Repairing the Dremel MultiPro 395 High Speed Electric Drill.

17 December 2019

The Dremel MultiPro 395 finally called it a day when I was drilling hard to make a square hole on an wooden board. This was after 15 years when I first bought it for about SGD$100/= .   It was made in Mexico.   Today, the same tool advertised in TaoBao was for SGD$22/= including shipping. I have a similar one from other Chinese makes that cost only SGD$14/=.

Double click to go to TaoBao website

As it was such sentimental tool that I decided to repair it yesterday.  I made a video just to share for fun. There are many other video available in YouTube but none was able to tell me much about the tool.

TroubleShooting

In the past, a repair by changing the brushes and brush mounting did the trick but this time, it does'nt work. For the first time, I broke open this speed controller. There was unusual findings. The components was working using multi-meter testing method.

I’ve almost gone for an in-circuit testing for the TRIAC using an external 12 volt source. Upon further searching for other faults, I found it was a bad connection at one of the field coil connector.

The Fix

The socket of the field coil was not easy to remove to repair the connection. A direct connection using one strain of wire from a multi-core cable fixed the problem. The wire soldered and connected the field core wiring to the socket as shown in this video. 

More Technical Information about the Dremel MultiPro 395.

1. Like any other tools, Dremel has a field coil and armature. In addition, it has also a simple and durable speed controller. The existing controller has a TRIAC BT134W that has a rating of 600Volts @ 1 Ampere. It has a DIAC like any other motor controllers. It has also a mystery an epoxy sealed component. It appears to be a thyrestor for over voltage protection of the TRIAC. The circuit is as shown

2. AliExpress has a newer speed controller for about SGD$3/= including shipping. This new controller has a ST Z9M TRIAC which works exactly like the BT134W. One can find the datasheet of ST Z9M and BT134W in here and here.

These TRIACs are very cheap.  It cost only a few cents each in AliExpress.

3. The DIAC is a glass like component connected directly to the Gate of the TRIAC

4. One can also hack the controller by short circuiting the TRIAC's terminals MT1 and MT2. This will will connect the field and armature to the supply source. An external speed controller will be required.