Kenwood Chef A701a Electronic Speed Controller Retrofit

Following on from my previous post, I gained another addition to my mixer family: a kenwood chef A701a.

I got this as spares/repairs and when I opened it up, one thing was obviously wrong: it seemed to have swallowed half a bag of flour:

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Once I got it cleaned up, it turned out the fault was just a wire loose. The solder joint had snapped, I suppose. With this soldered back on, it lived again!

Running it at a low speed, I could smell the residual flour burning off the 450 ohm wirewound resistor. This got pretty hot in low speed operation because at lower speeds it alternates between having this resistor in series with the motor, and shorting it out. From my research previously, this resistor seems to be the most common failure in these mixers. Aside from that, it’s wasting a tonne of power and making the motor hotter than it needs to be, so it worried me a little from a motor reliability perspective too. I realise it had worked fine for 40+ years and the chances are it would continue to be fine, but I thought it would be a fun project to try and retrofit the A901 TRIAC motor controller to this model.

The other reason I wanted to make this modification was that the design required the physical switch on the motor controller board to switch the full motor current. There’s some suppression circuitry which I suspect is partly to reduce the stresses on that switch, but it still gave me the fear – you could even see it sparking as the motor ran! The TRIAC controlled version, on the other hand, only carries the TRIAC gate current through the switch so the stress on the switch is more or less zero.

This snippet from the service manual shows the wiring of the mixer:

I can only assume the chopper circuit is placed in between the field and rotor windings to take advantage of the interference filtering offered by the windings of the motor – since it’s series wound it makes no odds electrically where it’s placed. The same is done on the A901. I haven’t found a service manual for this but I’ve reverse engineered the following schematic:

Since the A701a and the A901 are very similar and the motors seem to be more or less the same, I figured I’d just reproduce the A901 circuit verbatim and see if it works. I found this old solder terminal board on it lying around, and I thought it seemed ideal for this circuit. I already had the components as spares for my A901s. The TRIAC in the original circuit is unknown and has some non-standard marking, so I took a punt on a BT137-800E. In this simple circuit I suspect all triacs with a sufficient voltage and current rating are much of a muchness, and the 800V, 8A rating of this seems like it should be plenty.

I disconnected the motor controller circuit on the mixer and placed this new circuit in instead. It works! Here I’m just using a crocodile lead to simulate the switch actuated by the governor so there’s no actual speed control.

I captured this interesting video too, where you can see how the governor works:

With the basic circuit working, I set about figuring out how to fit it. First, I ripped out the old motor controller components, just leaving the board with the switch:

Next, the TRIAC needed mounting. It didn’t seem to get noticeably hot at all, but since the mixer has plenty of space inside it, I thought I may as well give it some heat sinking just to be on the safe side. I used one of the holes in the board to mount it onto a random bit of aluminium angle I found lying around, with a thermal pad to act as a gap filler:

I found this random bit of metal which conveniently fitted in the pillar which supported the capacitor, and retained an M3 screw:

With the circuit bolted down, annoyingly the TRIAC leads didn’t reach to any of the solder terminals:

This may have been a blessing in disguise though since I’m not sure I’d have been comfortable with the solder joints being exposed to the potential stresses caused by vibration in the mount. Anyway, I wired up the TRIAC with short lengths of wire instead. I’ve also wired in the switch.

And now putting it back together and soldering the various motor/mains input/switch wires:

And it works! Another happy mixer! It gets a lot less hot at low speeds now

I’m pretty happy with this result! If I end up getting another one, I’m tempted to do a bit more of a polished job of it by designing a PCB to go on the old board, and possibly make it easier to upgrade more of them. It seems like A701s appear with broken motor controllers from time to time, and the wirewound resistor is expensive to find a replacement for. Replacing just a PCB could be a good alternative…