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Thursday, September 10, 2015

Beogram 4000: Adjusting the AC Motor Voltage and Measuring the Voltage Differences Between the Phases

Today I adjusted the AC motor of the Beogram 4000 that I am currently restoring. The manual prescribes to adjust the main phase of the motor to 6V (RMS) and the frequency to 42.3 Hz for 33 RPM. This is easily done with an oscilloscope. To see both phases the probes can be simply connected to either side of the phase capacitor:

Adjustment procedure: First set the 33 RPM trimmer on the control panel to its null position (i.e. vertical). This ensures that the adjustment of the RPM offset potentiometer on the main PCB can now be adjusted to yield a 42.3 Hz motor signal when the control panel trimmer is nulled out. This ensures that the user can both trim the pitch for the same frequency deviations in + and - directions. Once the frequency is set the MOT potentiometer can be adjusted. It is important to do it in this order, since the frequency affects the amplitude of the signal. We are working with an analog oscillator here...;-).
Once everything is adjusted the oscilloscope should show about this:

The yellow trace is the main phase and the green trace is the 2nd phase. It is interesting to note that the 2nd phase has a smaller 5.3V (RMS) amplitude. This concludes the adjustment.

When switching the motor to 45 RPM one sees this:

Note that the 2nd phase is now much larger, about 8V (RMS), while the main phase dropped a bit to 5.6V (RMS). The manual also wants 6V (RMS) here, but there is nothing one can do about this since the MOT trimmer determined both frequencies, and 33 RPM is probably for most people the much more important speed. 

There is an interesting detail I noticed about the RPM trimmers on the control panel. The 45 RPM trimmer is nulled when the correct frequency of 57.3 Hz is adjusted with it:

When I took the control panel apart for converting it to LED illumination, I noticed that the red inserts that allow the adjustment of the trimmers with a screw driver are different for 33 RPM and 45 RPM: The driver slot is at an off-angle for the 45 potentiometer, while for the 33 trimmer it is symmetrical. Again, the designers gave the 33 speed priority and made a small compromise for 45: The consequence of this off-angle is that the 45 speed can only be adjusted by about a quarter turn to higher speeds, while one can go about a half turn to lower speeds. All this was probably dictated by the need for using standard resistors and trimmers in the Wien oscillator feedback circuits, which prevented them from designing the circuit in a way that the 45 trimmer would be exactly in its center position for an exact 57.3 Hz.
With modern resistor values one could easily fix this since nowadays one can buy resistors with almost any value. Apparently not so in 1973. Of course if one would now fix this by replacing the resistors, the trimer slot would be at an angle, which would not look good. What a conundrum! Will I be able to sleep tonight?? This is Beolove!..;-)

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