Dead Random Orbital Sander: How Can I fix It? - Woodworking Talk - Woodworkers Forum
 
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post #1 of 8 Old 05-10-2013, 04:57 PM Thread Starter
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Dead Random Orbital Sander: How Can I fix It?

My old Bosch Random Orbital Sander died while I was using it yesterday. The motor just stopped after I had been using it for about ten minutes.

I will probably have to fix it myself. do you have any suggestions on what I should "look for" when I take it apart?

I was using the sander plugged into a Lithium Ion Battery with a pure sine wave converter, which was probably NOT a good idea in retrospect. Apparently the voltage can drop when using that (although it is a PURE sine wave inverter, so I guess that counts for something.)

Just to make sure, when I got home I tried plugging it into a wall outlet and it still wouldn't work. pressed the trigger and the motor wouldn't turn on.

Do these things have "fuses" that can be easily replaced or something like that?

thank you if you can give any suggestions on how I would diagnose this bad boy.
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post #2 of 8 Old 05-10-2013, 06:28 PM
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If it's old I would first inspect the switch to see if the motor is getting power. Do you have a volt meter to check it? If that checks out I would check the brushes and armeture to see if they are alright. Sometimes the brushes get so worn there is not enough spring pressure to make proper contact. Usually with those the machine starts making a fluttering noise or visually see more than normal sparks in the inside before it quits though.
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post #3 of 8 Old 05-10-2013, 07:07 PM Thread Starter
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Thank you Mr. Neul:

"If that checks out I would check the brushes and armeture to see if they are alright"

I will look into that. Hopefully I can find a video somewhere on how to examine the brushes in an electric motor because that is something I have not done.

Just one more question: These things don't have a "circuit board" or some fancy electronics inside, do they? Do they usually just have a few simple electrical parts inside?
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post #4 of 8 Old 05-10-2013, 07:31 PM
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<<Just one more question: These things don't have a "circuit board" or some fancy electronics inside, do they? Do they usually just have a few simple electrical parts inside?>>

If it's variable speed, probably. If it's not, maybe.

Checking the brushes (should be easy) and switch like Steve said is a good place to start.

Bill
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post #5 of 8 Old 05-10-2013, 07:33 PM
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They typically don't have anything fancy, just a universal motor. There is a minuscule chance there could be a circuit board, but I would be surprised to see one.

If your inverter not providing full voltage was the problem you should have felt, and smelled the problem, as the windings got hot.

As was said, it is most probably the switch, or the brushes.

The brush holders are typically on the outside and pretty easy to get to. Depending on how much respect you have for electricity you can check the switch by pulling the brush covers, withdrawing the brushes, turning on the plugged in sander, and checking the voltage between the brush leads. If the switch is good there will be 120-volts potential across the leads. Just be careful, because if there is 120-volts, it is enough to do serious harm.

If you don't have 120-volts showing at the brush leads the switch is most probably the culprit.
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post #6 of 8 Old 05-10-2013, 08:24 PM Thread Starter
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I think I have too much "respect" for electricity to try that.

However, I will try to research online to see if there is a safe way to test how you mentioned. I am pretty sure I would electrocute myself until I see how to do it about a zillion times first.

Is there no way to use a tester of some kind WITHOUT having the sander plugged in?

Studies have shown that having a ladder in the home is more dangerous than having a firearm. That's why I own 10 guns... in case some maniac tries to sneak a ladder into my house...
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post #7 of 8 Old 05-10-2013, 11:42 PM
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You can try testing for continuity with an Ohm meter, and the power off.

With the power off you can test to see if the switch has no resistance in the closed (on) position, and infinite resistance with the switch in open (off) position. The problem you can have with this technique is that the contacts can be dirty, and show no resistance with the low current of the ammeter, but heat up and have high enough resistance when trying to carry a working load, to not actually work.

There are probably instructions online on trouble shooting a universal motor.
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post #8 of 8 Old 05-11-2013, 08:27 PM Thread Starter
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Thanks very much for the explanation.

I did not know that it was called a universal motor.

Thanks again.

Studies have shown that having a ladder in the home is more dangerous than having a firearm. That's why I own 10 guns... in case some maniac tries to sneak a ladder into my house...
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