Powermatic Potentiometer Value

[CPeter James]

Does anyone know the value of the potentiometer used on the Powermatic 3520 and 3520B lathes? I have to replace one and can't find the value of it anywhere.
Thank you
Peter

[Sam Beagle]

You should call powermatic directly. I had a VFD issue for my lathe too and they easily helped me

[John K Jordan]

Does anyone know the value of the potentiometer used on the Powermatic 3520 and 3520B lathes? I have to replace one and can't find the value of it anywhere.
Thank you
Peter

I'm out of town and can measure the resistance when I get home if you still need it. However, if I remember the circuit diagram correctly as having the low voltage (10 volts I think) applied across the two outer legs of the pot so the wiper supplies a relative voltage to the speed circuit, then the exact value of the potentiometer shouldn't matter much as long as the resistance is not so low that it draws too much current and pulls down the suppy voltage. (if actually wired as I remember the wiper will supply the correct voltage based on position, regardless) If you have old linear (not audio) pots around you might just try one, 5 or 10k or somesuch. A pot like that won't hurt anything.

You might also check Doc Green's 3520 notes on his website to see if the gives the factory resistance value. Or if not, send him an email and ask.

JKJ

[CPeter James]

I found it on Doc Green's web site. It is 5k ohm just as the Automation Direct units that I use. We are going to build a remote control pad with both a big red kill button, and the remote speed control. The remote speed control will be either on the panel or on the remote pad and the option controlled by a double pole double throw switch.
The fwd/rev will stay on the main control panel.

Thank you for the tip.
Peter

[Brice Rogers]

Peter,
When you get the new 5K pot, I would appreciate if you could post the markings that are on the pot. I would like to know if the pot is a wire-would or if it is something else like ceramic, etc. I own a G0766 and have gone through a couple of cheap pots. I would like to know the manufacturer that Powermatic uses. Hopefully the pot is marked with more than just Powermatic's own part number.

[John K Jordan]

.... We are going to build a remote control pad with both a big red kill button, and the remote speed control...

That's a good idea. I have a remote emergency kill switch on my 3520b but the speed control would make it more useful.

BRICE, I'm curious as to what failed on your 766 pots. Did you take them apart, was it a cracked resistance film, burn throush, poor connection to a terminal, dust buildup? Is it wired into the circuit similar to what Doc Green shows for the PM?

JKJ

[Brice Rogers]

The pots on the G0766 lathe are 10K wirewound. Working from memory, the wire was quite fine. I measured it optically at just a few thousandths.

The first one that I took apart has a mechanical interference that kept the wiper from touching correctly with the wire-wound resistance element. When I did a post-mortem, I also noticed some debris inside, suggesting that their manufacturing process was lacking both in cleanliness and in mechanics. On the second one (a 10K pot) I found that one of the wires had broken - - perhaps from wear. I think that there are others who have reported symptoms that are consistent with a broken wire.

It appears that the Griz 0766 is wired the same way as Doc Green shows - - a reference voltage on one end of the pot, the other end grounded, and the wiper in the middle doing voltage division. So, as you may have mentioned earlier, the value of the pot probably isn't overly critical as it is a voltage divider. The pot is obviously a linear pot based on half scale equals half rpm (I suspect that most wire-wound pots are linear as it is easier to make them).

I'm wondering if dropping to a lower resistance value (perhaps 5K or 1K) might help some G0766 owners. My theory is that the wires might be thicker and wear better. Also, there are some pots that claim a much higher life. They are either polymer film or cermet (a ceramic). That is why I asked the original poster to provide info on the markings on his new pot. I don't hear a lot about Powermatic pots failing.

[Bill Boehme]

The value isn't critical, but 10,000 Ω (10K ohms) is a good value and is one of the most commonly available values. Additionally, make certain to get one that has a linear taper and not an audio taper. There are several different materials ... carbon film, wirewound, and conductive plastic. Be sure to get the conductive plastic type. Check the shaft type ... It's probably a quarter inch fluted type. Also get the same type of mounting ... probably threaded bushing.

Wirewound pots are mainly meant to be used in a set-and-forget or power dissipation application. Carbon film pots are good, but when used in an application where they are constantly being adjusted they will become noisy which can mean erratic changes in motor speed. This is the ideal application for conductive plastic. There isn't any power dissipation issue and the rated life is many million cycles.

[John K Jordan]

Wirewound would be my last choice for this application.

With 10k across 10 volts the current is small - if substituting a much lower resistance such as 1k it might be best to compare the voltage with a 10k resister in place to make sure the 1k didn't pull it down.

I wondered if fine dust might get inside and be a problem. I've disassembled and cleaned several lathe switches that failed due to dust, one from my 1st Jet 1642. I cleaned, lubed, and sealed the switch completely against dust. I wonder if that would help the Grizzly pot problem.

JKJ

The pots on the G0766 lathe are 10K wirewound. Working from memory, the wire was quite fine. I measured it optically at just a few thousandths.

The first one that I took apart has a mechanical interference that kept the wiper from touching correctly with the wire-wound resistance element. When I did a post-mortem, I also noticed some debris inside, suggesting that their manufacturing process was lacking both in cleanliness and in mechanics. On the second one (a 10K pot) I found that one of the wires had broken - - perhaps from wear. I think that there are others who have reported symptoms that are consistent with a broken wire.

It appears that the Griz 0766 is wired the same way as Doc Green shows - - a reference voltage on one end of the pot, the other end grounded, and the wiper in the middle doing voltage division. So, as you may have mentioned earlier, the value of the pot probably isn't overly critical as it is a voltage divider. The pot is obviously a linear pot based on half scale equals half rpm (I suspect that most wire-wound pots are linear as it is easier to make them).

I'm wondering if dropping to a lower resistance value (perhaps 5K or 1K) might help some G0766 owners. My theory is that the wires might be thicker and wear better. Also, there are some pots that claim a much higher life. They are either polymer film or cermet (a ceramic). That is why I asked the original poster to provide info on the markings on his new pot. I don't hear a lot about Powermatic pots failing.