TLDR: Diagnostic method guidance sought for magnet + coil triggered triac speed controller circuit. Clueless Mechanical Engineer needs coaching.

A friend gave me an old food mixer to fix. She said it smokes and smells.

I am a mechanical engineer by profession and pride myself that I can fix anything .... eventually ... but this one is stumping me so I thought I'd ask for help from the forum. Also, I'd like to learn more about this type of stuff.

There is a movable controller board, with two pins, whose proximity to a rotating magnet on a brushed ac motor varies when you turn the speed dial. The whole board tilt so the pins get closer to the rorating magnet. I think this triggers a diac-triac which chops the sine wave.

This sounds crude .... but my 2019 Kitchen Aid uses a Watt Governor, like a steam engine, so this is high tech in Mixerland!!

When I plugged it in it worked apart from a graunching from the gearbox and the aforementioned smell.

I fixed the 'box and pulled the board. There were some dead looking capacitors so I replaced all the big caps. I also replaced the triac and a high power resistor which looked toasted.

I reassembled it and it no longer smells but I've lost speed control.

ARGH!

I pulled one leg of each cap and resistor and double checked they are right capacitance/resistance. All OK and same as numbers / codes on old parts.

I checked all continuities. All OK.

I could not test the new triac but I assume it is OK.

I think I need to put a scope on it and figure out what is going on but I have no clue really - I'm a mechanical engineer.

I tried a scope across the thru hole winding which I think is supposed to sense the flux from the pins pointing at the rotating magnet but all I got is a mess. The winding is 39 ohms so that seems sensible and the pins appear to connect to a core thru the winding as a kind of flux supplying core. The two pins are electrically continuous, so I assume there is no break in the flux path. So why is the coil signal a mess? Do I need to disconnect one end of coil to make sure the problem is not from elsewhere? No idea.

*** I'm looking for general rules on how you trouble shoot a circuit like this. I'm pretty clueless and I'd be very grateful ***

Do I start at the triac and work back? Or start at the coil and work forwards?

I'm guessing the coil waveform should be at rotational frequency and weak ... weaker as it moves away from the magnet.

There is a little transistor and a bunch of diodes which I guess are there to "perk up" and "square up" the coil signal before it reaches the diac-triac.

I'm guessing that the triac output should be a sort of chopped up ac mains wave.

But I am unsure of this and have no clue what the intermediate wave forms should be, or even whether the 'scope is the right diagnostic tool.

Thanks for any advice.

I have a circuit diagram from another model which I think is correct for this one, too.

 

Welcome to AAC!

In a perfect world, we begin by looking up the make and model of the machine.

The next step would be to locate the circuit schematics. Failing at that, the hard work of reverse engineering the circuit board begins.

 

I have the circuit. I found it on the internet.

It is a Kenwood A901P but I think the board is the 902 type. 901P does not normally have PCB.

I replaced C1 C2 C4 R1 & TRIAC.

My problem is, not knowing the best way to trace it back.

I now think the coil is OK. I think the scope is on the blink. I put an AC meter on the coil and the indicated voltage varies nicely with rotation of the dial from about 1.5V to 0.4V, which sounds plausible enough.

What to do next? What is the general procedure for trouble shooting. If this was a car engine, even a modern one, I'd have a sort of play book in my head; but I never worked on a triac speed controller before.

 

When I plugged it in it worked apart from a graunching from the gearbox and the aforementioned smell.

A short-circuited part of the motor winding could account for uneven running (hence the sound) and also the smell.
Are the brushes and commutator in good condition?

 

A short-circuited part of the motor winding could account for uneven running (hence the sound) and also the smell.
Are the brushes and commutator in good condition?

Yes. I put new brushes in it, cleaned the holders and inspected the commutator. All good. It does not smell any more.

I got the scope semi fixed and can see that the triac gate signal does not vary as you vary the coil strength by moving the board, so something is wrong in between.

I am going to rebleep the board for continuity, recheck all the components and replace any that I have in my box of bits, to be on the safe side.

In a slight plot twist, I've noticed there are some reverse-engineering errors in the circus diag I found on line. The circuit is like mine but one or two of the resistor values don't match the colour bands in his photo or on my machine. The colours in his photo match mine and mine measure what I think they should so I think it is just cut/paste errors on the diagram.

He has done the universe a favour by uploading it so I'm not grumbling.

 

My first suspect would have been the triac.

Since you have already replaced this, then the methodical approach would be to trace the signal that triggers the triac using the oscilloscope.

The problem and danger here is that you are working at LINE voltages without isolation.
What oscilloscope and probe are you using?
How are you connecting the ground clip on the oscilloscope?

Do not proceed with testing using the oscilloscope until we clarify how the oscilloscope is grounded.

 

When testing the gate signal I have the scope grounded on the triac heat sink.

Probes are clipped on before power up from wall switch. Probes are insulated.

What is the correct procedure? Other than "be careful" obvs.

The scope is a simple non name chinese hand-held, which sits on the bench.

I have now replaced and confirm tested all resistors, capacitors, triac.

There is a varying AC voltage from the coil depending on board position re magnet. So that is hopeful.

Once I confirm correct test procedure, I shall trace signal from triac.

There is still a small transistor to worry about (coil amplifier, I guess) a diac and a bunch of diodes ...

Thanks.

 

If your test instrument has a grounded 3-pin power plug then the instrument (e.g. oscilloscope) is already grounded. Care must be taken as to where you can connect the grounding clip of the oscilloscope probe.

If your oscilloscope is a portable battery-operated device with no connection to AC mains supply, then the oscilloscope is said to be "floating" or isolated from AC mains. In this case, it would be safe to connect the probes as you would when using a handheld test meter, provided that you do not exceed the maximum measuring voltages of the test instrument.

 

That is the case. Floating and as a precaution unattached to me.

I can forsee instances when the earth side of the BNC connector could still be "unfriendly" so I'm just leaving it on the bench.

 

When I twiddle the speed control, the coil output varies but the diac and triac voltage signals and duty cycles do not alter.