Potentiometer Help

I have a craftsman lathe ( model 351.217170). It uses a potentiometer with integral SPST switch for speed control. Today, I turned the pot off to inspect my sanding and then it would not turn back on. The safety feature on this lathe requires that the potentiometer be off before the main switch is applied. If it is not off, the lathe won't start. Once you have it running, you can leave the main power switch on and rotate the potentiometer to off to stop the lathe. Then rotate back on to start back up. The whole time the main power switch must be on.

My symptoms are no motor noise at all. Fan runs with main power switch on. Potentiometer SPST switch is reading 17K ohms when off and then goes to 0 ohms when on. The pot resistor that controls the speed varies as expected from near 0 ohms to 10K ohms at full speed. The 17K seems low for an open circuit and may not allow the safety circuits to operate correctly. All measurements are made connected to the circuit board. So circuit board could be influencing resistance measurements. Not so easy to disconnect from circuit board.

Anyone have experience with the pot on this lathe? Do most think it's the pot? If not then it's likely the control board and may be screwed as parts for this lathe are not available.

I have found 10K pots with SPST switches for cheap enough. I guess I could just try it. Any help would be appreciated.

Tough problem. I have a 351.21717.
It really doesn't sound like the pot to me. Especially when you're getting good ohm readings like that.
I had a couple control boards go bad years ago and was lucky enough to find replacements.
I'll look at my pot in the next week or so and see what exact reading you be getting

Doesn't sound like the pot switch to me, but if you want to be sure, just temporarily disconnect one wire and remeasure the switch. You'd have to disconnect it to replace it anyway....

I suspect that you can buy a new (NOS) pot from Sears. What is unique is the switch along with the pot.

Your readings seem pretty normal. The 17K ohm may just be what is on the other side of the pot. That is, the controller circuitry.

So, perhaps you invest $15 or 20 in a NOS pot. If it doesn't fix it, then perhaps your controller has crapped out.

I disconnected the pot from the controller board and it measures what I would expect: infinity when off and 0 ohms when on. Looks like it's the controller board, which means it's likely dead for good. So far, no luck finding a controller board.

If it uses a DC motor, speed controllers are readily available although they are not likely to fit exactly like the original. If it is a 3 phase motor, variable frequency drives are also readily available (usually for a lot less than OEM price).

Tom,

Something that you might try as there is no down-side and a slight possibility of uncovering the problem is to

1. Check and exercise all of the push on connectors and reseat them.
2. Get a magnifying hood or glass and carefully examine the circuit board looking for
a. Components that look like they are suspect (smoke, burn marks, bulging and leaking capacitor, etc.)
b. Stare at the circuit board traces with a bright light looking for "cold solder joints" (soldered components where the solder doesn't
look shiny and smooth and also cracked circuit traces (sometimes circuit trace may crack after a few thousand heat/cool cycles).
3. Go through the steps to turn the lathe on and with your voltmeter, see if there is any voltage going to the DC motor. If you are lucky enough to uncover this, you may have bad brushes in the motor.

I glanced at the on-line manual and noticed that this is a brushed 90 volt motor - - like a treadmill motor. You might be able to jury rig a treadmill controller. Sometimes treadmills are given away for free on Craig's list and the controller still works.

But before you examine the circuit board, make sure that the lathe is unplugged. Use caution when measuring live voltages. If you don't feel comfortable with any of this, then don't do it.

Quote:

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Originally Posted by Brice Rogers

b. Stare at the circuit board traces with a bright light looking for "cold solder joints" (soldered components where the solder doesn't
look shiny and smooth and also cracked circuit traces (sometimes circuit trace may crack after a few thousand heat/cool cycles).

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Good debugging list!

I can't tell you how many circuit boards I've fixed with cold solder joints. I use a light and a magnifier. Sometimes I'll just go over the board with a soldering iron and touch and remelt each joint that doesn't look perfect. I once fixed air conditioners on two of my vehicles in the same day with this technique.

Cracked traces can be difficult or impossible to see. Since they may pull apart enough to quit when cooling a heat gun to warm the board can sometimes find them. You have to work the board while powered up, though.

Also, freeze spray is my friend on a board that works until it warms up. Occasionally a faulty component will quit when it gets hot. You can sometimes pinpoint the problem child by freezing areas until the board starts working then narrow it down to one component. I fixed one of my expensive MIDI synthesizers that way. Of course, this is another debugging technique where I have the board powered while testing. Don't do it without knowing what not to touch.

As well as reseating the connectors, reseat any chips in sockets. And check any cables that might have broken internally or at the connector, from flexing or even vibration. Flex them back and forth while powered up and see if something starts working. (Again, be careful or die.) Suspect stranded wire that is soldered to something - the strands can be stressed at the point to where the solder has wicked.

Oscilloscopes are cheap now - anyone doing a lot of this might find one valuable to trace circuit boards, used like a glorified multimeter with the advantage of tracing signals as well as voltages.

JKJ

John Jordan, you added a bunch of great ideas.

I saw on the GGMG that the original poster ordered a G0766. That's a nice step up from the Craftsman lathe. When I was googling it, it seems like the control circuit board was a common failure component.

Yep the controller board looks to be dead. I'm going to give it to a fellow club member that has the same lathe so he can use it for parts... as long as his controller board holds up. I could not find a drop in replacement and I have no interest in reverse engineering it to work with a different controller board. I never had much money in it to begin with ($75). All that Craftsman lathe did was get me hooked. Because now that I've spent $100s on turning tools and accessories I have to have a lathe to use them on. I was hoping the Craftsman would last me a few more years until I retired. I had planned to then get either a Powermatic 3520C or a Robust. Well I can't wait that long. So I had to settle for a little less fancy and bought the Grizzly G0766. Still a lot of lathe and I've had great luck with Grizzly. Great bang for the buck, even if not the fanciest machines. Lathe is to be delivered on Friday. Looking forward to it, as I know it is quite a step up from the Craftsman.

G0766 lathe came today. Nicely packed. I don't have it together yet, as I'm waiting on my son to come over to help me stand it up. I did take the crate apart, pulled the legs out, took the tail stock out, and inventoried all the smaller parts. I also cleaned up the cosmoline. Glad they only used it sparingly. Only took a few minutes to clean it off. Quite different from my planer. That took me about 2 hours to get all the goo off of it.

Thus far, all things are looking great.

Still waiting for my Craftsman to take its last breath. I've had it for nearly 20 years. My next lathe will be homemade