My Beogram 5500 has an issue. After I fixed the tonearm—which used to get stuck halfway across—a new problem has emerged: when a record starts playing, the tonearm tracking doesn't work; it stays in place, causing the stylus to remain in the same groove and skip. Essentially, the tonearm isn't moving forward. What checks should I perform? Where are the sensor and the light bulb for the tracking circuit located?