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PM Control Circuitry, Part 3

This is a continuation of Part 2. A few more details are provided about the inverter, and suggestions are offered for the course of action when the inverter appears to be graveyard dead. Because it is likely in such case that you will have to remove the unit for repair or replacement, a few tips are given that may be helpful in removing and re-installing the unit.

Possible sources of replacement parts are given at the end.

A Totally Unresponsive Inverter

The assumption here is that when you apply power to the lathe, nothing happens. No relay click is heard and the display on the inverter does not light up. The RPM display on the front of the headstock may or may not be lit up. Nothing happens when you turn it ON with the Start/Stop switch.  

The first thing to do is to make sure that power is being applied to the lathe. The RPM display may provide this information.

RPM Display

The RPM display is a unit that is almost completely separate from the inverter. The only connection between the two is that the display gets its power (240 VAC) from the two input power terminals of the inverter. If the RPM display is lit up, this tells you that power is being supplied to the inverter.

In the presence of a fault, it could happen that the RPM display fails to light up while the lathe itself, the control circuitry and inverter, is still fully functional. The only difference would be that you will not have an RPM indication.

If the display fails to light up, check the splices (wire nuts) in the power wires that run from the inverter to the display. They are located inside the headstock in the vicinity of the control panel.

On the other hand, the displaly may be lit up and fully functional at the same time the inverter is dead as a doornail. In this case, the display will indicate an RPM if you turn the lathe spindle by hand.

Check the Input Power to the Inverter

Several things could cause a loss of input power. A circuit breaker could have tripped, which sometimes happens for no apparent reason. Corrosion at the prongs of the plug or inside the receptacle can create an open circuit. A remote possibility is that the line cord itself could be defective. If you have a switch for turning the power ON and OFF to the lathe, it may have malfunctioned in some manner.

Open the door on the inverter to gain access to the wiring. Identify the various groups of conductors and locate the black and white wires that go to the input power terminals.

Apply power to the lathe. Set your voltmeter to measure AC Volts and then measure the voltage at the input terminals. You should read something on the order of 240 V.  Disconnect power from the lathe.

If no voltage is found, your next objective is to find the problem in the wiring leading up to the inverter: line cord, plug, receptical, and so forth. Hopefully, when you restore power to the lathe, it will function normally.

Gear Up to Remove the Inverter

If power is being applied to the inverter and it is still unresponsive, there is nothing else you can do except remove it for replacement or repair. This task may look intimidating because of all the wires (16 in all) that must be disconnected, but they appear in groups, which organizes the chaos to some degree. You have the input group, the motor group, the control group, and two pairs. And that’s it.

My suggestion is to make a diagram of the three terminal strips and indicate which color wire goes to each terminal. Some people prefer to take a picture, but I think making a diagram is better. If you make a good diagram, there is no need to label each wire as you disconnect it.

You do not have to remove the grommets that surround the cables or bundles where they pass through the metal box at the bottom. The entire box is removable after the wires are disconnected from the terminals, and it will stay with the lathe.

The box is secured by two screws. When the screws are removed, it will slide out. Before you remove it, however, note very carefully how it fits into the case of the inverter. It can be very confusing to re-install if you don’t remember exactly how it was situated.

Find the two screws that hold the box in place. They are far toward the back of the inverter case, a full 2” from the front of the box. Two other screws are in the vicinity but they are recessed in plastic and are only 1.5” from the front. Don’t mess with these.

The screws holding the box may be very tight and hard to remove. What you must avoid doing is stripping out the screw heads. Use a good-quality screwdriver that fits the screws perfectly. Press really hard against the screwdriver as you apply pressure to loosen the screw.

Once the screws holding the box are out, you will find that you must disconnect the wires before the box will slide out. This is no big deal.

One pair of wires goes to the braking resistor mounted inside the headstock case. You must disconnect the wires but you do not have to remove the resistor.

Before you begin, check one more time to see that power has been disconnected from the lathe. Then go for it. It’s not such a bad job as you might think.

Once all the wires are disconnected, remove the four mounting screws that hold the unit on the back of the headstock and the unit will be free and clear.

Obtaining replacement parts

If you discover a problem with the Start/Stop switch, see the article on this website that describes the switch in detail: Powermatic 3520 Switch Replacement.

The Forward/Reverse switch is a single-pole, double-throw switch that is commonly available at big-box, hardware, or auto parts stores. Any switch having three terminals is likely to be this type. The terminals should be “spade lugs” instead of solder terminals or screws.

Here is a likely possibility:

Search:  All Electronics   CAT # STS-95


The pot is a 5 k-Ohm  (5,000 ohms) unit rated at 2 watts. (A 1/2 watt unit will work just fine.) The shaft diameter is 0.25” and the required mounting hole is 3/8”. Other characteristics you may see are linear taper, 10% tolerance, solder lug, and panel mount.

Pots are generally available only from electronic parts supply houses. In searches, use the word potentiometer instead of just pot.  Here are two offerings that look good:

(1)   Search:  allen bradley 2022005507

      Allen Bradley Manufacturer’s #:   RV4NAYSD502C


(2)   Search: newark electronics 01F2643

      Honeywell Manufacturer’s #:  RV4NAYSD502A


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