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Thread: Question about a circuit board

  1. #1
    Senior Member Buffco's Avatar
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    Question about a circuit board

    I have a Rockwell Sonicrafter X2 oscillating tool, as shown:



    It's just about my favorite tool. I've cut nails with it, grout, wood, hardiboard, you name it. I initially bought it to cut the bottom of door casings to install hardwood floors, but I also use it to rough out sheetrock or osb for outlets. I'd rather use it than a jigsaw. I've had it about 3 years. Recently, it started slowing down intermittently, then speeding back up again. It got to the point where it only ran in slow speed unless I squeezed the crap out of the end of the grip.

    So I tore it apart, suspecting the variable speed switch or brushes. (Note: I HATE variable speed potentiometers on tools. Nothing but problems.) Anyway, that wasn't it, the brushes were fine and plenty long. When it got to where it wouldn't even run, I figured I'd tear into it deeper. Can't break what's broken, right?

    Anyway, in the end of the tool, just aft of the rotor, there's a plastic "tray" that holds the circuit board. I cut out all of the rubberized stuff that covered up all of the components... Found all of the usual suspects, diode, some capacitors, a resistor or two, the main transistor, and some little blue piece that I didn't recognize right away. This blue piece fell off the board as I removed the rubber coating and I saw that the three electrodes had snapped. Here's what the piece is:



    It's a potentiometer. The brass piece is a screw that turns an internal pot. Now, the slotted brass stud was covered in a dab of silicone, rendering it immobile. I pried the silicone off and sure enough, you can spin the brass screw. I left it in it's original place and took some ohm readings across the 3 electrodes. After looking up the model number, I found it's a 50k ohm potentiometer. The readings across leg 1 and 2 were roughly 16k ohms, and leg 2 and 3 was 36k ohms. I spun the screw and took another reading. Leg 1-2 was now 18k ohms, leg 2-3 was 34.

    My question is this.... What reason is there to put a potentiometer in a circuit if it's not to be spun, lowering or raising the output? This brand of pot has different set resistances according to which item number you get, from 10 ohms to 2 million ohms.

    https://www.bourns.com/pdfs/3296.pdf

    Anybody here know? Side note I failed to mention, I took a 2 year electronics course about 12 years ago to get hired on as an electrician at the railroad... Why they taught us electronics instead of high voltage, I don't know, but that's what it took to get the job. So anyway, I have enough working knowledge about circuitry to be dangerous. I've already ordered the exact same pot and I'll resolder it in place. For 6 bucks with shipping, even if it doesn't work, I'm not out much.

    Just curious as to why a pot is put in if it's not to be used.
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    Last edited by Buffco; 09-10-2017 at 03:10 AM.

  2. #2
    Senior Member cpj's Avatar
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    Re: Question about a circuit board

    Possibly to balance something out. Maybe X resistance was needed, maybe it was Y. With a pot you make it what you want.
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  3. #3
    Senior Member Buffco's Avatar
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    Re: Question about a circuit board

    I figured that was the case, just never knew pots would be used in that way. If this fix works I'm going to be ecstatic.

  4. #4
    Senior Member Teach's Avatar
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    Re: Question about a circuit board

    It was probably there to fine tune the variable speed output before the tool was buttoned up at the factory. I assume the hand grip has some sort of squeeze switch to control the speed? You might be able to make yourself a hotrod tool with a little judicious adjusting of that pot. Look up "Wheatstone Bridge" for one application of a variable resistance.
    Jerry
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  5. #5
    Senior Member Buffco's Avatar
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    Re: Question about a circuit board

    Quote Originally Posted by Teach View Post
    It was probably there to fine tune the variable speed output before the tool was buttoned up at the factory. I assume the hand grip has some sort of squeeze switch to control the speed? You might be able to make yourself a hotrod tool with a little judicious adjusting of that pot. Look up "Wheatstone Bridge" for one application of a variable resistance.
    Jerry
    No, it has a rotary button that you click from 1-6. I don't mind a squeeze button variable speed as bad, but the tools with a click-switch potentiometer I've owned have usually always been trouble. This picture is the best I could find that shows the speed adjustment, located behind the hand, at the end of the tool:



    I always run it at full speed anyway so I really don't need a variable speed. If I was using it to sand I guess it would be handy, but I'm always cutting with it.
    Last edited by Buffco; 09-10-2017 at 03:40 AM.

  6. #6
    Senior Member Teach's Avatar
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    Re: Question about a circuit board

    I use a solid state variable speed control that the tool plugs into and the speed adjusts with a knob. It works on any brush-type AC electric motor like a jigsaw, angle grinder, etc. It comes in very handy when I'm using my electric die grinder as a tool post grinder on my lathe.
    Jerry
    Hide and wail in terror, Eloi- - - -We Morlocks are on the hunt!
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  7. #7
    Senior Member LMLarsen's Avatar
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    Re: Question about a circuit board

    You cut your nails with that thing? Damn, boy...
    Quote Originally Posted by Shane View Post
    A gun is a tool, no better or no worse than any other tool: an axe, a shovel or anything. A gun is as good or as bad as the man using it. Remember that.
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  8. #8
    Senior Member shush's Avatar
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    Re: Question about a circuit board

    The children's hair?
    Quote Originally Posted by cjp View Post
    ..... Oh dear God, I've admitted to liking something Limey.
    I'll never hear the end of this.
    Quote Originally Posted by Jayhawker View Post
    ...But seriously Shush....

  9. #9
    Senior Member Buffco's Avatar
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    Quote Originally Posted by LMLarsen View Post
    You cut your nails with that thing? Damn, boy...
    I ain't no Nancy boy.

    Sent from my XT1585 using Tapatalk

  10. #10
    Senior Member tennmike's Avatar
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    Re: Question about a circuit board

    Quote Originally Posted by Teach View Post
    I use a solid state variable speed control that the tool plugs into and the speed adjusts with a knob. It works on any brush-type AC electric motor like a jigsaw, angle grinder, etc. It comes in very handy when I'm using my electric die grinder as a tool post grinder on my lathe.
    Jerry
    I've rescued a few variable speed tools with one of those. They're actually pretty simple to build. A tirac, potentiometer, and a few other parts, and a knob for the potentiometer, and a case, some soldering on a circuitboard, and you're in business.

    Edit to add: In Buffy's case, the potentiometer is a max current limiter that sets the max high speed of the tool. Potentiometer is a cheap way to do that instead of using two separate resistors.
    Last edited by tennmike; 09-10-2017 at 05:16 PM.
    "Tyranny is wonderfully ingenious in the art of inventing specious phrases to spread over its nefarious designs." From the book Tyranny Unmasked by John Taylor of Caloline County, VA. 1821

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