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I hope that someone may be able to help me with a problem i have-

My good lady wife dusted down her trusty Kenwood mixer last night to get started on the Christmas cake (apparently it must be done this weekend or doom will ensue) On attempting to use it she found that when switched on the motor would start then stop then slowly start etc etc- generally misbehave.

Here is a short video of the misbehaving motor:

https://youtu.be/7Mmma936F_w

I stripped the beast and checked the brushes- all ok and so took further advise from a "kenwood expert" who says my speed control is likely goosed (a common problem it seems).

He recommended replacing the triac, a couple of 4.7k resistors, a 220uF capacitor and a 150nF resistor.

Although i can't see any obvious signs of distress on the board such as blown / leaking cap's or charred resistors i am not capable of testing the triac with the limited equipment i have so thought it best to go ahead and replace the components suggested by "the Kenwood man"

The control board is actuated by a rotary dial - the spindle passes through the board and the speed control (Potentiometer?) we have a direct mains voltage input, a motor output and a thermistor input (thermistor is attached to the motor to prevent overheat)

Upon turning the dial the motor speed should increase progressively depending on dial position- The machine also has a function whereby you can turn the dial in the opposite direction for a "pulse" of full power- this full power is applied until you release the dial and it returns to 0 (stop) via some sort of spring action in the potentiometer?

image of board needing repair

better quality image

I only have a Maplin in my area and can't seem to find a like for like match on the capacitor which should be a 150nF 275v x2

The only available 150nF capacitor at Maplin is a Wima Polypropylene Metal Foil 1250V 0.15uF Capacitor which is obviously 4.5 x the voltage of the one i want to replace, other than that the closest match i can come up with at Maplin is a 0.1uF 275V X2 RFI Metallised Polypropylene Capacitor.

Can anyone tell me if either of these two capacitors are likely to be acceptable?

I thank everyone in advance for their time, i'm sure you all get fed up with people like me popping up out of nowhere asking for help but it really is appreciated!

Regards,

Rob

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    \$\begingroup\$ You need to make sure that you replace that capacitor with an "X2" rating with another one with an X2 rating, as explained in AC voltage ratings for capacitors, so the Wima cap you mentioned would not be suitable. \$\endgroup\$ – Andrew Morton Nov 4 '16 at 11:45
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    \$\begingroup\$ The Wilma cap is not shown as an X2 rating, however the description states " Wima Polypropylene Metal Foil 1250V 0.15uF Capacitor •Extremely high pulse duty•Self-healing• Internal series connection• Very low dissipation factor• Negative capacitance versus temperature•RoHS compliant (2002/95/EC) \$\endgroup\$ – Simmyoto Nov 4 '16 at 11:52
  • \$\begingroup\$ This capacitor is ideal for high pulse, high frequency applications such as switch mode power supplies, converters in drives and power electronics, deflection systems in monitors and TV sets and electronic ballasts" .. would this self healing property coupled witht he fact it's rated to 1250v in a 240v circuit not cover the X2 specification? \$\endgroup\$ – Simmyoto Nov 4 '16 at 11:52
  • \$\begingroup\$ @Simmyoto -- the X2 spec is much, much harsher with regards to surge withstanding \$\endgroup\$ – ThreePhaseEel Nov 4 '16 at 22:20
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The precise value of an RFI suppression capacitor like that is not terribly critical, so using a 100nF X2 cap instead of a 150nF one should get the circuit up and running for now at least. You can order the 150nF/275V X2 cap from most mail-order suppliers though -- I'd try Farnell, given where you're at.

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IME, a higher-voltage capacitor would be a suitable replacement. Except in some cases of high-capacitance electrolytics, using a higher-voltage capacitor carries no significant risks or disadvantages.

Note that typically special kinds of capacitors are used with circuits connected directly to the power mains. These are "X capacitors" and "Y capacitors". But using a much higher voltage capacitor with a (presumably) double-insulated kitchen appliance is probably safe.

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  • \$\begingroup\$ Many thanks for the Info Richard, the appliance is indeed double insulated, ThreePhaseEel suggests that using the lowed nF cap would likely be k in the short term until a direct replacement is sourced, Thanks to your information along with everyone else's i have ordered both cap's from Maplin and will probably go ahead and use whichever is the closest patch size wize to fit the board, The main point is i now have confidence in using either for the short term to get the circuit running. \$\endgroup\$ – Simmyoto Nov 4 '16 at 12:08
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I suspect the wiper on the big black VR1 pot for speed control is intermittent, which sort of matches your description.

slowly start etc etc- generally misbehave.

Beside it is an internal VR2 trimpot . Perhaps contact cleaner or WD40... on the VR1. Nothing looks like it has overheated.

Reminds me of a Piher brand pot around 1K I once knew around that vintage.

Plastic caps are pretty reliable.

Can you describe the malfunction more clearly?

e.g. ( Hysteresis ? dead spots? late surge start at 30%? then smooth then erratic 75 to 100%?

My Mother usually started Xmas Cake 3 mos before. others got so many they were aged 1 yr +3 mos ;)

If all erratic responses come via the speed control, then a new Pot or WD40 may work. New Piher pots are round. http://www.piher.net/sensors/index.php?page=shop.product_details&flypage=flypage.tpl&product_id=24&category_id=1&option=com_virtuemart&Itemid=31

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  • \$\begingroup\$ [LINK] (youtu.be/7Mmma936F_w) - Here is a video of the shenanigans \$\endgroup\$ – Simmyoto Nov 4 '16 at 12:43
  • \$\begingroup\$ I should mention that the mixer has speed adjustment along with a "pulse" mode whereby turning the dial in the opposite direction it applies full power for as long as you hold the dial in the pulse position- this pulse position is sprung so the pot resets to 0 if released. \$\endgroup\$ – Simmyoto Nov 4 '16 at 12:48
  • \$\begingroup\$ Seems to have a fast thermal error ( slows down in a matter of seconds) probably the smallest semi cct. parts such as ZD1.. got a DMM RLC meter? ZD1 is a low voltage zener used to get DC offline and create a circuit for triggering Triac with phase shift.. It could also be the cap that drives the ZD1 \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Nov 4 '16 at 13:47
  • \$\begingroup\$ or as the Kenwood man says change the triac and parts... if the leakage triac current has risen that would pull down the zener voltage more and perhaps cause erratic triggering \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Nov 4 '16 at 13:52
  • \$\begingroup\$ Yes i have a DMM....what would you like me to test? \$\endgroup\$ – Simmyoto Nov 4 '16 at 14:09

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