Help needed - Scroll saw variable speed not working

[tlt]

Hi guys,

I've got a Delta SS350 scroll saw that's been quite serviceable for years now. But lately, the variable speed has not been working at all. No matter where I turn the knob, it always runs at full speed. I've looked at the diagram, but am not sure where to begin.

Diagram here:
http://www.ereplacementparts.com/del...505_14004.html

For most tools, running full speed wouldn't be a problem. But with the scroll saw, if I could even just get to run at the low speed all the time that would be fine.

Any help and suggestions are greatly appreciated.
--tlt

[Mr__Bill]

The quick answer, I expect you need this circuit board. However you may be able to repair it. The most likely culprit is the SCR, that black thing on the big heat sink, they tend to fail open and that is full speed. The easiest repair is variable potentiometer, the round thing on the end of the black wire with the white connector on the end where it attaches to the circuit board.

I would first do the sniff test and see if anything smells burnt or looks like it's burnt or overheated, if it fails the sniff test then just order the part. Check the potentiometer next, that can be done with a Multi-meter looking for changing resistance as the pot is turned, unplug it from the board and hold the MM leads to the pins. If it's the pot that is easily replaceable, your local Radio Shack or electronics parts place should have one that can be soldered in. If the problem is on the board ie. the SCR an electronics tech may be able to do a component level repair. This assumes you know someone who will do it free, if you pay for labor it will cost as much as a new circuit board.

You could also bypass the board and use a router speed control.

Perhaps someone here can give you better instructions on how to trouble shoot the board.

Good luck.

Bill
on the Sunny Oregon Coast

I am assuming the diagram you linked to it the correct one for the saw.

[Uncle Cracker]

Agree with the above assessment...

Note also that a little money can be saved if you decide to replace the whole circuit board (part #899174), by going to M&D Mower (m-and-d.com) or toolpartsdirect.com or dewaltservicenet.com. They all want $78.79 for one.

The potentiometer might also available by itself (part #899175) from some sources, although not from Delta or those others listed above....

[LCHIEN]

It soulds like the final driver stage of the electronics has shorted out and is passing full power through without obeying the control signals.

for a normal person w/o access to electronics repair techniques (solderg, PCB repair, oscilloscopes) the easiest thing to do is to simply put an inexpensive electronics speed control like for a router, on it. That'll probably be OK since it'll control the full power level.

A router speed control can be had for around $20-30, repairing the board by replacement ($70) or taking it in for repair (probably $70 plus labor) would cost more than the used value of the scroll saw.

[tlt]

Some great ideas here. I'll take a peek at the circuit board and just make sure nothing is obviously disconnected -- that is the extent of my electronics expertise I have a router speed control lying in a box somewhere, so will give that a shot. I'll post results so that it gets archived in the search for the next guy.

Thanks for the help,
--tlt

[tlt]

Quick update here. I attached the standard duty router speed control from mlcs, as shown in the link:
http://www.mlcswoodworking.com/shops...d_control.html

With the control on, the scroll saw pulses, from slow to high speed, to slow again. Or if I turn the router speed control down, it just hums without moving at all. Did I get the wrong kind of speed control?

[LCHIEN]

Quick update here. I attached the standard duty router speed control from mlcs, as shown in the link:
http://www.mlcswoodworking.com/shops...d_control.html

With the control on, the scroll saw pulses, from slow to high speed, to slow again. Or if I turn the router speed control down, it just hums without moving at all. Did I get the wrong kind of speed control?

well, that is the kind of speed control I would have tried.
I guessed that it would have worked, but I think my guess was wrong...
Sorry...

[BobSch]

Since the internal speed control doesn't work you might try bypassing it.

Pull the black and white wires for the motor and power cord off the circuit board and connect them together, black to black and white to white.

Then try the router speed control.

[tlt]

Thanks for the ideas guys. I've got an EE visiting in a couple of weeks, and will see if I can get him to bring his soldering gun along. I'll post results for the next guy.

In the meantime, this may be an excuse to pick up the Dewalt scroll saw I've been drueling over for a while now...

[BobSch]

...In the meantime, this may be an excuse to pick up the Dewalt scroll saw I've been drueling over for a while now...

Why is the best solution always the most expensive one?