A Beogram 4002 (5501) turntable, previously restored, began experiencing a problem where the OFF function would randomly activate during playback as if the button was pressed.
Prior to delving into the solution, the circuit diagram is presented for reference.
Transistor TR17, within the ‘Electronic Switch’ section, is responsible for initiating the turntable’s OFF function. When its base is grounded, the collector rises to approximately 18V, causing the arm to lift and return to its resting position. This can be triggered in three ways: through two mechanical switches – the “End Switch” (ES) located beneath the carriage and the “»” key on the keypad – or via the end groove detection system (“RUN-OFF STOP”). The End Switch activates when the carriage reaches its furthest left position.
The end groove detection mechanism utilizes transistor TR20. When the end groove is detected, TR20’s base is pulled high, grounding its collector and activating the OFF function.
An ‘analog rotary encoder’ forms the core of the end groove detection. It relies on light from bulb (OIL1) passing through four holes in the carriage pulley, creating fluctuations detected by a photoresistor (OR2). These light pulses briefly ground OR2, interrupting transistor TR21 and raising its collector voltage to around 19V. These voltage pulses charge capacitor C33, which, in conjunction with resistors R88/89, pulls up the base of TR20, activating the OFF function. Notably, bulb OIL0 only illuminates when the tonearm nears the record label, preventing premature RUN-OFF STOP activation between tracks.
The system avoids triggering RUN-OFF STOP during regular playback on the last track, even with OIL1 illuminated, due to the network of C34, R91, and D30. This network discharges C33 in the absence of light on the sensor. While the pulley rotates slowly during track playback, discharging dominates. However, rapid light flashes from the end groove tip the balance towards charging, activating TR20.
Returning to the random stop issue: After confirming the functionality of the ES and OFF switches, signal tracing from TR17’s base into the RUN-OFF STOP circuit revealed the following oscilloscope readings:
The traces represent:
- Yellow: TR17 collector (trigger for measurement)
- Blue: TR20 base
- Green: TR21 collector
- Red: TR21 base
These readings highlight random fluctuations in TR21’s collector voltage before the OFF event. These fluctuations correlate with the signal at TR21’s base, indicating its proper function. The conclusion: OR2 was randomly shorting to ground, causing voltage spikes sufficient to charge C33 and trigger TR17 when the voltage threshold was exceeded.
The diagnosis pointed to a faulty OR2 sensor. Since the specific B&O part was unavailable, a 3D-printed replacement for the entire bulb/sensor assembly was designed. Building on a previous LED bulb replacement, the new design replaced the encoder system completely.
[Image descriptions: A series of images showcasing the design and installation of the 3D-printed replacement part, including the LED, resistor, and photoresistor. The images depict the assembly process and the final installed sensor.]
With the new setup in place, signal measurements at TR21’s collector during LED illumination matched the expected behavior. Testing with records confirmed the repair’s success.