See the video below for a visual of the issue. Needless to say, it is annoying. First, I don't think the spindle is running at full speed at all (25k), and when I set the speed to 500 above it's minimum (which is 3500 or so), it shuts off. Anything other than full speed does the revving up and down of the spindle speed as seen in the video. It is an AMB 1400 DI spindle. It is wired to a 30A 240 circuit (the spindle operates on both 50 and 60Hz). Connections to the controller.. I have tried them all.. The spindle has 3 wires. Ground (0V according the the documentation) - I have connected this to the ground terminal next to the spindle control, as well as the ground next to the 24V out terminal which is near the external driver terminals, and for shits and giggles, to the - terminal on the power supply directly Spindle control, 0 - 10V is connected to the variable spindle terminal And a 10 - 26V connection (according to the manual for "For Power Supply). This one I have connected to the 24v output terminal near the external driver blocks, as well as directly to the power supply.. I'm wondering if I got a dud... Any ideas... Edit: removing the video, not at the request of anyone (and the video itself is not published, it requires a direct link), but after talking with the vendor, I chose to remove the video to avoid the possibility that someone may view the video in a negative context of the product. I believe we have narrowed down the problem to the board per the comment Peter made above. I will continue to troubleshoot though. I had planned on replacing the controller anyways.