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Nova Comet II Controller Repair or replacment

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Jun 19, 2019
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Houston, TX
Greetings, I really hope someone can help me.

I have an older model Nova Comet II lathe with extended bed. I can no longer get the lathe to run. I suspect that something has gone wrong with the controller. (I have pulled it open, dusted it out, verified that the fuse is still good).
When I called Teknatool International, they confirmed that the signature was that of a failed controller. Because this is an older style, their replacement controller will not connect (my controller is hard wired to the motor). New controllers are module. Teknatool also will not provide any specs on the wiring to allow me to try and use a new controller and just add the connector to the wiring to the motor.
Their solution is to buy a new lathe, which I do not find acceptable.

Does anyone have troubleshooting steps to try and recover the existing controller (find out what part failed and replace), or have a detailed drawing of what the new controller would need to connect to the old motor (cut the wire, add in the appropriate connector).

Thank you for your help!

R. Kent Hodson
Houston, TX
turning pens for the last 3.5 years, and now with a better lathe, bowls and handles for the last year.
 
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From what I've read, the usual suspects of no operation are the potentiometer, the power switch, or the control board. If you can pull the control board, look for obvious burned components and if you can't repair it, find a good technical electronics guy. We have a Makerspace in our city, some of those guys are absolutely brilliant with electronic controls.
 
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Some of the motor repair shops have experience with VFD controllers if they have a technician that is skilled on the electronics side.
A good electronics technician or electrician should be able to use a multimeter to narrow the problem down to the component level.
Are you sure you have power going to the controller? When it comes to electrical troubleshooting you always need to start from the source
and verify each system until you get to the machine. (power panel, breaker, fuses, circuit wiring, receptacle, plug and power cord)
The other possibility is the motor may have failed, you can usually do a basic test on a motor with a multimeter to verify if it is good or bad.
 
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I can verify that I have power at the outlet (light is also on that plug).
since this is all integrated - taking it somewhere (once I can find a place) means taking the entire lathe, or pulling out the motor since the motor and controller are wired together.
thanks for the ideas!
 
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I had no idea what a Nova Comet II was until googling.....$440 at Walmart. Makes you wonder how much to invest in fixing it.

It's not clear whether this machine has a three phase motor and VFD or a DC motor and related controller. Either way a bad motor controller could be replaced with a generic unit in the $150 range. If the motor is the problem forget it, probably not a standard mount type. In my area the few electric motor repair guys still in business start at $100/hr and up. The electricians we use in the family construction business won't do side jobs because of licensing and liability issues.

As was previously suggested, finding a hobbyist type electronics guy would be the way to go.
 
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There is probably a plate on the motor that gives its specs. It is dc if the photos on the internet are correct. Something like the KB Electronics KBMD-240D will probably work as a replacement. You can get one from amazon for $130. If you are comfortable taking it apart, I would guess that the control is just attached to the motor and you would still be able to find the wires actually going to the motor. Check to see if they are getting power to eliminate problems with the motor. Have you checked to see if the on off buttons are working properly?
 
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You could test the motor with a treadmill controller to verify the motor is good or bad. Most treadmill controllers are rated in the 1,2,3 horsepower range which would most likely run the motor on your lathe. If your lathe runs on a DC motor you can source aftermarket controllers for around a $100.00 on line or you could strip one out of a treadmill if you are wanting to save a few dollars. You can usually find free treadmills on Craigslist which usually have a 2 horsepower electric DC motor and a DC controller that runs the motor.
 
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You could test the motor with a treadmill controller to verify the motor is good or bad. Most treadmill controllers are rated in the 1,2,3 horsepower range which would most likely run the motor on your lathe. If your lathe runs on a DC motor you can source aftermarket controllers for around a $100.00 on line or you could strip one out of a treadmill if you are wanting to save a few dollars. You can usually find free treadmills on Craigslist which usually have a 2 horsepower electric DC motor and a DC controller that runs the motor.
The treadmill control panels I have seen are a real rat's nest of wires. Not sure it would be any easier for the inexperienced person to wire that in to a DC motor than trying to troubleshoot a circuit board. Don't some treadmill controllers need a speed pickup on the motor? Never seen an external speed pickup on a lathe motor.
 
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"Never seen an external speed pickup on a lathe motor."

Grizzly has a speed pickup on the spindles of their lathes with VFD's. I got a free VFD because of that. A local guy was having issues with his Grizzly lathe so they replaced the whole VFD setup under warranty without attempting a diagnosis. It turned out the problem was only dust obscuring the speed pickup sensor. The guy gave me the old VFD.

Diagnosing and repairing electronic devices has gotten so expensive it's becoming "throw it away".
 
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I appreciate the various responses. Yes, cost is part of the key - since I can replace the whole thing for about 450 plus tax, then it does not make sense to take to a commercial electrician. I am looking around for a hobbyist electrician, but no luck so far. This has a DC motor - 3/4 hp/5.5 amps/1 phase/class e/ 1700 rpm. I can tell that the switches are working and power is getting to the switches, since the switches only work with the power on. On its last couple of turns I could hear a bit of crackly in the controller area - that does not guarantee the motor is not the problem, but I still lean on the controller being the issue.
I looked more closely at the controller - no sign of something burning out. but, the good news is that there is just 3 wires in (white/black/green), and 3 wires out to the motor (white/black/green), so pretty much any controller should be able to connect.
Guy - thank you for the specific specs on KB Electronics KBMD-240D. That is exactly what I needed to keep digging into this.
 
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"Never seen an external speed pickup on a lathe motor."

Grizzly has a speed pickup on the spindles of their lathes with VFD's. I got a free VFD because of that. A local guy was having issues with his Grizzly lathe so they replaced the whole VFD setup under warranty without attempting a diagnosis. It turned out the problem was only dust obscuring the speed pickup sensor. The guy gave me the old VFD.

Diagnosing and repairing electronic devices has gotten so expensive it's becoming "throw it away".
My fault, I meant to say DC lathe motor. Sorry!
 
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The treadmill control panels I have seen are a real rat's nest of wires. Not sure it would be any easier for the inexperienced person to wire that in to a DC motor than trying to troubleshoot a circuit board. Don't some treadmill controllers need a speed pickup on the motor? Never seen an external speed pickup on a lathe motor.

Many of the treadmills use a very common DC control board as the heart of the system, these controllers are used by many hobbyists to make a variety of variable speed machines. There are a number of videos on YouTube that cover the different components on these controllers and how to bypass a few of them and what types of potentiometers to use when retrofitting the controller. Most DC controllers fail from overheating the control board, heat sinks are the most important component for these types of DC motor controls.
 
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There is probably a plate on the motor that gives its specs. It is dc if the photos on the internet are correct. Something like the KB Electronics KBMD-240D will probably work as a replacement. You can get one from amazon for $130. If you are comfortable taking it apart, I would guess that the control is just attached to the motor and you would still be able to find the wires actually going to the motor. Check to see if they are getting power to eliminate problems with the motor. Have you checked to see if the on off buttons are working properly?
Thanks for the specific part number to go looking for. That really helps.
 
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Thanks for the specific part number to go looking for. That really helps.
Good luck with it. I found the third party seller on amazon, Motor Control Solutions to have good customer service if you decide to go with the kB control. They sell several brands and may have something else that would work better for you.
I just added a remote to a used lathe I bought that came with a kB vfd. It needed a signal isolator board to operate via remote and motor control helped me identify the part I needed.
 

Bill Boehme

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It's not clear whether this machine has a three phase motor and VFD or a DC motor and related controller.

It's a DC motor with a simple thyrister type device to vary the voltage to the motor. You won't find lathes with three phase motors controlled by inverters until you get into the full size lathes, but even some of the low end full size lathes will have DC motors with simple thyrister controlled power supplies. I don't know if anybody suggested checking the brushes yet, but that is the most common problem with DC motors.
 
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I want to thank everyone for the ideas on how to solve this problem. I went with Guy's suggestion of the KB Electronics KBMD-240D controller and a 8A fuse. What I did not realize was that the fwd/rev switch is a separate purchase, so I still need to do that. It took a couple of tries to wire up the motor so that the spin direction was correct, but I have a working lathe again. The only other thing I added when wiring this up was a 6A thermal reset circuit breaker - I mounted it in the side of the KBMD controller.
 
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Kent,

A lot of credit to you for fixing the problem. I probably wouldn't have done that, choosing to look for a new lathe instead. One thing I would recommend for the future, should this fix not work is looking for a different brand midi lather. I own a new model Nova Comet II, only used the lathe less than 2 years before its controller went bad and the lathe was dead. Because I had no choice and the warranty is only 1 years for the controller, I ended up replacing the controller from Teknatool for $150. However, if this one only lasts about 2 years I wouldn't bother a second time. Since Teknatool doesn't seem surprised by the failure, it's likely that it's a very common problem. I am doing research on other midi lathes and there are other comparable lathes out there which have larger motors and outboard turning capability. Thus, in short, while the Comet II is an inexpensive lathe and performs well for beginners, it may be worth the extra money the other brands cost to get a more durable lathe.

Great job fixing it though.
 
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Ive got a feeling the controller on your Comet was under rated for the motors horsepower. Ive seen that on a cheaper industrial machine, controllers were biting the dust regularly until a savvy electrician noticed the board was rated a half horsepower and the motor was one horsepower. Changed to the right one and the problem went away. An under rated board will work but get hot and will not stand up to continuous use. If it does it again look up the specs on the board and see if you can get one rated a step higher.
 
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Gary,

What you say maybe true but I probably won’t bother. If the controller doesn’t last I will seek another lathe with a better warranty. If a company will only warranty something for a year it’s probably likely to fail sometime around then. The lathe was a good starter lathe for a beginner who isn’t sure whether turning is something to get into but I wouldn’t recommend it for an established turner. It’s a little under powered for bigger pieces even though it’s capacity is 12” bowls.
 
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I appreciate all of the other thoughts that are being added here. This will not be my forever lathe - I really like working with bowls and 12" and 3/4 hp is a little light for this overall. Good learning point for me right now. The new controller can handle up to a 1hp motor (mine is only 3/4), and with the 6A thermal cb I put in, I will hopefully keep from pushing this beyond what the controller can handle for years to come. I also have a son working on a smaller lathe from Harbor Freight - mostly making pens and wands, but in time he will need more capability. In the meantime, I plan on learning and saving up for something closer to a 2hp / 16 or 18" capacity.
 
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You would probably want to fix it then sell it for what you can get to help finance your new lathe. Better yet sell it before it decides to fail and get what really fires you up.

Selling it while it's working now is probably the smart thing to do because if it fails again there is no point in fixing it again. I wouldn't get another $150 out of the sale price. I really don't feel I need a full-size lathe at this point in my life. I was considering the Laguna Revo 12/16 because I don't seem myself turning many bowls larger than 12", but an extension to turn outboard is available. I realize that a lathe this size has its limitations but the Laguna's motor is a little larger and the outboard capability for bowls a little larger than 12" seems attractive. I'm still doing research. What I would really like is a Robust Scout which would give me larger capacity and I'm sure is a much better lathe, but at around $4,000, that's a little beyond my needs at my age.
 
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I just bought a 12/16. Got the extension so I don't have to keep removing the tailstock. Been a great lathe so far, definitely beefier than the nova comet I had or king I looked at but also costs more. The only thing I dislike is the push button spindle lock. I much prefer a spindle lock that you can lock in place like the one on my nova saturn. I turn all my ornaments and stuff on the laguna now. The saturn is now for bowls, table legs and someday hopefully hollow forms.
 
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I just bought a 12/16. Got the extension so I don't have to keep removing the tailstock. Been a great lathe so far, definitely beefier than the nova comet I had or king I looked at but also costs more. The only thing I dislike is the push button spindle lock. I much prefer a spindle lock that you can lock in place like the one on my nova saturn. I turn all my ornaments and stuff on the laguna now. The saturn is now for bowls, table legs and someday hopefully hollow forms.

The Laguna seems "beefier" except for the Laguna stand/base. It seems light duty for the lathe; all sheet metal. Did you buy the stand/base also?
 
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Instead of a beefier midi lathe you might consider a short bed bowl lathe like a Nova Saturn or Grizzly G0838. If looking to do hollowing with a captive/articulating tool holder a full length bed is needed.
 
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I do thank everyone for their ideas and advice. The Comet II lathe was sold and I have replaced it with a Laguna Revo 18/36.
Now I get to learn a new set of adjustments with the larger lathe.
 
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The motor is likely a 90 volt DC permanent magnet motor which means there are just 2 wires required to operate it. The two wires are the ones connected to the two brushes, so if you can isolate them just connect them to a 12 volt garden tractor battery and if it runs then the motor is OK. Note reversing a DC motor is nothing more then switching polarity on the 2 wires.
All that said it probably ain't worth it.
 
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