Club Mud has seven electric potter's wheels (plus two kick wheels), there for the members' use, but also used by students taking classes sponsored by Maude Kerns Art Center. Recently, two of them stopped working, within days of each other.
Tea tackled the first one, followed by Don and Karen, then Tea again, who finally determined the problem was with a loose connection in the wire bringing power to the motor. Replacing it (and fixing a broken-off ground wire) brought it back to life. They couldn't find anything obvious the matter with the other one, so they dragged it off out of the way with an "Out of Service" sign, and I promised to take a look at it later.
So I came in yesterday with screwdrivers, wrenches, and a multi-tester, determined to, as Matt Damon said in The Martian
, "Science the s#*t out of it."
Here's what I did.
First, I diagrammed the wiring in the control box
, just so I could see where the power went in what order. (And not mess things up when I started moving wires around.) In the box were three cords: power in from the outlet, power to and from the foot pedal, and power out to the motor. There's also a power switch, circuit board, fuse, reversing
switch (this wheel turns both ways), a connector block between the circuit board and reversing switch, and a separate connector block for ground wires. Don't know for sure how everything works, though I suspect the circuit board is a rectifier, converting AC to DC. Then I:Checked for power.
The motor is a variable speed DC motor, so I used the multi-tester to determine that no DC current was actually reaching the motor. So the motor is probably
not the issue. More on that later.Checked the fuse.
Swapped the fuse out with one in another wheel, which still ran. Swapped fuses back.Checked continuity.
I used the continuity tester to make sure cords and switches were actually conducting electricity. Power cord was okay. Reversing switch was okay. Power cable to motor was okay. Took apart the foot pedal (speed control) and checked its cable. Was okay. Then I checked the rheostat, a sliding switch that changes the amount of current to the motor. I got no current through at the top position, nor midway down. I finally got a reading way down at full extension. Not sure whether this is a problem or not.
So it's not a broke wire or switch that I can see. Probably not the motor, but I can determine that for certain with a little bodging.
Our seven wheels are all the same manufacturer, Brent, but wildly different ages and models. Some things they have in common--the same fuse, for instance. Others, like the control box and foot pedal, I'm not so sure. Best I can do is pick one with the same motor and hope the rest is compatible.
I find exactly one wheel with matching motor, and get Jon's help lifting it up onto the table. I disconnect the power leads from its motor, and attach them to the other wheel
. Yeah! It spins; theory confirmed, the motor is fine. To double check, I hook up nonworking controls to working wheel, which doesn't then work.
Now I need to narrow it down to either the circuit board or the pedal
. The pedal connects to the circuit board with two spade connectors; a third goes to the ground block. I mark the connectors for white and black wires on each board (as it happens, they're in different order), then disconnect both pedals. Fortunately, the wires are long enough to reach into the other box without pulling the whole cable free.
Plug in, switch on, press pedal. Hooray! Wheel spins! It's definitely a problem in the original pedal.
At this point, I take a break and call Georgies to see if they have a replacement pedal rheostat in stock, and how much it'd cost. They'd have to order one in, and it looks like we'd have to pay for pedal and all, for around $200.So I stop and think a bit.
There's actually three control switches in the pedal: two rotary switches, blue and red, that control low and high speed limits of the wheel, and the rheostat, which as I mentioned, is a sliding switch, controlled by a forked lever anchored to the axle of the pedal with a set screw. I wonder whether one of them is the problem.
I reconnect the original pedal, turn the rotary switches back and forth to determine their limits--about 210°--then set them right in the middle of their range. Power up and test. No change. I notice that the rheostat controller is set to go from top to middle of its range. You may recall that the continuity tester didn't show any current across it at that range. I loosen the set screw--which already seems slightly loose--and rotate the lever down so it goes from middle (at off) to bottom (at full). Plug in, switch on, press the pedal and it spins!
Flip the reversing switch and it spins the other way.I fixed the wheel!
I replace the bottom of the pedal. Reassemble the control box and attached it to the frame. Put the bottom plate back on the motor and wrestle the wheel back off the table. Last of all, I take off the "Broken" sign and replace it with "Fixed."
What happened to begin with? I have two theories. Either someone tried to adjust the wheel speed range from the rheostat rather than the rotary dials, or a loose set screw allowed the axle to rotate within the the end of the rheostat lever, moving it out of its effective range.