Dewalt How do I take apart dw708 guide/slide rail

Dewalt DW708 Manuals

Check the polarity of you power source.

switch only the wires thats fitting now on the carbon brushes on each other, and its done .

Curious if you have tested the switch to make sure it is the problem? Also, have you checked the armature & brushes (make sure the brushes move freely in the brush tubes and squarely contact the armature)?
The DW708 is prone to bearing issues and the field is not uncommon either.

It would be unlikely to have a new switch fail that soon unless there is another issue causing it.

Have you checked the manual. My Dewalt Dws780 was not turning on, then I checked the manual and able to start it. There was some sort of plug issue with my saw but thankfully it sorted out quickly. I got this saw from this video recommendation.

http://www.ereplacementparts.com/dewalt-dw708-type-crosscut-miter-saw-parts-c-1009_1163_2604.html

I assume you are asking about the Dewalt
DW708 TYPE 3 Crosscut Miter Sawas seen here
http://www.ereplacementparts.com/dewalt-dw708-type-crosscut-miter-saw-parts-c-1009_1163_2604.html

Have you taken it apart yet? You will need screwdrivers, spring clip pliers, small bearing puller and a pipe same diameter as the inside of the bearing; for replacement of bearing.
Be sure to take out the brushes before taking out the armature.
Part 165-90
You most likely will have to replace part 87 too.
The bearing rests in there and gets worn too.
Serious bearing/vibration problems causes the end cap part 84, to wear out.

There should be a lock down pin near the pivot that needs to be dis engaged. 

here is a link to the owners manual:
http://www.dewaltservicenet.com/documents/English/Instruction%20Manual/386762-02,DW708.pdf

you need to remove the guide fence with the screws on the back. then you remove the throat plate and there is the bolt is that keeps it all together.

Just took one apart last week, not really difficult

i use a framing square off of the back fence with the blade down. when it is square, i check the other side of the fence and loosen the screws on the plate with the degree increments and set it to zero