BeoloverRPM: Precise RPM Measurements on Beograms

*************   The new BeoloverRPM is available directly at the Beolover Store!  *************

An essential concern with turntables is their RPM accuracy and stability. Variations or inaccurate RPM can seriously diminish the listening experience. Unfortunately, many DC-motor Beogram 4002 and 4004 models develop significant RPM stability issues as they age and need their motors rebuilt. For this reason I developed the BeoloverRPM device that allows to qualify the RPM performance of Beograms for extended periods of time. This allows to verify if there are intermittent RPM issues etc..., which are notoriously difficult to diagnose and demonstrate just by listening to records.

Like all custom parts featured on this blog, the BeoloverRPM device is available to other enthusiasts. Just send an email to beolover@gmail.com or use the contact form.

I recently updated the BeoloverRPM with new capabilities and a new, more professional look. The new BeoloverRPM can be placed on the aluminum panels for a quick and convenient RPM check

but it can also be configured for frame mounting when working on a Beogram:

This was achieved by a design where the frame adapters can be removed from the main device:

The BeoloverRPM comes with two frame adapters that have different spacing between the 'legs' that straddle the enclosure wall. B&O used slightly different enclosure designs with varying wall thickness.

Another significant update is that the BeoloverRPM now has two data acquisition modes:

The modes can be selected via buttons on the device. The 'slow' mode performs similar to the previous designs. It shows the RPM and min/max values on the display, and sends a RPM snapshot to the serial port every 10s (faster if the RPM changes quickly).

The 'fast' mode is new: It graphs all datapoints as they come in whenever a rib passes under the sensor. The display on the BeoloverRPM shows a rolling snapshot of the last 64 datapoints. The data is also sent out via the serial port. Here is an example measured on a Beogram 4004:

The main graph shows the data output from about 60 platter rotations. The graph shows a repeating pattern modulated by a sinusoidal envelope. Closer inspection yielded that the pattern repeats every 24 points, i.e. everytime the platter makes a full turn (there are 24 ribs on the platter). This can be explained by minute changes of the rib spacing (it varies by about 0.1%).

The sinusoidal modulation is a result of the feedback based RPM control of the DC platter motors of the later Beogram 4002 and 4004. Feedback control always results in an oscillation of the actual value around the set point. And that is what we see here. The analog feedback mechanism used by the later Beograms is based on time constants defined by capacitors and resistors, and this results in sinusoidal RPM variations.

Fascinating stuff!...;-).

I made a short video about the new BeoloverRPM. Enjoy!

Terminal Software:

For Apple Macintosh I tested WCHSerialPort. It is available on the App Store for free. A nice feature of this app is that it can directly 'print' into a file, i.e. the data can immediately be imported into a graphing app or excel for visualization.

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