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.

Latest addition to my collection on the right

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:

It was everywhere

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!

Broken wire – the neutral wire from the motor

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:

The input chokes don’t exist in my model. Also nor do any of the supression capacitors, except the 0.1uF across the resistor (value not labelled but it’s 450 ohm)

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:

The switch in series with the gate is the one controlled by the centrifugal governor. It gets pushed open when the motor hits its target speed.

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…

16 replies on “Kenwood Chef A701a Electronic Speed Controller Retrofit”

Hello, thank you for the excellent information regarding the alternative TRIAC circuit that you came up with.
Does the Triac circuit improve the max speed ( i.e to make it faster ) , there doesn’t seem to be any adjustment for high speed ?
It seems that it could use a little more rpm’s especially when whisking egg whites for example, on later models they seem to be faster.

Hi Charles,
Glad you found the post interesting! The triac doesn’t affect the max speed – that’s still limited by the mains voltage. With the mixer turned the max speed, the triac’s basically always on. I don’t think there’s any easy way to increase the speed unfortunately.

Hello, thanks for posting the circuit diagram. I had a cap & resistor blow out on my Australian-built A703C a few days ago, and it was useful to be able to verify the values. As it turned out mine had a pair of 56R resistors, instead of 56R/220R.

I was a bit surprised to see the mechanical governor fitted in my machine (according to Wikipedia the A703C was a full-electronic governor), so maybe it had a replacement motor at some time?

All is well and I’m operational again now.


Interesting! I wonder if that’s from a botched repair before? Like you say, it could be a bit of a frankenstein’s monster made of parts from various mixers. Anyway, glad you fixed it and that I could help. Thanks for sharing!

With regard to “Frankensteinians”, your A701A could prove confusing to any future owner who tries to repair it “by the book”, too! 🙂


This is a good idea, and an interesting read. I notice your new board only has one capacitor, whereas the A901 circuit has two. Is there a reason for this?

Thanks, glad you found it interesting! My board does actually have two capacitors on but one’s on the other side. I hadn’t realised you can’t see it in any of the pictures.

That would explain it! Have you had much use out of the modified machine? A901s tend to be a bit “lumpy” at low speed, I wondered how this compares. The A901E introduced a more elaborate speed control circuit (also used on KM200 series), which uses a magnet on the bottom of the cooling fan to induce a voltage in a pickup coil on the speed module as a form of feedback. It’s a lot smoother than the triac circuit and enables the low speed to be turned down to ~45 rpm. There are some A701A motors with this setup in existence, but they’re extremely rare!

I’ve used it a little bit! But yes, it still has the same “lumpiness” problem because of the mechanical governor. I’ve also tried the A901E and you’re right, it’s much smoother! I’d never heard of that being done with the A701A – that’s interesting to know! I wonder if it’s some kind of hacky retrofit like I’ve done here, or a short lived model?

By the way, I designed a PCB for this little circuit: Kenwood Chef A701a Electronic Speed Controller Retrofit – Part 2

Alex, I wondered if you ever put together a commercially available kit including PCB for this Retrofit kit?

Where can I buy the potentiometer for the speed control?
Or what is the resistors electrical resistance?
Best regards

Thanks for posting the upgrade solution for the A701A. I have previously repaired an A901 which has an electronic control circuit and had my original 701A languishing in the cellar. After finding your article I have just completed removing the original wire wound resistor and capacitor circuit and fitting the triac circuit – fortunately I saw the comment about the capacitor connected across the mains supply which was hidden in your picture – although given in the circuit diagram. it fits neatly behind the tag board.
I would suggest that anyone repairing an old A701A machine should disassemble the speed control circuit breaker and inspect the contact breaker points. I only did this after finding the machine only worked intermittently after I had connected the triac circuit. The contacts were heavily corroded and pitted and needed polishing up with some fine emery cloth – the circuit now works and the mixer is working.
Hopefully the low gate current to switch the triac will mean less wear on the circuit breaker. I notice that this is not listed as a separate component so might be hard to replace without cannibalising an old machine.
I have found that the planet gear turns faster than the 60 rpm suggested at minimum speed. My other observation is that there is quite a lot of sparking from the (newly replaced) brushes on the commutator in the motor as it turns – not sure whether cleaning the commutator would be a worthwhile task.
I hope to get a lot more mileage from this machine which I reckon is 49 years old. Several much younger appliances have bitten the dust in much less time.

I have an A-701 and looking for how to solve the sparkle, I came across your project.
I’m putting it into practice right now!
Thank you!

I have a Kenwood KM260 and the motor is quite lumpy at low speeds, I have tried to find a motor upgrade kit (caps and resistors) but can only find the whole PCB for 60 quid!
Does anyone know the values/reference of an upgrade for the caps and resistors?

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