Technician Work

Our treadmill gave up a few days ago. I took the motor down to the local motor shop for a check-up, mostly because that was easier than reverse-engineering circuit boards (for which I had no schematic). Alas, the motor checked out fine.

Reverse-engineering a circuit board takes patience and careful inspection. Since I do electrical engineering design work for a living, the function of the boards is clear enough, but the circuit details have to be ferreted out the hard way. At my age, that means a magnifier and an Ohmmeter, drawing a schematic as I go. These old through-hole designs are much easier to deal with than modern surface mount circuits. The original designer placed some handy LEDs in the circuit to give indication of various functions for troubleshooting technicians who would have to make a living doing service calls. That was great, because it drastically limited the scope of what I had to trace out. In this case, the PWM LED would blink, so I could tell that the input speed-control signal was good. But there was no output to the motor control board on the wire that is supposed to set the speed of the motor. I was able to measure the forward voltage of all the transistor junctions, except the final opto-coupler output (4N35 / U3). Careful inspection of the white wire that plugs in to HD5 revealed a skinned place where it had gotten trapped between the top of the housing and a heatsink. This short would have easily taken out the optocoupler output circuit.

I ordered a part. The shipping will be much more than the $0.60 part cost. It is way less than the price of a new treadmill, of course. My wife is very attached to this particular one anyway. I am grateful to God that the problem wasn’t more time consuming to discover. You can see from the schematic and this picture of the circuit board that I didn’t have to get anywhere near drawing out the whole thing.

I hope you have enjoyed this temporary nerd-break in the usual fare on missionary sojourn.

Friday Night Update: The UPS guy pulled in behind be when I drove in from work today to deliver the optocoupler that I ordered from Digi-Key. After replacing it, the motor on the the treadmill started doing its normal spinny thing again. My wife is very happy! I’m not just a nerd. I’m a nerd hero!

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Jon

Awestruck admirer of the Sovereign of the universe, from whom all rights and responsibilities of men derive.

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