Bosch MUM54A00 kitchen machine

[roblkc3]

Hello!

I'm actually trying to fix my wife's kitchen machine, a Bosch MUM54A00, which suddenly stopped working (one day she wanted to turn it on, but nothing happened).
I'm pretty confident that I won't be able to reassemble it, since I broke 90% of the plastic clips that hold the cover together. Nonetheless, I would like to understand what's wrong with it anyway --- let's transform the bad luck in a good learning opportunity, since it's the first time I repair something that connects to voltage mains!

The electronic board seems to have (by the look of it) a small power supply that converts down the 240 V to power up the small microcontroller (painted in white, looks like a PIC), and has a triac (the one with the heatsink).
On an separate board there's the control logic for the knob that selects the speed that one wants the machine to operate, and a couple of connectors coming from safety switches (e.g., to ensure that the bowl is properly inserted.
I've short-circuited the two pins of the bowl safety switch (after checking their behavior), and plugged the board to mains. I've then measured the voltage between the red and the blue cables that are usually plugged to the motor (as in the picture) and got 213 V.
I've then plugged the mains directly to the two spade connectors of the motor, and verified that the motor happily turned.
However, once I've plugged the red-blue cables of the electronic board to the motor, nothing worked anymore --- measuring the voltage on them I have 20 mV..... ?
What can explain this behavior?

Thanks a lot in advance!
Best,
Rob

[fzabkar]

Your PCB appears to have a transformerless power supply. These rely on the impedance of a series capacitor (approx. 1uF, 250VAC) to reduce the mains voltage, and this is then regulated by a zener diode.

Check the DC voltage on the zener or the associated electrolytic capacitor. If the zener is not shorted, then test the 1uF (?) capacitor. It may have aged.

[roblkc3]

Thanks for your reply! 
I've checked all the capacitors and none are shorted (though I haven't desoldered them yet to check their ESR). I had already checked them visually to verify that no electrolitic went clearly bad. Also, the diode stays at a 28 V (and the associated cap too).
I haven't done any more checks since working around mains voltage makes me quite nervous (as I said, not being used to.... but still, this fear should give me a healthy dose of respect and thus possibly avoid dumb things...)
Do you think that simply by having a (possibly significantly) larger ESR could lead the board to being that sensitive to having a load attached (basically, going from 213 V to 0 V)?
Thanks again and have a nice day!

Best,
Rob