Beogram 4002 (5513): Replacing the RPM Scales Light Bulbs with LEDs and a Further Improvement of the RPM Stability

After rebuilding the DC motor and the main PCB of the Beogram 4002 (5513) that I am restoring right now, I replaced the incandescent light bulbs that provide the back light for the RPM trimmer scales in the control panel with SMD LEDs. This shows the replacement boards that can directly be soldered into the solder points of the light bulbs:

Here is an impression of the installed boards:

These assemblies are available to other enthusiasts. Just send me an email. This video shows how to install the LED boards.

The interesting aspect here was that the user accessible RPM trimmers in the control panel were the remaining part of the DC motor control system that I did not touch so far, and there were still some minor spikes in the last 24 hrs RPM measurement. So I was curious where these spikes originate and after replacing the bulbs I measured another RPM spectrum. This shows the four spectra that I measured so far in comparison. The top spectrum is the latest one with the LEDs installed:

The bottom spectrum with the large spikes was measured initially before I started working on this unit. The second spectrum was measured after rebuilding the motor. The third spectrum was measured after replacing the RPM relay and the RPM main trimmers on the circuit board. This one was already very good and mostly within the ± 0.05% 'corridor' that is allowed per service manual, but there were still some small spikes every few hours. After the LEDs were installed the spikes were gone, too. 

I am currently hypothesizing that the reason lies in temperature related effects. The original light bulbs get pretty warm and over time the entire RPM panel heats up. This may cause minute movements of the RPM trimmers that are in this panel, resulting in RPM drift that is subsequently compensated by the control system. This results in brief speed variations until the set RPM is established again. With regard to the question why this is not seen in all Beograms that still have the original bulbs installed (like this one, for example), well, I think that the trimmers can be in different states of oxidation and/or contamination, and that some react more sensitively to temperature changes than others.

At any rate, it seems that this Beogram's RPM issues have been cured. On to the remaining restoration tasks!