Hi,
I recently bought a lathe that had been converted to CNC.
I know pretty much nothing about mach3 or CNC lathe conversions and am having issues getting it to run right.
It worked briefly but I had to move and it seems that the hall effect sensor that is used as the tachometer was ripped out. It was originally glued to the piece of aluminum pictured.
Now whenever I try to run the lathe at any RPM it defaults to its minimum speed (about 60rpm) and gives the message: PWM Bumped to minimum setting

The original hall effect sensor is pictured. I have tried buying a similar one from eBay (https://ebay.to/3lzcNKS), as well as this model from amazon https://amzn.to/3mOS7Oq but still have the same issue. I have checked to see if anything else was disconnected but this seems to be the only thing out of place.

The board that was connected to the hall effect sensor is a PMDX-126 (Revision B)
If it helps at all, the output voltage for the blue wire is 12.76v and 3.57v for the brown wire.

If you have any suggestions, that would be great!

Hello MicrowaveMan,

The OH137 should work if properly wired and if it sees the correct magnetic field changes. Here is a hookup drawing made from the datasheets for the PMDX-126 and the OH137.

The OH137 datasheet doesn't show how the device should be oriented in relation to the magnetic field. You will have to experiment. Wire the OH137 to the PMDX with it hanging loose. Monitor the output with a voltmeter while positioning a strong magnet near it in different orientations. The voltage should change to a value near zero when you find the right one. Once you see it work try it with the spindle magnet.

It's a unipolar sensor so it need to see a field of at least a certain strength to turn on, then the field needs to be reduced below a much lower level to turn it off.

Hi MN300,

Thank you so much for the reply.
The symbol with the arrow is that a BJT transistor? Does the size matter, and should I leave the locations of the original wires in place or switch the brown wire.
It is currently outputting 3.57v and not the  12v/5v options shown in your diagram? The wires worked in these locations before but I am not sure how it was wired at the spindle end.
All I found were the remains of a resistor and OH 137 sensor.

The PMDX-126 manual describes the 12U as
Unregulated +12V power supply (Referenced to the GND pins on this connector).
If you aren't measuring approximately 12 volts from 12U to a GND pin there is a problem with the PMDX-126.
However all is not lost, the OH137 will also run on +5 volts. If the +5V measures OK use that instead of 12U.
If 12U is not used for anything else, internally or externally, you should be able to run without it.

The transistor on the diagram is a leftover option from the examples in the manual. Just wire the OH137 as shown with no extra transistor.

Thank you once again for the help!
I really have no experience with any of this stuff.

I wired up the OH137 as shown in your diagram.
When no magnet is present, the brown wire (number 3 pin) shows 0V.
When I introduce a magnet to the side of the OH137, the full voltage of the blue wire (pin 1 ) is measured.
I have not connected the number 3 pin to the PMDX-126 because it already reads 4.5V at the terminal. Is this correct or is there an issue with the board?
These were the terminals that were originally used.

Your voltage readings don't make sense, I am wondering if you have the OH137 wired correctly. I can't tell from the photo where wires are connected.

Please check your wiring.

OH137 pin 2 should connect to a GND terminal, the nominal color for this is blue. Your photo is showing black but of course it will work regardless of color as long as the correct two points are connected.

OH137 pin 1 should connect to a +5V terminal, the nominal color for this is brown. All voltage readings should be made with the negative meter lead connected to a ground terminal. You should measure 5 volts with the positive lead connected to OH137 pin 1 at all times.

OH137 pin 3 is an open collector output, that's like a switch to ground. You will read 0 volts there until it's connected to something that pulls up that line. That something is the input number of your choice.
First check the voltage at that input with nothing connected to that terminal. The manual doesn't say what it should be, please let me know what you find.

Connect that input terminal to OH137 pin 3, the nominal color for this is black. The input voltage should change to 0 when the magnet is sensed.

I checked the wiring and am now getting something different.
Maybe I should order some more OH137s to double check.

Testing in a power supply with Pin 1 to +5V and pin 2 to GND, I read 2.3V on pin 3 with no magnet. When a magnet is introduced I read 0V.

On the PMDX-126 board with no OH137 connected to it, just measuring the terminals, I read +12.5V where the blue wire is connected, and +4.5V where the brown
wire is connected based on the picture.   Shouldn't the terminal with the brown wire read 0V with nothing on it?

Testing in a power supply with Pin 1 to +5V and pin 2 to GND, I read 2.3V on pin 3 with no magnet. When a magnet is introduced I read 0V.

When testing the OH137 a pullup to a positive voltage should be connected to pin 3. If you have a resistor somewhere in the range of 1K to 10K connect it from +5V to pin 3. The output should switch from 5V to near zero.

On the PMDX-126 board with no OH137 connected to it, just measuring the terminals, I read +12.5V where the blue wire is connected, and +4.5V where the brown
wire is connected based on the picture.   Shouldn't the terminal with the brown wire read 0V with nothing on it?

I can't comment on this because I can't see which terminals those wires are connected to. Whoever installed those wires may not have used to the standard color convention.

Here are some check of the PMDX-126 you can do.

With the meter negative lead to a GND terminal and the positive lead to +5V you should read  +5V.

With the meter negative lead to a GND terminal and the positive lead to 12U you should read about +12V. It's unregulated so it could be several volts higher than 12V.
 
With the meter negative lead to a GND terminal and the positive lead to an input you should read a positive voltage. The manual doesn't say if the inputs are referenced to +5 or +12.

I know it's been some time, but I just tried what you said, and it looks like the tachometer is now working. However, the "PWM bumped to minimum setting"  is making it impossible to control the spindle speed.
The minimum setting appears to be 40rpm and is stuck there.
Is there a log for errors, or anywhere you would suggest checking to fix this issues?