Need Help Deciphering Circuit Board Schematic

Posted on Feb 5, 2014

The wire speed DC motor is constantly receiving about 20 volts, regardless of the state of the switch on the nozzle handle. I checked the switch with an ohm meter. The switch works fine. I even disconnected the switch from the circuit board. The motor is still going. By looking at the physical circuit board, I have come up with the following schematic in EAGLE CAD, which I believe to be 99% accurate.

Need Help Deciphering Circuit Board Schematic

Click here to download the full size of the above Circuit.

My friend replaced two components, one was in a TO-220 package, and the other was in a TO-92 package. At this time, he is unable to find the components he removed from the board, so it is anybody`s guess as to what they were. The components that are in the circuit now are an LM7812C voltage regulator (the TO-220 package) and an MPF 102 601 N-channel RF amplifier (the TO-92 package). (In the schematic, I replaced both with standard NPN transistors. I have no idea if that makes any sense. ). There are also two "black and silver" diodes on the board, and two "black and orange" diodes on the board. They are labeled as such on the schematic. That is the reason I`m posting. I need help deciphering this schematic, because it doesn`t make any sense to me. For instance, the welder is DC only, no AC side. So the line voltage comes into the welder, goes to the power switch, through a transformer, and then through the power diodes. The wire speed control circuit board is then connected here, to this rectified power. So why is there a rectifier on the circuit board Also, there are diodes towards the bottom of the schematic, which point up (i. e. , allow flow from neg -> pos, but not pos -> neg (unless they`re Zener diodes. I`m not sure). I have included the schematic I made in PDF form, as well as a "semi-schematic" that I received from the manufacturer, as that is about all I can fit into 97KB. I need help to make sense as to how this...

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