The Beogram 4002 (5524) turntables I’ve been repairing had a recurring issue: during the start-up sequence, the arm wouldn’t consistently lower at the 30 cm mark. Occasionally, the RPM would unexpectedly switch to 45 right after pressing start.
Troubleshooting was challenging because the problem was intermittent. I replaced all the electrolytic capacitors, the RPM relay, and trimmers, as detailed on my blog. To test, I installed the boards in my own Beogram 4002 (5513). Initially, everything functioned perfectly, and I played a few records without any problems. However, the arm began lowering prematurely before reaching the record’s lead-in groove. After several attempts, the RPM started shifting from 33 to 45 as soon as the carriage moved after pressing start, all while the arm continued to set down too early.
To identify the root cause, I took measurements to confirm the record detection circuit was functioning correctly, which it was. My attention then turned to the position sensor circuit, where I discovered an anomaly.
[Image of circuit diagram]
Here’s how this section of the circuitry operates: The position indicator plexiglass bar, marked with black lines, controls TR17. When a black mark passes between 4IC1 and the IR diode 4D1, 4IC1’s resistance significantly increases. This causes the voltage at TR17’s base to drop, turning it off. Consequently, the collector voltage is pulled up to 21V through R85. This, in turn, pulls up the base of TR7, enabling C20 to charge via D22 and D23, which are at approximately 15.5V from TR6’s collector when the sensor arm detects a record.
However, this charging process creates a brief but strong current surge through D22/23, dropping the voltage at D22’s cathode to nearly zero for about 50 milliseconds. This negative pulse activates the arm lowering mechanism, causing the arm to lower when the bar indicating the 30 cm set-down point passes between 4IC1 and 4D1.
When no black bar is present, TR17 remains on, keeping the collector voltage at 0V and preventing the arm from lowering.
The premature arm lowering was traced back to TR17 not fully turning on, resulting in an ambiguous voltage of 10-13V at its collector instead of a clean 0V. Measuring the base voltage at TR17 while 4IC1 was covered by a black marker revealed a lower than expected voltage of 0.63V, compared to 0.69-0.7V on a properly working PCB. This discrepancy highlighted the circuit’s sensitivity and how certain tolerances could lead to an unclear signal at TR17’s base.
To rectify this, I replaced the 4.7k pull-down resistor R86 with a 10k resistor. This change reduced competition with TR17’s EB path, allowing for a stronger pull-up via 4IC1. After installing the 10k resistor, the system functioned correctly and consistently, with a stable 0.7V measured at TR17’s base.
It’s worth considering why the RPM occasionally changed during this malfunction. The connection to IC2 via R46 provides an explanation. When a record isn’t present but the wide 45 RPM bar passes between 4IC1 and 4D1, IC2 can be activated. In this scenario, TR8 is off, allowing voltage to build up at IC2’s base if TR17 is also off (resulting in 20.5V at the collector). It appears the 10-13V present at TR17’s collector is sufficient to trigger the RPM switch when no record is detected, as is the case before the detector arm reaches the record during the START process.
This experience highlights the intricacies of analog control systems. While a microcontroller-based design might have simplified things, the Beogram 4002’s lack of such technology ensures its longevity. Unlike microcontroller-dependent designs, like the Beogram 8000/8002, which become obsolete due to proprietary firmware, the 4002’s design can be maintained indefinitely.