed!