Repair Craftsman electronic radial arm saw?

[JPG]

I just caught up with this thread...

It seems that if you measure voltage on the motor brushes and it is wired like you show in the schematic, then it ought to turn. Can you turn it by hand? Make sure it is not binding. If the armature winding (the part the brushes connect to) is open circuit, then it won't run. You can check that with an ohm-meter across the place where the brushes make contact.

The motor looks like a "universal motor" (runs on both AC and DC)] but somehow the schematic posted looks a bit "funny" with the field winding (stationary part) in series with the AC.[/B] It's definitely not a stepper motor (I used to work for Superior Electric, so I am really familiar with steppers).

The two black square devices are definitely Hall sensors; you need two to determine if the motor is going clockwise or counter clockwise.

The diodes (cylindrical objects with stripe) should probably be checked, but like others said, no signs of burning.

The key is the motor; if you can get it to turn, then there is hope for fixing the controller.

Good luck. Don't give up!

The last schematic posted(by me) is quite clear. The '120v' input needs to be DC, not AC to allow direction control. The bridge(4 diodes) connects the same polarity to the rotor windings regardless of the incoming polarity. The stator windings however are reversed with the reversing polarity. A series connected universal motor is not uncommon.

I have added 'test points' and expected readings to the schematic.
I do wish you could verify the termination of the shiney metallic encased overload device.

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See post 61 this thread

1) the overload device is probably a fuse and is the most likely culprit.

2) stator windings(including 'fuse?)

4) rotor windings(including brushes)

3 I goofed - same as 2

A 'low reading' is less than 100 ohms. If you get different readings, tell us. They might be 'clues'.

If the 'fuse' is open(blown) we must try and determine why it blew.

I must comment on your measuring unknown voltages with a multimeter. Tis not good practice to try to determine voltage type(AC/DC) by attaching the multimeter and setting the multimeter to ac/dc ranges. Unless you know how the meter will respond to the different voltage types, the readings are of little value.

The resistance readings can be affected by the connected wiring etc., but with the low readings expected, that should not be a problem.

I suggest one(8) other check. The diodes. Each one should give a low reading with the multimeter connected one way, and a very high reading with the multimeter connected reversed - 4 diodes - 2 readings = 8 readings.

[JPG]

Is there a part number and manufacturers name on the motor.

Are there any other identifying bits of information on the motor like operating voltage and current.

https://forum.shopsmith.com/viewtopic.php?p=93441&postcount=9

second pix

[JPG]

Make sure it is unplugged before doing any of the resistance measurements below!

I'd start at the "AC in" terminals you show in one of your earlier pictures. Put the multimeter on "ohms" (lowest range if you need to set it) and you should see a few ohms (close to zero since the motor is mostly wire). If it looks "open" (very high ohms), reverse the leads and see what you get.

To check the armature, remove the brushes if you can and put the multimeter probes on the commutator (the thing the brushes ride on, slotted copper thingy) where the brushes contact and you should see low ohms there. If not the armature may be open circuit. That might be "game over".

Good hunting.

Consider this a resistance check "0". i.e. do it before those three I suggested.

[johnm]

Keith, you were close, but you got two diodes reversed.

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The pink? thing across the armature is probably a capacitor, but may be a resistor.

Missing is the metallic object mentioned before.?????

I think I've sorted this schematic out and how the motor is supposed to operate. First a bit of a tutorial...

The motor is a "universal motor" so if you wire the armature and field windings in series, it will always run in the same direction whether AC or DC is applied to the terminals. To reverse the direction, you need to physically reverse the connections of *one* of the windings. The direction of motion has to do with the direction of current in one winding *relative* to the other winding.

For this application, we need to run the motor in both directions, and we need to physically reverse those connections without actually reconnecting (like you would with a relay) since that gets messy. That's where those diodes come in on the circuit board...they ensure that the current in the armature winding always flows in the same direction, regardless of the polarity of the current into the "AC in" terminals (which determines the direction of the current in the field winding).

What this means is that the voltage applied to the "AC in" terminals by the control box is really a DC voltage (or DC average), and the control box should reverse the polarity to make the motor spin one way or the other. If you apply pure AC to those terminals, the motor won't spin, since it is trying to reverse direction every half cycle.

You can test this out by applying a DC voltage to those "AC in" terminals once they are disconnected from the control box...an auto battery charger would probably work just to see if it will spin.

That tells me the problem is in the control box. There's probably a defective bridge rectifier or set of rectifier diodes in there, or maybe a blown fuse for the motor circuit itself. Just trace the power wires (black and white) back to the circuit board on the box and see what's around there (another photo?).

Sorry for the long-winded reply. Good hunting.

[pennview]

A couple of thoughts on this subject. With the saw unplugged, attach a 12 volt car battery (or battery charger) to the two terminals on the lift motor to see if the motor is at fault.

If you've given up on the electronics aspect of the saw, you could likely convert it to mechanical operation from the top of column using an extension attached to the screw lift mechanism. A piece of pipe or tubing, or perhaps a long socket extension coupled to the screw would likely allow you to raise/lower the arm.

[dusty]

A couple of thoughts on this subject. With the saw unplugged, attach a 12 volt car battery (or battery charger) to the two terminals on the lift motor to see if the motor is at fault.

If you've given up on the electronics aspect of the saw, you could likely convert it to mechanical operation from the top of column using an extension attached to the screw lift mechanism. A piece of pipe or tubing, or perhaps a long socket extension coupled to the screw would likely allow you to raise/lower the arm.

Conversion to manual may be more complicated than that. I think that, in addition to depth of cut, miter angles and bevel angles are also done by the control unit. I THINK!

[tdubnik]

OK. After a few more tests and a major discovery.

First when trying to get an ohm reading on the power input points; I couldn't get anything. The meter doesn't move at all.

By taking an ohm reading at the brush terminals and rotating the armature I got readings that were normally 3 ohms but as I turned the armature the ohms would read infinity at one spot.

Time to break out the magnifing glass as my eyes don't see as good as they used to. This is what I found on inspection. The armature has 16 of those little copper plates around it. Each has a fine wire connected to it from the armature windings. As you can see in the picture (taken through a magnifing lens) at least one is burnt into.

Does this mean game over?

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[tdubnik]

Conversion to manual may be more complicated than that. I think that, in addition to depth of cut, miter angles and bevel angles are also done by the control unit. I THINK!

Dusty,

They are measured by the control unit but they are adjusted manually. I have tested these functions and they seem to work. There are sensor units for both bevel and miter that still seem to function correctly.

A proeblem I might have if I try to convert to a top mounted manual elevation is that the miter sensor is located in the column cap. I would have to make sure I could convert without destroying the miter sensor system.

[johnm]

OK. After a few more tests and a major discovery.

First when trying to get an ohm reading on the power input points]13310[/ATTACH]

Put one lead of the multimeter on the segment with the burnt spot and the other on another segment and see what you get.

If the solder is just loose, you might be able to resolder.

[JPG]

You have at least two wires separated. It looks like one has been contacting the armature.

Careful reconnecting the separated wires properly(you should see a repeating pattern of the rotor wires path to the commutator(copper thing the brushed contact) may bring it back. Far from easy!!!!