DW788 - Speed control problem - Scroll Saw Woodworking & Crafts Message Board

markdavd · 09-30-2008, 04:55 PM

I've been reading this board for years, but this is my first post. I've read about about the various issues with the DW788, but can't find anything similar to my problem.

I've had the saw for about 4 years. The other day, the speed started varying without the control being adjusted. It also started randmly jumping to full speed (or higher than full speed) in the middle of a cut. When this happens, if I turn it off and back on again, the speed resets itself to what it was.

I checked the brushes and they look like they have a lot of life left. I opened the case and nothing seems to be loose.

The only thing I can think of is the control board being bad, but before I spend the money to replace it, I want to see if anyone has any other ideas.

Thanks

minowevie · 09-30-2008, 10:55 PM

Hi there Mark, I wont to welcome you to the forum, so glad you after reading us for years, desided to finaly join. really glad for sure.
as far as you quistion. I don't have a Dewalt. but I have had that problem. on my Hegner. I have a v speed, and it is under my saw. and it gets full of dust sometimes. I too was amazed when I couldn't turn the speed down. so Neal Moor told me to check for dust in the nob. and shuzammmmm, that was it exsakly. so now I just blow it out often. and it works just fine. I even cover my saw to keep the dust from sanding, and other woodworking tools dust from getting in the other parts that need oiling. hope this helped some. your friend Evie

marshall · 09-30-2008, 10:58 PM

Mark : Never had this problem with my 788 , but it will be interesting to view this post to see how and what the out come will turn out . Maybe even a dirty switch , maybe blow it out with some compressed air ? I'm sure that someone here will be able to help you , be patience they will come up with something that will help you out !!! Good Luck !!! Marshall

david804 · 09-30-2008, 11:04 PM

Mark
I also have a 788 and haven't experienced the issue that you have but being in the electronics repair buisness for 28 years I would also guess the actual speed knob is where the problem is most likely. You can try blowing it out in case it is just dust or you could try electronic contact cleaner in the control. This will require that you unplug the saw and remove the knob from the top then spray the cleaner in the control probably just spray around where the shaft enters the pot then work the control through its full adjustment a couple of times to clean the wiper and surfaces. now you can reassemble and plug it back in and see if this helped the problem.
David

markdavd · 10-01-2008, 02:59 AM

Thank you for your responses.

I tried cleaning out the knob with compressed air and electronic contact cleaner to no avail.

I next opened the case and blew the control/circuit board out and that didn't help either.

Anyone else have any ideas?

lucky788scroller · 10-01-2008, 03:04 AM

Sorry, I have no idea.I suppose it is possible the circuit board controlling the variable speed could go bad for some reason.I hope you get it squared away. If you adjust it to its fastest setting does it still automatically change irradically?

markdavd · 10-01-2008, 05:18 PM

It doesn't matter what the speed setting I have it set at - it still varies on it's own and sometimes jumps to high speed.

I spoke last night with a retired electrical engineer and he says it sounds like the speed control has a short so it looks like I'll be replacing the control board. It looks like they sell the control board and potentiometer (speed control knob) together, so I'll be replacing both.

I'll let everyone know what happens.

Jim Finn · 10-01-2008, 11:35 PM

I have that same saw and the switch went bad in a month. I had it replaced and I then intalled a foot switch and covered the switch on the saw with saran wrap. Works for me.

steveh · 10-12-2008, 04:52 PM

The trouble with this saw is that unless its all together, you cant test it, if its all together you cannot get at the control board for testing. "has a short" to describe your problem is like describing weather as "having temperature". What you have is an intermittent connection that is probably affected by vibrations of thesaw.Since you cleaned the variable control, and blew out all the sawdust, you may want to look at the soldering connection on the bottom of the board where the three wires from the speed control knob connect. This sounds like you may have an intermittent connection there, that is a poor solder joint. while you have the board out, inspect the soldering with a magnifying glass. If you wiggle the wires on the top of the board, see if there is any movement on the connection to the copper trace. if there is you may haveto resolder it. I haven't had to repair my board yet, but I have looked at it. I have worked on many intermittent electronic problems and circuit boards over the last 40 years. Sometimes the problem can be cleared up by simply disonnecting and reconnecting push onconnectors to re-seat them.
Scrolling is a lot more fun than trouble shooting electronic problems.
Hope this helps

markdavd · 10-13-2008, 01:45 PM

Steve,

Thanks for the suggestions. I replaced the control board and pot (speed adjustment knob) last week and it didn't fix the problem. I guess I'll have to find the time to carry it into a repair center.

In the meantime, I've pulled out my backup saw.

09-30-2008, 04:55 PM	#1
markdavd Member Join Date: Jul 2006 Location: Dallas, Ga Posts: 65	DW788 - Speed control problem I've been reading this board for years, but this is my first post. I've read about about the various issues with the DW788, but can't find anything similar to my problem. I've had the saw for about 4 years. The other day, the speed started varying without the control being adjusted. It also started randmly jumping to full speed (or higher than full speed) in the middle of a cut. When this happens, if I turn it off and back on again, the speed resets itself to what it was. I checked the brushes and they look like they have a lot of life left. I opened the case and nothing seems to be loose. The only thing I can think of is the control board being bad, but before I spend the money to replace it, I want to see if anyone has any other ideas. Thanks

09-30-2008, 10:58 PM	#3
marshall Just love Being Here Join Date: May 2008 Location: Jonesboro AR Posts: 2,113	788 speed control problems ????? Mark : Never had this problem with my 788 , but it will be interesting to view this post to see how and what the out come will turn out . Maybe even a dirty switch , maybe blow it out with some compressed air ? I'm sure that someone here will be able to help you , be patience they will come up with something that will help you out !!! Good Luck !!! Marshall __________________ Usually busier than a cat in a sandbox !!!!!!!!!!! { Dewalt 788 only }

10-01-2008, 03:04 AM	#6
lucky788scroller Senior Member Join Date: Sep 2005 Location: wisconsin Posts: 6,260	Sorry, I have no idea.I suppose it is possible the circuit board controlling the variable speed could go bad for some reason.I hope you get it squared away. If you adjust it to its fastest setting does it still automatically change irradically? __________________ Dale w/ yella saws

10-01-2008, 11:35 PM	#8
Jim Finn Senior Member Join Date: Feb 2008 Location: Lubbock Texas Posts: 120	I have that same saw and the switch went bad in a month. I had it replaced and I then intalled a foot switch and covered the switch on the saw with saran wrap. Works for me. __________________ But that is just my opinion,I could be right

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09-30-2008, 10:55 PM	#2
minowevie Senior Member Join Date: Nov 2005 Location: california Posts: 6,398	Hi there Mark, I wont to welcome you to the forum, so glad you after reading us for years, desided to finaly join. really glad for sure. as far as you quistion. I don't have a Dewalt. but I have had that problem. on my Hegner. I have a v speed, and it is under my saw. and it gets full of dust sometimes. I too was amazed when I couldn't turn the speed down. so Neal Moor told me to check for dust in the nob. and shuzammmmm, that was it exsakly. so now I just blow it out often. and it works just fine. I even cover my saw to keep the dust from sanding, and other woodworking tools dust from getting in the other parts that need oiling. hope this helped some. your friend Evie

09-30-2008, 11:04 PM	#4
david804 Member Join Date: Apr 2008 Location: Crittenden Co., Kentucky Posts: 84	Mark I also have a 788 and haven't experienced the issue that you have but being in the electronics repair buisness for 28 years I would also guess the actual speed knob is where the problem is most likely. You can try blowing it out in case it is just dust or you could try electronic contact cleaner in the control. This will require that you unplug the saw and remove the knob from the top then spray the cleaner in the control probably just spray around where the shaft enters the pot then work the control through its full adjustment a couple of times to clean the wiper and surfaces. now you can reassemble and plug it back in and see if this helped the problem. David

10-01-2008, 02:59 AM	#5
markdavd Member Join Date: Jul 2006 Location: Dallas, Ga Posts: 65	Thank you for your responses. I tried cleaning out the knob with compressed air and electronic contact cleaner to no avail. I next opened the case and blew the control/circuit board out and that didn't help either. Anyone else have any ideas?

10-01-2008, 05:18 PM	#7
markdavd Member Join Date: Jul 2006 Location: Dallas, Ga Posts: 65	It doesn't matter what the speed setting I have it set at - it still varies on it's own and sometimes jumps to high speed. I spoke last night with a retired electrical engineer and he says it sounds like the speed control has a short so it looks like I'll be replacing the control board. It looks like they sell the control board and potentiometer (speed control knob) together, so I'll be replacing both. I'll let everyone know what happens.

10-12-2008, 04:52 PM	#9
steveh Member Join Date: Feb 2008 Location: Qualicum Beach, BC Posts: 30	The trouble with this saw is that unless its all together, you cant test it, if its all together you cannot get at the control board for testing. "has a short" to describe your problem is like describing weather as "having temperature". What you have is an intermittent connection that is probably affected by vibrations of thesaw.Since you cleaned the variable control, and blew out all the sawdust, you may want to look at the soldering connection on the bottom of the board where the three wires from the speed control knob connect. This sounds like you may have an intermittent connection there, that is a poor solder joint. while you have the board out, inspect the soldering with a magnifying glass. If you wiggle the wires on the top of the board, see if there is any movement on the connection to the copper trace. if there is you may haveto resolder it. I haven't had to repair my board yet, but I have looked at it. I have worked on many intermittent electronic problems and circuit boards over the last 40 years. Sometimes the problem can be cleared up by simply disonnecting and reconnecting push onconnectors to re-seat them. Scrolling is a lot more fun than trouble shooting electronic problems. Hope this helps

10-13-2008, 01:45 PM	#10
markdavd Member Join Date: Jul 2006 Location: Dallas, Ga Posts: 65	Steve, Thanks for the suggestions. I replaced the control board and pot (speed adjustment knob) last week and it didn't fix the problem. I guess I'll have to find the time to carry it into a repair center. In the meantime, I've pulled out my backup saw.