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There are two magnets mounted on the flywheel of the lower drum. One is a north-pole magnet and the other is a south-pole. As the drum is spinning, those two magnets pass over the Pulse Generator sensor and thus the PG signal is formed which is that square-wave signal you see on your oscilloscope. The PG signal is used to tell the servo circuit which head is in contact with the tape. If the relation/alignment of the magnets vs the position of the video heads on the upper drum is messed up, the switching point will occur at the wrong timing, causing the symptom seen here.
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04-04-2022, 07:22 PM
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