Kenwood Chef A901 repair

I was in the market for a kitchen mixer a while ago, so when I saw a kenwood chef A901 in a charity shop, I jumped on the opportunity to own a piece of vintage kit like this.

As I walked down the road carrying it home, a woman stopped me to say that she recognises the mixer because it’s the first mixer she bought when she moved in with her husband in the ’60s! I’m not sure it’s that old so she might have been thinking of a slightly different model (maybe the A701) but it still goes to show how well the design has stood the test of time.

It served me well for a little while, before it suddenly started smoking and sounding weird while I was using it. Without the motivation to debug it at the time, I put it back in my cupboard as a project for another day. I feared it would be a failure in the motor insulation so the mixer might be a write-off.

Anyway, I revisited it recently and found that I was a fool to put it off for so long! The fault was a very common one and I fixed mine with little difficulty. Having put the effort into figuring out this fault, I thought maybe it would be fun to fix up some more. So I grabbed another one listed as “spares and repairs” off ebay, and sure enough, it had the same fault. Here’s a few highlights from my fixing it:

First, the top cover needs to come off. I found on my way in that a few parts were quite dirty/greasy. A little bit of soapy water cleaned these up nicely though.

Once the top comes off you can see the motor and the gearbox.

Next we need to get into the other end of the motor housing to slide the motor out. This turned out to be very frustrating. One of the screws was seized pretty badly. My screwdriver kept camming out and the screw wouldn’t turn. Even with penetrating oil, I couldn’t get it out. I was preparing myself for the possibility that I might have to drill it out but out of desparation, I tried a long shot: heating it with a hot air gun. It turned out this loosened it just enough to get it open!

With the motor out of the mixer, you can see that the problem is exactly what you’d expect: a failed capacitor.

In fact, I even found the other half of the capacitor rattling around in the housing!

The failed capacitor was a 150nF X-rated but there was also a 47nF capacitor which I figured I ought to replace too. I had some 47nF and 100nF X-rated capacitors in my parts box so I paralleled up both to replace the 150nF.

While I had the motor out, I figured I’d also check the brushes since these are the first thing to go on the motor. With about 14mm left on each side, these still seem perfectly adequate and don’t need replacement.

Almost ready to put it back together! Before I put the motor assembly back in the housing though, I decided I didn’t fancy putting that screw back in which I nearly wore the head off of trying to get it out: it might never come out again if the mixer needs maintenance in the future! I had a look around the workshop for any suitable replacement screws but I couldn’t find any with the same thread. It looked like some kind of self-tapper. The original screw was slightly smaller than M4 though, so I tapped out the holes to M4 and fitted it back together with M4 bolts instead.

Here’s the motor assembly back in the case with the new capacitors:

I tested it and it worked! But at the lowest speed it was a bit too slow, and it pulsed a lot. The reason is because of the ingenious (if, by modern standards, quite primitive) motor speed control. The motor axle has a kind of leaf spring arrangement on it, whose edges get flung out by centrifugal force (in their reference frame, before any pedants correct me) as the motor spins. This causes the top of the spring to push up on a switch on the control board, and stop the triac firing. So it spins up to a certain speed, then starts skipping mains cycles. It’s essentially an electrical version of a Watt governor or a hit-and-miss engine. Amazing! The effect is very pronounced at low speeds and with no load, since the motor’s inertia makes the control loop quite underdamped but adding a load makes it less noticeable. The speed is set just by adjusting the position of the control board relative to the rotor. To increase the minimum speed a little, I just loosened the spring loaded screws a little until the minimum speed seemed about right, without excessive pulsing.

Here’s the hinge mechanism put back together with nice socket head M4 machine screws, rather than the nasty phillips head self-tappers it had originally:

With it all put back together, there was one final step. The rubber feet on the bottom were completely worn down. It turns out you can buy replacements online, and the gap between the bottom of the mixer and the work surface may contribute to motor cooling. A corkscrew comes in handy for pulling out the old feet:

And it’s all happy again! That was quite a satisfying saturday project, now to find someone in the market for one of these… And maybe get some more and do the whole thing again!

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