Kenwood Chef A902 A904 AC Motor Speed Control Module




Introduction: Kenwood Chef A902 A904 AC Motor Speed Control Module

I collected some good informations about dimming motor speed using feedback.

This will give you a regulated speed.

Also I added my experiance and all my availble data for free which took about 50 hours of work.

Step 1: Using Magnet to Control the Motor Speed



The last day I received a damaged Kenwood chef mixer A902.

I noticed that this old appliance uses a tricky method to control the rotation speed.

It uses a feedback from the pick-up magnet.

Step 2: Faradays Law of Induction

This is how the magnet makes fulx control.

Step 3: How Kenwood Controls Motor Speed?

As soon as the magnet begins to close-up to the coil, so it induces more magnetic flux by rotation, so the circuit reaches the trigger value in slower speed.

So, when we need to increase the motor speed we pull the control board away from the magnet (which is a ring fixed beside the cooling metal 12 blades fan hub).

Step 4: How to Calibrate the Motor Speed Knob.

After troubleshooting the speed control module. Special care should be carried out when calibrating the speed of the motor.

The aging of the magnet will reduce the generated magnetic field induced by rotation. So, the board should be calibrated to a closer position to the motor. You can detect that in case the motor reached its maximum speed before the main speed control knob reaches the highest position (6).

You can adjust that by putting the knob to the highest position (6) then turning the two calibrating screws clockwise until the speed starts to slow down.

N.B. These two screws which are fixed by two springs, should be balanced. Also no need to disassemble the unit for calibration. Just turn the machine upside down, and look at the adequate place inside the protective perforated black cover. They should be located on the opposite side of the speed control knob.

Step 5: Using Triac - Diac to Make the Dimmer

The Triac will cut the main power sine wave as soon as it reaches the trigger level. (Dimmer)

Now, I read a lot about damaged Triac PH 71719. Any Triac with the same pin assignments and the same rate can work. Like: BT137. (8A, 600V)

This machine consumes no more than 600 Watts at start up. Or in case of using hard pastes.

The written specifications are: 240V 375W. So BT136 (6A) is enough.

Step 6: Why the Triac Could Be Damaged?

The main cause of damaging the Triac is Brushes' issue.

The Carbon brushes wear by time. And they produce conductive black smear on the holding sockets and the motor copper collector which increases the power consumption, spikes and reduces the efficiency. So to prevent such issue it's better to maintain the appliance by a qualified person each 5 years by normal home use.

Step 7: How to Clean and Check the Brushes?

1- Slide the brown cover as illustrated.

2- Push the holding tooth

3- Then release and rotate the cover and remove it as shown

4- Use a screw driver to unscrew the holding screws.

5- Remove the brush from its holding spring. And mark one side of the brush.

6- Clean the brushes and the internal and external sides of the holding sockets. Also clean the copper collector by Alcohol. And leave them to dry.

7- Take care to replace the brush in the same direction in reverse order, when finished cleaning. Because the brush sides wear differently.

8- Tighten the screws and put the brawn cover in reverse order.

Step 8: Other Components Which Should Be Tested or Replaced

Some capacitors could be damaged by aging but please take care not to change them with commercial types as these capacitors are from type X2. Which is long surviving, but the better one is from Y type which is safer in electric insulation if damaged., By the way and as these capacitors are embedded and protected into the machine so X type is enough.

The speed control module also contains a 275 v 593 BC varistor, it will be damaged if the main current reached more than 275 volts. In normal condition, it should give high impedance.

Step 9: Kenwood A902 A904 Schematic Diagram and PCB

I added a top view of the PCB (component side) to help you solving the issue.

Also I added the cable colors the the scematic

There are 4 wires for the motor:

Two are coming from the motor brushes (marked as Orange - Gray)

and two are coming from the field coils (Red - Blue)

the main power (light Blue - Brown)

Step 10: What Is the Green Hidden Button Used For?

If you raised up the main arm of the appliance you can detect a green button.

This is the overheating protection. it will be released in case of overheating.

In such case:

1- Just turn off the system

2- and wait for about 10 minutes

3- Then tightly push this green button.

4- And restart the appliance.

In case of reoccuancy, so you should look for hardware issue such as smears on the brushes, worn brushes. or unsufficiant grease.

Step 11: Maintaining the Bearings - Kenwood A901 Service Manual

Maintaining the bearings:

Grease degrade by time. And you should clean it by

1- Disassembly of mechanical parts

2- Then using grease solvent such as gasoline then

3- Waiting to dry before adding new suitable grease.

N.B. Please, be careful not to contaminate the plastic parts with gasoline

4- Then reassemble by reverse order.

More helpful information can be found inside the attached Kenwood A901 Service Manual

Step 12: Add or Replace Rubber Feet for Your Appliance

Kenwood mixer usually comes with rubber feet.

They will be damaged by time.

I addad an llustrated pictuers to explain how to add non-standard rubber feet.

This will help you adding rubber feet for any other appliance using threaded-Inserts collected from old laptop covers. by the use of heating gun.

I fixed these inserts as shown in the attached pictures using dental self curing Acrylic. which comes as (Liquid - Powder)

Ask a dental lab to teach you how to use this. :)

You can google it, "DIY using self curing acrylic resin"

Prepare the resin mixture then put the screw with the threaded insert in place.

Also you can use two components fast set Epoxy.

Wait to harden

Then unsrew and clean all excess by the aid of knife. then reassemble.


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    6 weeks ago on Step 2

    Amazing work, thank you for all the work you put into this!

    someone can help me here - I need to rebuild the speed controller from
    scratch, and the inductor coil is damaged beyond recognition, so my
    question is:
    Does anyone here have an indication in regards to the coil properties?
    Inductance / number of windings / wire diameter / resistance / dimensions?
    I can't find any info about that online.
    I'd really like to get my Kenwood fixed...


    Reply 6 weeks ago

    Unfortunately, I don't have this appliance anymore. but I still have all my documentations.

    I used and old free program called paint shop pro 5, and one of the taken pictures of the main board.
    A wire of the coil can be noticed in this picture. Also in the same picture there is a TO220 package Triac

    After some surfing in the net I found the dimensions of the Triac.
    Then after knowing the dimensions of my picture I magnified the wire X100 just to be able to measure it.

    I made a multi layered picture and I embedded the entire information I just collected

    By the use of some mathematical equations I revealed that the coil's wire diameter is about 0.16mm

    As the thickness of the total winded copper in this coil is about 4.5 mm (see the picture), it means that there are about 28 layer of wires
    The reel length is about 11 mm, so each layer should contain about 68 turns of wire

    The total is 28 X 68 = 1900 turns

    I appreciate if anyone who has the main coil reel, if he can measure it by millimetre to adjust my given information about this coil specifications.



    Reply 5 weeks ago

    Hi Soubhi, thanks a lot for the comprehsive answer!
    I will start with your measurements, see how I go. When I have found a good result, I will let you know my findings.




    Question 2 months ago on Step 4

    Where can i get oring magnet for kenwood mixer KM250


    5 months ago on Introduction

    Just to say a HUGE thank you from the UK for all the hours you spent putting into this, its amazing and I am so , so grateful.

    I do not think we appreciate members like you that really put hard work not only into fixing something but taking the time to post, edit, photograph etc.

    Its so appreciated and really gives me hope in such uncertain times that people like us and our "world community' are so important, helping each other and sharing information.



    1 year ago

    Really good to see a rundown on keeping these machine going and going and going.
    We have one over 30 years old and it failed. Burnt resistor was the culprit. It was then that I discovered the speed control. It is cleaver that they use the mechanical/magnetic system as against using a standard potentiometer speed control as I am sure a potentiometer would go noisy and fail after a few years.
    I replaced the burnt out resistor with a higher wattage one, new triac and new mains rated capacitor. Works a dram again.


    Reply 7 months ago

    Please, it's better to replace all resistors with the same wattage.
    the motor drain more power if the brushes are producing high sparks.
    To reduce the sparks try to clean the rotor cone, the brushes and the brush holders.

    Also, ludricate the two ends of the rotor by the use of adecuate grease.


    Reply 6 months ago

    Just replacing resistors with the same wattage without regard to need does not make sense. If A10 watt resistor is needed in one place would you also use a 10w resistor used as a transistorr bias?
    If a motor is sparking excessively then that should be cured. Cure the cause not the symptom. Make sure the snubber capacitor is correct. Replace the brushes or spring if it's a separate item.


    Reply 1 year ago

    This machine was made to survive.
    Also it comes with metal framework which is impossible to see, in new designs.


    7 months ago

    I removed the zener diode. it says PH BZX79C 39, I suppose its 39V . I have kenwood major km250 with the same pcb and values of each component as mentioned in the circuit diagram. the main capacitors are originally of evoq rifa - now available under KEMET brand available with MOUSER. i am replacing all components as my motor does not start even after replacing the triac, two main capacitors, 56ohms resistors, checked the windings too, the cutout and switch is fine too, brushes are fine too.

    Only thing the philips diac (db3/db4/db6) values are not available on the removed component., neither are they available with any one, can anyone help?


    Reply 7 months ago

    The Diac value could be TH-32. Find the attached image below.
    The Zener diode is 39 Volts

    To solve this issue follow these steps;

    1- Unplug the AC power cord.
    2- Push the main switch to ON position and check the connectivity using OhmMeter on Beep position. It should Buzz (low Ohm).
    3- Check the TH switch, this is an overload protection switch. Puch its green knob, and check if it Buzz using the OhmMeter, so it's OK. But if not, so try to shorten its pins, and try to restart the appliance by plugging the AC power.
    4- If everything is OK then desolder the Triac.
    5- Connect the MT1 and MT2 place. (the other pins which are not connected with the DIAC via R3.
    6- Try to plug the AC power and to start the motor by turning the main swtich to the ON position. if it worked so your problem is in the dimmer circuit section.
    7- In case if the motor failed to start, so try to exchange the motor with a lamp. and repeat the last step, if the lamp worked so your motor itself is damaged.

    Screenshot_20201113-110928_Adobe Acrobat.jpg

    Reply 7 months ago

    Thanks a ton will check and update.


    Question 7 months ago on Step 11

    Hi! I also have a electrolytic capacitor in place of C5 , philips 63v, but value is not clear. pl help?


    Answer 7 months ago

    In my board it's 2.2uF /100V
    N.B uF = Micro Farad


    Reply 7 months ago

    Thanks , I desoldered its 4.7mF 63V 85C 030 Philips , manufactured by OEM VIshay, still manufactured now under 030/031AS series.for
    Kenwood major (KM250)


    8 months ago

    Great run-down. I'm at the start of trying to fix a blown A902 but Capacitors C6 and C7 are blown away (only wires left). Your diagram says they are RY1n 250V but do you know the 𝜇F ratings of these caps please? It would be most appreciated. Thanks
    I plan to refresh other caps, trial and resistor too. And upgrade the motor. Should be good.


    Reply 8 months ago

    Thank you,

    RY1n is 1nF = 1 nanofarad
    Please be sure to clean-up the copper collector and brushes before replacing these two capacitors. They are used to reduce the sparks which are generated by powering up the motor.
    N.B if the sparks remain too high, this could be happen in case of a blown rotor.