No DC motor Beogram 4002 or 4004 restoration is complete without replacing the RPM indicator incandescent lightbulbs with LEDs. The purpose is not only longevity, but also DC motor stability. Since the user accessible RPM trimmers in the RPM indicator panels are actively participating in the motor control, their operating temperature is of concern. Light bulbs emit most of their energy as heat and therefore significant temperature changes occur. Replacing the light bulbs with LEDs lowers the heat load dramatically and therefore the trimmers do not change their temperature as much. This can significantly decrease RPM changes of the DC platter motor. See here for a summary of DC motor RPM issues and their cure.
So, after rebuilding the PCBs of this Beogram 4004 (5525) it was time for this procedure. Here is an impression of the original light bulbs in the RPM control panel:
The left one was exchanged at some point with a different type. The correct bulbs are difficult to find these days, one more reason to switch to LEDs. This shows my custom designed replacement boards:
These assemblies are available to other enthusiasts. This video shows how to install the LED boards. They fit precisely into the gaps in the plastic assembly that holds everything together:
The boards feature red/green LEDs tuned to emit light looking like incandescent light. I elected this design strategy to make sure that red photons are present for the illumination of the RPM indicators, which are made from red plastic. Amber LEDs such as used in some restorations of classic audio components when replacing bulbs are not suited for this task since they do not emit red light. LEDs are usually very monodisperse in their emissions, which makes them different from incandescent light that contains a wide spectrum of wavelengths.
The design of the boards permits to reinstall the bulb covers, retaining the original look even 'under the hood':
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