Dremel 395 Type 4 Variable Speed Switch Faulty

[mstoer]

I have an older Dremel (had for over 20 years now).  The variable speed quit and it just runs full out once you slide switch to 5 (maximum).   I've opened it up many times, initially cleaning the contact surfaces of the switch and lately been trying to figure out what parts are in the circuit.   The photo shows the PCB (small ceramic slab, approx 11mmx25mm) and lucky me, they potted one component.  The newer Dremel switch PCBs seem to have a slightly different design/layout and they don't pot the components.   I'm guessing there is a diac (not a diode), a capacitor and under the black blob maybe a triac? Has anyone tried to repair one of these? Any suggestions on how to remove the potting compound?

I do have a new Dremel, but I'd like to fix this on if I can. 

[Paul Moir]

.. and under the black blob maybe a triac?

Almost definitely; they seem to just use a glorified light dimmer speed control.  It's a little hard to tell, but is there three wires going into it?  The top one looks like it might just go beside the blob.
Anyway, since it's dead, why not grind it off with the other dremel to see what's going on.
Interesting other repair:
http://danjovic.blogspot.ca/2013/11/reparando-o-controlador-da-dremel.html
Perhaps doing a direct die improves thermal conductivity to the ceramic.

[mstoer]

Here is a closer view.  I started to scrape away at the epoxy with a scalpel and hit the other half of the metal that looks like some kind of hold-down. It does not look like the tab on a typical SMD component.   The ceramic is only 0.64mm (0.025") thick, so I'm trying to be careful not to break it.  The cap is a 1206 for size comparison.   There are 3 traces leading away from the epoxy in the direction of the cap.  The right most one goes to the cap, the middle one to the diac and the left one to the black "T".

If I don't get any scraping I'll try the other dremel I guess.

[Paul Moir]

Thanks,
To me it makes sense that the "hold-down" (which it may very well also be) is the MT1 contact rather than using a wirebond.  It seems to fit with the typical speed control schematic.