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PM Control Circuitry, Part 2

Troubleshooting


In most cases, when a PM 3520 will not start and run, the problem is in the control or power circuitry rather than in the inverter. This is good because such problems are easy to fix. You may have to replace a switch, but that is far simpler and less expensive than having to deal with the inverter.


The approach in this article is to wire around or bridge sections of the control circuitry in an effort to get the lathe to run. If that attempt is successful, we can conclude that the inverter is OK and then begin looking for the problem in the control circuitry. However, if we are unable to make the lathe run, we will conclude that the problem lies with the inverter itself. In that case, the only option will be to remove it for repair or replacement.


The following checks and tests are simple to perform even if you do not have experience in such matters. The control circuit voltages are low and harmless. Only a simple volt/ohmmeter is required along with a couple of clip leads and a short “ test wire.”


If you don’t have a voltmeter, perhaps you can borrow one or you may consider buying one – the simple ones are not expensive. (Check at auto parts stores.) Another approach is to invite a friend who has one to come to your shop and lend a hand.


Before you begin, read and study the description of the circuitry given in Part 1. The better you understand the circuit, the better will be your chances of finding the problem and making the repair.



Basic Observations


Suppose you apply power to the lathe, the RPM display lights up, but nothing happens when the Start/Stop switch is turned ON. What do you do then?


If the main 240 V power is applied to the lathe when the Start/Stop switch is already in the ON position, the lathe will not start. If after applying power you try to start the lathe before the inverter has had time to boot up, the lathe will not start. In both cases, switching the Start/Stop switch to OFF and then back to ON will restore normal operation.


The lathe will not run if the speed control is turned all the way down. Rotating the speed control knob clockwise to increase the speed will start the lathe provided the Start/Stop switch is ON.


If the Start/Stop switch is intermittent, the lathe may run after clicking the switch a time or two, or it may just bump every now and then but fail to run continuously. In this case, the switch should be replaced.


If the speed control pot is worn or dirty, the slider may not make good contact with the resistance element so that no voltage is applied to the speed control terminal of the inverter. Changing the setting may move the slider to a clean spot where it is able to pick up the voltage. This will cause the lathe to run, but perhaps erratically. In such case, the pot will need to be replaced.


Move the Fwd/Rev switch to reverse and try the Start/Stop switch again. If the lathe will run in one direction but not the other, the indication is that the Fwd/Rev switch is bad.



Are you feeling lucky?


Because the Start/Stop switch has an established history of failures, the obvious first step is to determine if it might be the problem in this case. To do this we must have access to the control circuitry.


Remove power from the lathe. Remove the screws holding the control panel on the front of the lathe, and carefully pull the panel out of the case of the headstock. If the tangle of wires are taped into a bundle, remove the tape and separate the wires to gain a little more freedom. Arrange whatever support is needed so that the panel rests in a stable position. You can now test the switch.


Setup for the test:  Set the Start/Stop switch to OFF. Set the speed control to low midrange. Apply power to the lathe.


The test: Switch the Start/Stop switch to ON. Because there is a problem in the circuitry, the lathe will not run.


Take the test wire and connect it from the contact on one side of the switch to the contact on the other. That is, bridge the switch with the test wire. If the lathe runs, the indication is that the Start/Stop switch is defective (open).  



An Annoying Little Possibility


If the lathe does not run, there is another thing we should check. We must be sure the Start/Stop switch is not stuck in the ON position. This would be a rare occurrence, and I cannot imagine how it could happen without previously having the lathe fail to turn OFF, but in the world of troubleshooting, all things are possible.


If power is applied to the lathe with the Start/Stop switch already in the ON position, the safety built into the inverter will prevent the lathe from running. Cycling the switch from ON to OFF and then back ON will restore normal operation. But if the switch is stuck, it can never be turned OFF so the lathe will never run until the switch is replaced.


We need to verify that the switch is not stuck in the ON position.


Procedure: Remove power from the lathe. Disconnect the yellow wire from the center terminal of the Fwd/Rev switch. Turn the Start/Stop switch ON.


Set your meter to check either resistance or continuity. Then check the continuity of the Start/Stop switch by connecting the meter probes directly across the switch. Because the switch is ON, the meter should indicate continuity or a resistance of less than about 2 ohms.


Switch the Start/Stop switch to OFF and repeat the check. In this case, the meter should indicate an infinite resistance or no continuity. If it shows continuity or a low resistance, the switch is stuck in the ON position and must be replaced.


If the switch passes these tests, it is good, and we no longer have to worry about it. Otherwise, it should be replaced.



Bridge around the Start/Stop/Direction Circuitry


If we can get the lathe to run by bridging a part of the control circuitry, we will then know that the inverter is OK and the problem lies with the controls. This will be a major step toward finding the problem.


The following test consists of using the test wire to temporarily ground terminal 4 of the control inputs, which should make the lathe run in the forward direction. This amounts to bridging across the entire start/stop/direction circuit.


Go to the back of the lathe and open the access door of the inverter. Locate terminals 4 (white wire) and terminal 10 (blue wire) of the control terminals.


Setup for the test:  Start/Stop switch OFF. Set the speed control to low midrange. Apply power to the lathe.


The test: Take the test wire and temporarily touch it to terminal 10 (ground, blue wire) and terminal 4 (run forward, white wire). When you make the connection, the lathe should start and run in the forward direction.  


In a similar manner, move the test wire to terminal 5 (run in reverse, black wire). The lathe should run in reverse.


If the lathe runs as described, the inverter is OK. The problem lies in the control circuitry. This is a good thing.


If the lathe does not run in this test, there are two possible causes:


(1) The speed control circuit could be the problem. A speed control voltage must appear at terminal 2 (green wire) for the lathe to run.


(2) There is a problem inside the inverter.



Speed Control Test


Because a speed-control voltage must be applied to terminal 2 (speed control input, green wire) in order for the lathe to run, the next step is to verify that a voltage is indeed present at that terminal.


Setup for the test:  Set the Start/Stop switch to OFF. Set the speed control to midrange. Apply power to the lathe.


The test:  Measure the voltage at terminal 2 -- Connect the black lead of the voltmeter to terminal 10 (ground, blue wire) and the red lead to terminal 2 (control voltage input, green wire).  The voltage should be somewhere around 5 V because the speed control is set to near midrange.


Vary the setting of the speed control and see that the voltage at terminal 2 varies accordingly. For example, if the speed control is set to its maximum value, the voltage should be about 10 V. Further, the voltage should vary smoothly when the speed control knob is rotated smoothly. If these voltages are found, we can conclude that the speed control circuitry is OK.  



Check the power supply voltage.


If no voltage appears at terminal 2, we must find out why. The first step is to measure the output voltage of the 10-volt power supply (terminal 1, yellow wire). With the same setup, connect the black lead to terminal 10 and the red lead to terminal 1. The voltage should be on the order of 10 V.  (I measured 10.6 V on my inverter.)


If no voltage is found, or if it is significantly lower than 10 V, disconnect the yellow wire from terminal 1 and repeat the measurement. If the voltage is now present and close to 10 V, the indication is that a problem in the potentiometer circuit is pulling the voltage down.


If the voltage is still missing or is very low, there is a problem with the power supply and the inverter will have to be sent out for repair.



Check the splices in the green and yellow wires.


Behind the control panel, the yellow and green wires are spliced with wire nuts to white wires that are soldered to the terminals of the pot. If the 10-V power supply is good but no voltage appears at terminal 2, check the splices. If either of these splices are bad, no voltage will appear at terminal 2 and the lathe will not run. If the splices are good, suspicion then turns toward the pot.



Check the Potentiometer.


Remove power from the lathe. Disconnect the yellow, green, and blue wires from terminals 1, 2, and 10, respectively, of the control terminals. This isolates the pot so that we can perform resistance checks to determine if it is good or bad.


You will need to make good connections to these wires in the following tests. I suggest using clip leads between the meter probes and the wires. Your chances of getting good readings are slim to none if you simply try to hand-hold the probes against the wires. The setup I used is shown in the photo at right.


Switch the meter over to measure resistance and measure between the yellow and blue wires, which is across the entire resistance element of the pot. The resistance should be about 5,000 ohms, which is 5 kOhms. (The position of the control knob doesn’t matter in this test.)


Next, measure the resistance between the yellow wire and the green wire, which is between one end of the resistance element and the sliding contact. The resistance will vary with the position of the control knob and should vary from near zero to a maximum of 5,000 ohms as the knob is rotated over its full range. Further, it should vary smoothly when the knob is rotated smoothly.


If the reading flickers back and forth to and from infinity as you rotate the knob, the indication is that the pot is worn or dirty and will need to be replaced. That is, unless the connections between the meter probes and the wires are not good. This is why you need the clip leads.


Repeat this test by measuring between the green and blue wires. The results should be similar with the exception that the resistance will increase when the knob is turned in the opposite direction.


If the pot passes these tests, you can assume it’s good. Reconnect the yellow, green, and blue wires to terminals 1, 2, and 10, respectively.


If the pot must be replaced, my suggestion is to use much smaller wire in lieu of the three white wires that connect directly to the pot. Also, the insulation on the terminals is not needed. Using smaller wires and skipping the insulation will make the job much easier.


In these tests, it is unlikely that you will find the power supply, the splices, and the potentiometer circuit all to be good. After all, the voltage at terminal 2 is missing or low, and there must be a reason for it. Hopefully, these tests will have revealed the problem.



A Reluctant Conclusion


If the speed control circuitry is found to be in good working order in the above tests, and yet the lathe did not run when you bridged the start/stop/direction circuitry, the problem more than likely lies with the inverter itself. It will have to be either replaced or repaired.



Voltmeter Test of the Start/Stop/Direction Circuitry


Even if there is a not-yet-repaired problem in the speed control circuity, you can still do a voltage check of the start/stop/direction circuitry to confirm that all is well in that area. Then, when the repairs are made to the speed control, your lathe should be good to go.


This test is possible because the voltage at terminals 4 and 5 (run forward, run in reverse) pulls up to about 16 V when the respective terminal is open. Recall that the terminals are “active low.” You ground the terminal in order to activate it. But when it is not grounded, it will pull “high,” which in this case is about 16 V.


Setup for the test:  Set the Start/Stop switch OFF, for the time being. Speed control setting does not matter. Apply power to the lathe.


Using a clip lead, connect the voltmeter black lead to the blue ground wire where it connects to the terminal of the Start/Stop switch. Leave it there for the duration of this test.


Do the test:


Measure the voltage at the end terminals of the Fwd/Rev switch. If everything is in order,

both terminals should show a voltage of about 16 V. This confirms that the connections are good from the switch terminals to the control terminals on the inverter.


Move the red lead of the voltmeter to the center terminal of the Fwd/Rev switch. The voltage there should be high, and it should remain high as you flip the switch back and forth. If the voltage is low, or if it goes low at one position of the switch, the Fwd/Rev switch is open and must be replaced.


Switch the Start/Stop switch to ON.


Again, measure the voltage at the center terminal of the Fwd/Rev switch. It should now be low (0 Volts) because that terminal is now connected to ground through the Start/Stop switch, which we have already confirmed to be good (the first test at the beginning of this article).


Recheck the voltage at the end terminals. One should be high (16 V) while the other is low (0 V). Flipping the switch should cause the high and low voltages to swap ends. This indicates that the Fwd/Rev switch is good, and that a conducting path exists through the Start/Stop switch, which is as it should be because the switch is ON.


If the voltages do not swap ends when you flip the Fwd/Rev switch but instead both become high, the indication is that the Fwd/Rev switch has a bad contact. It will have to be replaced.


Measure the voltage at the terminal of the Start/Stop switch where the yellow lead connects. With the switch ON, the voltage should be zero. When the switch is OFF, the voltage should be high.



Wrap Up


At this point we have checked both the speed-control and the stop/start/direction circuitry completely. By the time you reach this point, you should have discovered why your lathe will not run and should have identified the faulty components. All that remains is to obtain new parts and make the repairs.


Possible sources for replacement parts and search tips are given at the end of Part 3.


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