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Potentiometer Circuit Troubleshooting:

Observe the travel of the knob from .2 to 1.4 and verify that the voltmeter indication tracks the knob position continuously  reading .2 volts to 1.4 Vdc.  Failure could be a change in battery voltage or a change in R1, R2, or R3 resistance, or a failure of potentiometer wiper to maintain continuos contact with resistor. Potentiometer fixed resistance is 1300 ohms, battery internal resistance is normally 2 ohms, and S1 remained closed  during this simulation. Answer is at bottom of this of page.

Pot1.bmp (6974 bytes)

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Answer

The Pot is scratchy. The wiper arm breaks contact between the "0.4" and "0.6" points of the knob.