Electric motor / controller repair Hawk G426
#8
Reposting of this message with new title - Thanks, Admiral

The retirement village where I volunteer in the wood shop had a visitor yesterday. One of the residents stopped in and after several minutes of conversation he told me he used to do a lot of scrollsawing. He noted the little RBI saw we have currently. He said he has a larger Hawk (now RBI) unit which he would donate. We would, however, have to repair it. 15 minutes later we had brought the saw to the shop.

It is a Hawk Woodworking model G426.

The problem is the motor. When running at slow speed it runs continuously. As you speed it up (a variable speed model) the motor will stutter - that is run for a second, stop for a second, etc. When I checked the RBI website there is no fix for this motor / controller. They only sell a new unit for $350.00 Not an option.

My question is has anyone encountered this problem and been able to repair it. After talking to an electrician friend he felt it was most likely the controller.

Thanks,

Bill
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#9
Attached are photos of the motor controller (P.N. 479). It performs two functions - converts AC input to DC output and varies the output to control the speed.

As seen in the photos it is all discrete components which 'should' make it repairable if that is the problem.

I have sent a request to Bushton Mfg with this same information. But I will ask the brain trust here for suggestions as well.

I am assuming the controller is varying the voltage to control the speed but I could be wrong. Measuring the output voltage while turning the front panel potentiometer I see a change from 86.2 volts to 87.5 volts. I checked the value of the potentiometer with it removed from the circuit. I measure 9.85 ohms across the outside pins and it varies from 0 to 9.85 ohms from the center pin to the outside pins. Not sure of the value this potentiometer should be. That is one of the questions I asked of Bushton Mfg.

All connectors are tight and no obvious damage to the capacitors.

Any thoughts would be appreciated.

Bill


Attached Files Image(s)
   
   
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#10
As I mentioned in the first post, you can get new controllers for less than 30 bucks at amazon that should work as long as the output voltage and rated wattage matches the old motor/controller.

On that one you picture, I'd be suspicious of the bridge rectifier (thats the square thing with the diamond on top).  Its job is to convert the AC voltage into DC and the caps then are used to even out the spikes.  If one or more of the internal diodes opens up or shorts, then your output is gone, even if its reading 80 something volts.  You can try and source a replacement or just replace the whole thing.  See here: Digital voltage controller

[Image: 71t5DLO3GkL._SL1500_.jpg]
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#11
I would make sure the motor brush caps (if external) are tight-a stuck brush can make it run jerky-.
Brushes should slide freely in the holder/guide.
I have a small lathe with a 90 volt motor-the rectifier is mounted in the motor-I vary the speed with a 15$ HF router control.
Aside from the potentiometer wearing/getting dust in it, I'd look for a cracked trace on the circuit board.
9.8 ohms sounds awful low, I'd expect something around 5 or 10K ohms, in that function.
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#12
(07-06-2018, 04:00 PM)rlnguy Wrote: I would make sure the motor brush caps (if external) are tight-a stuck brush can make it run jerky-.
Brushes should slide freely in the holder/guide.
I have a small lathe with a 90 volt motor-the rectifier is mounted in the motor-I vary the speed with a 15$ HF router control.
Aside from the potentiometer wearing/getting dust in it, I'd look for a cracked trace on the circuit board.
9.8 ohms sounds awful low, I'd expect something around 5 or 10K ohms, in that function.

There are no external brushes on the motor.
I checked the potentiometer removed from the PCB.
If the expected resistance is in the 5 to 10K ohm range that could be an issue.

Thanks, Bill
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#13
(07-06-2018, 08:18 AM)wjt Wrote: Attached are photos of the motor controller (P.N. 479). It performs two functions - converts AC input to DC output and varies the output to control the speed.

As seen in the photos it is all discrete components which 'should' make it repairable if that is the problem.

I have sent a request to Bushton Mfg with this same information. But I will ask the brain trust here for suggestions as well.

I am assuming the controller is varying the voltage to control the speed but I could be wrong. Measuring the output voltage while turning the front panel potentiometer I see a change from 86.2 volts to 87.5 volts. I checked the value of the potentiometer with it removed from the circuit. I measure 9.85 ohms across the outside pins and it varies from 0 to 9.85 ohms from the center pin to the outside pins. Not sure of the value this potentiometer should be. That is one of the questions I asked of Bushton Mfg.

All connectors are tight and no obvious damage to the capacitors.

Any thoughts would be appreciated.

Bill

The controller you linked to appears to require DC input. Since the controller I am working with includes the converter it appears this would require a separate converter.

I will check the rectifier with an ohm meter.

Thanks, Bill
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#14
(07-06-2018, 08:18 AM)wjt Wrote: I am assuming the controller is varying the voltage to control the speed but I could be wrong. Measuring the output voltage while turning the front panel potentiometer I see a change from 86.2 volts to 87.5 volts.


How did you measure this voltage?  You would need to observe it with a scope, a meter will give you an average voltage.

DC motor controllers usually pulse width modulate (PWM) the output.  Reducing voltage will slow the motor, but will also reduce torque.  Pulsing the output controls average voltage and therefore speed but maintains torque.

Can you post a photo of the motor nameplate?  That would help us figure out alternatives.

It's probably repairable, but difficult to troubleshoot remotely.  Have you tried asking a motor repair shop?  They may be able to fix it at reasonable cost.
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