During testing of a restored Beogram 4002 (5513) turntable, I noticed the RPM drifting over time. Replacing the original RPM relay and the 5k RPM trimmers fixed this issue. However, a new problem arose: infrequent, sudden drops in RPM.

This image shows the original RPM relay on the main PCB:

The relay design I created for the Beogram 4000 also worked in this model. Here’s the replacement board:

And installed:

I also replaced the original 5k trimmers (top-left of the relay) with these:

Lead extensions were necessary to install them upside down for easier access to the set screws:

Top view:

To investigate the remaining RPM drops, I connected my oscilloscope to IC3, which manages the DC motor feedback loop. To observe the signals during playback, I soldered jumper wires to specific points on IC3:

This setup allowed me to connect my 4-channel oscilloscope:

Using the single-shot button, I captured the signal traces when the issue occurred. This image shows the fluctuation at 33 RPM:

And the more frequent fluctuation at 45 RPM:

Trace assignments (refer to the circuit diagram below): blue - pin 1 (AC induction signal from the DC motor feedback coils), yellow - pin 6 (pulse width modulated output from the Schmitt trigger, indicating motor power), green - pin 7 (RC network determining the Schmitt trigger’s pulse width), and red - pin 4 (voltage the motor receives relative to the TR2 stabilized 8.5V output).

The Beogram 4002 (5513) uses a 3-pole DC motor, meaning each rotor coil is fully powered every six commutator contact changes. The aberrations in the pin 1 traces, occurring every six wave cycles, seemed connected to a specific motor rotation cycle. Cleaning the motor’s commutator and brushes reduced the frequency but didn’t eliminate the problem. Replacing the motor from a parts unit is a temporary solution until I find a permanent fix. This highlights the inherent challenge with DC motors in turntables: they depend on a precise feedback mechanism for constant RPM, unlike AC synchronous motors that simply respond to a sine wave input.