Beogram 4000: An Unusual Keypad PCB

This post is about an unusual keyboard PCB of a Beogram 4000 that was recently brought to my attention. Normally, the two position scale illumination bulbs in a Beogram 4000 are connected in series between the 24V rail and GND. This shows the bulbs on a standard board.

Below the PCB is the circuit layout as shown in the service manual. The way the bulbs are connected in series creates a situation where the polarity of them is reverse.

This is perfect for the installation of my (polarized) LED boards for replacing the bulbs:

When installed, these boards need to point in opposite directions to illuminate the entire position scale, i.e. their polarity is reverse relative to each other, which matches the above standard circuit layout, and both boards light up:

BTW: The red LED is the back illumination for the 33 RPM trimmer.

However, recently I received an email from another B&O enthusiast whom I supplied with two of these LED boards for his Beogram 4000 restoration, stating that only one of the boards was working in his Beogram! Very unbeolovely! After some more communication it turned out that his Beogram 4000 had a different keypad PCB, where two 24V bulbs were connected in parallel between the 24V rail and GND, instead of the normal two 12V bulbs in series.

This arrangement resulted in the same polarity for both of the bulbs, and this caused one LED board to be connected in reverse polarity, i.e. the LEDs did not light up. This situation is shown in the top-right picture in the slide below. 

He sent me a picture of this strange PCB (shown on the left). After a bit of head scratching it occurred to me that my LED boards could still be used if a small modification is made to the board:

By (1) removing one of the jumpers that connects one of the bulbs to GND, and (2) moving the 24V terminal to the solder point where the jumper originally connected towards the bulb, the standard in-series circuit was restored.

After this change was made, both LED boards started working (bottom right picture).

Beolovely again!