Lathe Variable Speed not Working

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Forum topic by lab7654 posted 06-04-2014 12:40 AM 1445 views 0 times favorited 10 replies Add to Favorites Watch
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266 posts in 2245 days

06-04-2014 12:40 AM

Topic tags/keywords: lathe question electric variable speed

I acquired a Grizzly G8691 12×40 wood lathe a few weeks ago and only made it through one project before it started acting up. The variable speed is electronically controlled with a knob. It’s now stuck on full 3000 RPM, regardless of where the knob is positioned. I’ve done a little research and the only thing I’ve found is that the “MOSFET”s have gone out. I want to know if this is really the problem and if so how I can replace them. Thanks in advance.

-- Tristin King -- When in doubt, sand it.

10 replies so far

View TheDane's profile


5423 posts in 3661 days

#1 posted 06-04-2014 12:52 AM

A MOSFET is a transistor … unless you are experienced at troubleshooting solid state circuits and have the right tools, I would suggest a call to Grizzly tech support.


-- Gerry -- "I don't plan to ever really grow up ... I'm just going to learn how to act in public!"

View MrUnix's profile


6710 posts in 2197 days

#2 posted 06-04-2014 01:09 AM

Find out the transistor number, look up it’s specs and get a replacement from Radio Shack.. As long as it can handle the current/voltage, it should work. Funny story.. I was fixing the ice dispenser in our refirg, and accidentally shorted a wire that sparked and smoked. Turned out that it fried the mosfet transister on the control board along with about a half inch of the foil trace. Looked up the specs and then went out and dug through my box-o-electrical junk in the garage. Found a suitable transister but instead of being rated for 1 amp, it was rated for 10 and about 5 times the size of the one I blew. Rest of the specs were basically the same. Soldered the thing in and it’s still working just fine to this day. Spoiler alert: I was an EE major in college and have tinkered with electronics all my life, so it was a fairly trivial job for me. If you aren’t comfortable doing component level replacement, then I’m sure you can probably get a replacement control board from Grizzly. But it might not be cheap. It would have cost me over $150 to replace the control board on my refrig, all because of a $2 transistor.


-- Brad in FL - In Dog I trust... everything else is questionable

View lab7654's profile


266 posts in 2245 days

#3 posted 06-04-2014 02:25 AM

Well the lathe isn’t made any more so I wasn’t sure if replacing the board was even feasible. I got the lathe for free, and it’s useless for doing much more than sanding or finishing pre-turned projects, so I’d be willing to try a replacement

-- Tristin King -- When in doubt, sand it.

View Slemi's profile


116 posts in 1539 days

#4 posted 06-04-2014 03:50 PM

That “MOSFET” is actually a triac (SCS) or in some cases thyristor (SCR). You can find replacement in most stores with electronic components. If this one still has markings on it, write them down or even better solder the part out and take it with You to the store. They will surely have some replacement for it. Or if You prefer there is for online ordering.

View Shawn Masterson's profile

Shawn Masterson

1325 posts in 1946 days

#5 posted 06-04-2014 11:38 PM

I think you have a candidate for a treadmill motor swap.

View MrRon's profile


4769 posts in 3241 days

#6 posted 06-04-2014 11:44 PM

Replace the MOSFET and see if that fixes the problem. You have nothing to lose. I replaced one on a Century welder and that fixed the problem. Cost <10 bucks.

View TheFridge's profile


9460 posts in 1484 days

#7 posted 06-04-2014 11:45 PM

It’s not hard to swap out. Transistors typically have 3 leads coming off the circuit track straight up into the transistor or bolted/ screwed in to the board. Usually has an aluminum heat sink it’s mounted on. Fairly easy to solder and unsolder. There are special types of glue used to glue to the heat sink back on(pretty sure). All in all it’s pretty cheap and not that hard to do if.

-- Shooting down the walls of heartache. Bang bang. I am. The warrior.

View Pezking7p's profile


3217 posts in 1649 days

#8 posted 06-04-2014 11:58 PM

This may sound dumb, but are you sure the selector switch works? A switch is more likely to fail than a solid state device.

-- -Dan

View Slemi's profile


116 posts in 1539 days

#9 posted 06-05-2014 06:22 AM

One thing You have to be careful about if You plan to repair. If the triac is fixed to the casing of the lathe for cooling, then it is isolated!!!!!!! You need to save the plastic washer with which the screw is isolated from casing and there is also something behind the triac to isolate from casing. If You loose them You must buy new to replace, they are not expensive, but it MUST BE THERE!!!. And You will also need the white paste which is between triac and the casing, this is for beter thermal conductivity.

Don’t let all this discourage You, it’s not big thing to replace all this. Just a thought, You don’t drive car in first gear if the transmission don’t work. Tools deserve to be repaired!

View lab7654's profile


266 posts in 2245 days

#10 posted 06-05-2014 11:36 AM

The lathe is up north at the cabin, so I’ll have to look at it this weekend and see what I find. I’ll get some specs on the transistors. Hopefully I can fix the lathe, I was just starting to like turning…

-- Tristin King -- When in doubt, sand it.

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