Lincoln Precision 185 PC Board Problem

Lincoln will actually give you a "wiring diagram" of the machine. They won't give out schematics of the boards, though. It should be fairly easy to open the panel and have a look at what is behind there. The problem description sounds EXACTLY like a "dirty" pot. You can get spray stuff to clean volume controls at places like Radio Shack. You just wquirt a little of this stuff into the pot and turn it several times back and forth, then try it.

Jon

It's not a pot. The same problem is being discussed in sci.engr.joining.welding . I replied there as I have the same welder with the same general problem.

It's not a pot because the knob can be turned forever in either the clockwise or ccw direction, and when it goes wierd and the current display makes a big jump it then tunes around that new value.

So I think there must be an input device connected to the knob with some kind of digital pulse output, probably an optical encoder. My PT 185 is very lightly used and shows a bit of the same problem. My guess is some kind of design problem from the factory either with the input device or noise/debounce handling. The problem I see on mine is not bad enough yet to make me dig inside and try to figure out what is happining.

Are you sure it is an optical encoder? I'm wondering if it is one of the very cheap mechanical encoders, they have a printed wheel with the quadrature pattern and brushes like inside a pot to make contact with the wheel. After the contacts get dirty, they get intermittent. If it is a mechanical encoder, you should be able to detect the contacts opening and closing with an Ohmmeter. One clue is most mech. encoders will only have 3 wires, the optical ones will have to have at least 4.

You could probably patch in a better-quality optical encoder if the above scenario is correct.

Jon