Harbor Freight 7x10 lathe motor control issue

TheCase

TheCase

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Hi all - thanks for having me here at the forums!

I am trying to help my father figure out what is wrong with his machine.

The symptom: the motor runs faster than usual. You can adjust the speed dial, and it slows down for a moment and then quickly ramps up to full speed again.

I need help identifying the parts on the board (Google has been fruitless). It looks like the rather large resistor is burned out (or is it a fuse?). Anything else I should be looking at? Feels like a possible capacitor issue?

Thank you in advance for any advice that any one might be able to offer!

Here is an image of the motor controller board:

IMG_9277.jpeg
 
looks like that resistor is toast. if you look under the resistor do they have on the board what it's value is?
 
R1 - I actually took a closeup and forgot to post it! Anyone know the value/wattage?

IMG_9278.jpeg
 
I'm thinking it's probably a current sense resistor and would be a low value like 0.02 to 0.08 ohms at about 10 watts or more
Do you know what horsepower the motor is?
Usually those resistors are in series with the motor line so if it opened up the motor would stop. So it's likely still good (amazingly)

I see there are some small trimmer pots above, you might want try adjusting those and see if you can stabilize the speed behavior.
Put a sharpie mark on them first so you can go back to where they were. In particular, you want to look for one marked "IR comp" which
when adjusted correctly keeps the speed constant with varying motor loads.

Worst case you could replace the whole speed control with a KB electronics module for less than 100$
You also could check with Pete at www.olduhfguy.com who does repairs
 
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It would be helpful if we could learn what type of speed control it is- SCR or pwm/mosfet
Does the motor make a fairly noticeable hum when running? That would mean the former.
PWM types are usually somewhat less hummy I think
Also the absence of large filter caps on the board would indicate an SCR drive
 
markba633csi said:
It would be helpful if we could learn what type of speed control it is- SCR or pwm/mosfet
Does the motor make a fairly noticeable hum when running? That would mean the former.
PWM types are usually somewhat less hummy I think
Also the absence of large filter caps on the board would indicate an SCR drive
Click to expand...

No noticiable whine. I'm not sure what the original top speed looked like, but it seems like it is able to achieve top speed (scary fast) without any binding/hangups. I'll try and get some video of the behavior.
 
From the symptoms you described in post #1 I think I would suspect a broken trace on the board possibly around the burned resistor area.
I would inspect the back of the board and also try to measure the burned resistor if you have a multimeter
Be sure to unplug machine before doing any testing
The schematic diagram probably won't be found anywhere, unfortunately
IDing the two power devices on the large heatsink would help determine what type of control it is
 
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Check on Youtube for the sewing machine motor mod. Much more reliable than the original brushed motor.
 
Or consider one of Dave's kits:

The Custom Crafter Servo Motors, Mini Lathe kits, parts, repairs and Upgrades | Servo Motor Upgrade Kit

ServoMotorKit.quick Upgrade Kits convert mini metal lathe milling machine, Band Saw Motors servo motor service upgrade variable speed 2hp electric servo motor repair smithy granite 1324 lathe milling machine 1,500 w upgrade craftsman grizzly 1HP 750 watt metal lathe harbor freight clutch grizzly
www.servomotorkit.com www.servomotorkit.com

He has a ton of videos on his youtube channel (Custom Servo Motor Mods) and is a member on here and has his own forum section too: https://www.hobby-machinist.com/forums/the-custom-crafter-diy-servo-motor-kits.608/
 
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