Beogram 4002 (5523): Replacement of the RPM Trimmer Light Bulbs with LEDs

After rebuilding the PCBs of the Beogram 4002 (5523) that I am restoring right now, it was time to replace the light bulbs that illuminate the RPM trimmers with LEDs. This is not only important to ensure long term stability of the Beogram, but also to improve the RPM consistency of the DC motor. The RPM trimmers are part of the DC motor feedback circuit that holds the RPM constant. Since incandescent bulbs emit heat the trimmers heat up during play, which can result in RPM changes. The much colder running replacement LEDs alleviate this issue.

This shows the extracted RPM panel from the back:

Removal of the covers reveals the bulbs:

After unsoldering the leads the bulbs can be pulled out and the LED boards inserted:

This shows the LED boards. They are drop-in replacements for the light bulbs:

These boards are available to other enthusiasts. Just send an email or use the contact form to the right.

The covers fit back onto the LED boards:

The LEDs illuminate the RPM trimmer indicators with a natural incandescent-like sheen while letting the indicator appear red. This is achieved by using red-green LEDs that are tuned to emit light similar to the spectrum emitted by a light bulb: