Dremel 395 VSC

[alfa88]

My Dremel finally went bad after a sputtering. I can find a replacement switch/control cheap enough but I wanted to make sure it wasn't something else 1st. so I did some research. Hopefully this will help others. Btw the easiest way I found to bypass the board is to pull the control board out and short the 2 contacts. My values are for a 115V Dremel. The value I measured for a capacitor seems low from what I've read and there is a mystery part so if someone can enlighten me that would be good.

 

[(*steve*)]

I can't make out the mystery part.

Are you saying that the 30nF capacitor measured low, or that you're surprised it is so low? Or did you measure it to get it's value?

Can you post a raw picture of the board?

 

[alfa88]

Raw Pic

I took another look at the pic and was a blob. It turns out that on porcelain looks can be deceiving. I ohmed it out and found the area to be open. A repair could be made but it would be temporary at best. At least I made a nice schematic.

 

[Resqueline]

I'm not quite sure I follow your last post (or fully understand the physical layout drawing in the first), but one side of the mystery part/ blob is connected to MT2, not MT1.

 

[alfa88]

Clarification

Oops. Fixed the drawing. When I first took a look at the board (with an eye loupe) the trace appeared simply scorched. After examining the picture I noticed melted metal (blob). I then Ohmed out the trace and it was indeed open.

 

[Resqueline]

Ah, yes, the track on the board is definitely vaporised. It migh be fixed temporarily with silver paint but I don't expect it would last for very long before wearing through.
The mystery part is probably a bidirectional zener used for transient protection. High voltage spikes would trigger the triac, thus protecting it from overvoltage breakdown.

 

[alfa88]

Mystery Part

With nothing to lose I managed to pop the coating off the mystery part. It just looked like the resistive stuff that comprised the variable resistor and the 30k resistor hidden underneath the triac. It measured 5Megohms in both directions.

 

[Resqueline]

Hmm, strange, I don't see the use of an ordinary resistor there.. Maybe it's made of a VDR/ varistor material then? Is it laser trimmed? No tiny chip hiding in the coating?