Resurrecting a zombie MUM 5 (mum56z40) food mixer

My in-laws food mixer arced when plugged, running the motor even though the main switch was "Off". Some pcb milling and epoxy later, fixed.

CodeMonkey

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My in-laws food mixer was plugged in to mains power (turned off) and then suddenly started working. I'm guessing there was a small voltage spike on the power line which caused the arcing, but i have no evidence. The arc connected a live mains wire on the pcb with a relay, causing the engine to work.

First, see this log update - A zombie food mixer

Then, see this log update after the fix - It lives to fight another day!

You can see the problem and fix in the pictures of this project.

Lastly, as i didn't make a video of the device tear down, here's one my father-in-law found in russian - Repair of food processor BOSCH MUM5 ... replacement gear reducer - (no, we do not speak russian).

Files

2.jpg Visible arcing path, pcb bottom side. JPEG Image - 3.67 MB - 06/07/2018 at 09:58		Preview

3.jpg More arcing damage, top (components) pcb side. JPEG Image - 2.96 MB - 06/07/2018 at 09:57		Preview

4.jpg More arcing damage, top (components) pcb side. JPEG Image - 2.79 MB - 06/07/2018 at 09:57		Preview

5.jpg Dremel and 0.8 mm drill bits. I broke one while drilling, but could still use it broken. JPEG Image - 2.72 MB - 06/07/2018 at 09:56		Preview

6.jpg Milled "air gap", bottom side of pcb. JPEG Image - 3.83 MB - 06/07/2018 at 09:56		Preview

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It lives to fight another day!
CodeMonkey • 06/07/2018 at 09:52 • 0 comments

There, fixed it. Milled out the burnt pcb area with a dremel and a tiny drill bit (0.8 mm). Made sure not to leave any black area on the pcb, basically creating an "air gap". Then filled it with epoxy.
A zombie food mixer
CodeMonkey • 06/07/2018 at 09:50 • 0 comments

This was the state of the mixer before fixing the pcb - a zombie. Not dead and unresponsive, yet seemed to have a life of its own:

When plugged in the engine starts working, even though the main switch is in the "Off" position, you can see the arcing towards the end of the video.