Originally Posted by
Torchhead
The THC300 is a simple analog "window comparator" circuit. It takes a present voltage and has an over and under circuit with a dead zone of about 1.5V Any voltage over the knob setting cause the DOWN circui to trip and any voltafe under the preset cause the UP put to trip. About as simple as it gets. The UP and DOWN are hooked in MACH to the THC UP and THC DOWN INPUTS and act like an external jog command for Z even when X and Y are moving under code. Screen LEDs should show the UP/ down status. The external voltage divider card divides the raw arc volts by 7.3 :1 so you should be able to test the raw volts at the input tabs (Elctrode and Workclamp and then read that divided by 7.3 a the Divided volts output back to the THC300.
If you have the THC300D its a digital version of the same basic circuit but it displays both the real Torch volts as it cuts and the Preset volts when the torch is off . It also has Up and DOWN LEDs on it besides having the UP and DOWN in MACH. So tunr off the THC button in MACH and do a test cut and watch the Torch Volts. Then turn it on and make sure the UP led is ON when it climbs.