Microwave Turntable Repair

My microwave is getting older and sometimes doesn’t heat food as much as expected. I kept using it after an earlier test of its heating power was inconclusive. A few days ago, a new problem cropped up: after an audible mechanical noise, the turntable stopped turning. This led to uneven heating, and I thought maybe it’s time to get a new microwave. Before I spent money, though, I wanted to take a look at the turntable motor and see if I can apply any lessons learned from my earlier teardown of a similar motor.

This microwave was a Sharp R-309YK and I was pleasantly surprised there was design effort for ease of repair. An access panel is stamped into the bottom of the microwave held by four small tabs of metal.

Using a pair of pliers, I twisted off those four small tabs and removed the panel. We see the turntable motor identified as 49TYZ-A1 by Yuyao Yahua Mechanical & Electrical Co., Ltd. I don’t know how important it is to buy the exact replacement, there are a lot of similar motors in this form factor. The only significant variation I noticed was the shape and length of the output shaft.

Before I buy a new motor, I had nothing to lose by taking a closer look at this one. I applied 110V power and nothing moved. The problem is indeed here rather than somewhere else in the microwave.

Following precedent of my previous teardown, I opened up the faceplate to look for a mechanical obstruction or anything else that would explain why the motor wouldn’t turn. I thought maybe a gear snapped a tooth, but there was nothing of the sort. I removed one gear after another until I was left with only the rotor, which did not live up to its name because it did not rotate under power.

I picked up the rotor for a closer look, and I noticed a crack running across its magnet. Tiny pieces of magnet had chipped off the edge of the crack. After clearing out the tiny chips and dropping the rotor back in, it spun up under power. I guess a lodged chip of magnet was enough to keep the rotor from starting up? But the rotor made a lot of intermittent noise while spinning. The click clack noise sounded like a tiny part catching on a physical obstruction and tapping it. But I had no luck finding the culprit. If it’s another magnet chip, I couldn’t find it. Hypothesis: centripetal force acting on the cracked magnet opened it up to a C shape and pulling a corner far enough out it is barely tapping some other part of the motor. If true, that’s not good because it will quickly produce more magnet chips and stop the motor again.

Fortunately, I had kept the rotor from my previous turntable motor teardown. I had disposed of most of the motor but kept the rotor because I wanted to visualize its magnetic field. Using my calipers, I confirmed that all major dimensions were nearly identical.

It seems to be a drop-in replacement, spinning up without the click clack noise. I reassembled the motor and reinstalled it in the microwave. A quick test confirmed that my turntable is turning again with this salvaged rotor.

All I had to do was reinstall the access panel, which was designed so that I could turn it 180 degrees and insert tabs to fit into precut slots. It just needed an appropriately sized screw that could self-tap into sheet metal. I found one in my stockpile of fasteners, and we are good to go. I didn’t need to buy a new microwave today, I didn’t even need to buy a new turntable motor. I appreciate Sharp engineers for stamping in an access panel to make this project so much easier than it would have been otherwise.

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