RAS electric brake

[Rick Potter]

I have a DeWalt 7790 12" ElectroStop RAS. Bought it slightly used from a friend. The blade brake has never worked, and after 15-20 years of ownership, I have decided to finally look at it. Is anyone familiar with these motor brakes?

Rick Potter

[Jerome Hanby]

Weird, I just posted something similar. Is your brake electrical or mechanical?

[Rick Potter]

It's electrical, Jerome. I searched, and only found one post about this, and it was for a mechanical brake.

Your post about adding one to your saw was the trigger for me to get the factory one on my saw working.

Rick Potter

[david brum]

There are a group of guys who adore old Dewalt radial saws at http://forums.delphiforums.com/n/main.asp?webtag=woodbutcher&nav=start&prettyurl=%2Fwoodbutcher%2Fstart. They can give experienced answers for most vintage Dewalt RAS questions. It's a lot like OWWM, except just for those saws.

[Jerome Hanby]

If it's purely electric, then the braking action is probably going to be some method of turning the motor into a generator and putting a load across it. There could be all sorts of extra stuff in there to do the braking automatically (like some timing circuit to kick in a second after the AC gets cut), but the meat and potatoes will most likely be some high current rectifier diode and a low ohm high current tolerant load (likely some form of wire wound resistor). Seems like the weak spots would be that rectifier and the load. Either of those being "burned open would result in the brake not doing anything. If you have an ohm meter with a semiconductor testing setting (personally I use an analog Simpson meter), you can test the rectifier, with all power disconnected, by setting to the RX1 scale and putting the leads across the diode, noting the meter movement, then reverse the leads. You should only see movement with one orientation of the leads, nothing in the other. If that looks good, put the meter across the load. Just guessing, but I bet the resistance should be less than 20 ohms. If it's open (meter doesn't move), the load probably has an open in it and needs to be replaced. If all that looks good, then you could "rig" some way to activate the break manually (say with a push button). turn the saw on, let it spin up, then power it off, and push the button. If it brakes, then it's the activating circuitry and I don't have any general advise there, you would just have to look at it.

All of that is based on the theory that it is that kind of braking. If it's some mechanical or electromechanical breaking system (some type of friction braking), disregard all of that.

Assuming this is a fairly old saw, there might be a schematic out there someplace for it. That would be a great start. Even a manual for installing the brake (I saw one for some old Dewalt while I was researching building a brake) might be a good start. A big advantage here is this saw probably predates any consumer product level digital circuitry, so this should all be pretty simple analog electronics. Much easier to work on without a schematic (and much easier to draw your own!).