Beomaster 8000 Display Brightness Adjustment for New LEDs

This post describes what I did to adjust the brightness of my refurbished beomaster 8000 display units (see previous posts) to the brightness of the original displays (as compared side-by-side with my second 8000):

After studying the circuit diagram for the display PCB (#8) I tried to adjust the brightness of the display with trimmer R15. Unfortunately, even fully turning it down only reduces the display voltage from the originally prescribed 5V to about 4.5V (measured at TP 14). This did reduce the brightness a little, but by far not enough.

This led me to change R14 from 4.7k to 3.3k, and R16 to 1.5k. This pushed the voltage range of R15 down to about 3.6V-4.7V, while keeping the voltage divider intact (to not affect the "reset" circuit). Consequently, the supply voltage for the LEDs also dropped. This allowed a significant reduction of  the brightness. Adjusting the display voltage to 3.85V yielded a brightness similar to the original displays (maybe a bit higher, but I always felt the original displays are a bit low for daylight situations, i.e I decided to leave it at that; if desired the new voltage divider allows to go considerably lower with the intensity, but care needs to be taken to avoid dropping the keyboard strobe below ~2.7V). Here is a cutout from Diagram 3 from the service manual showing the discussed components:

It is interesting to note that R14/R15/R16 also control the strobe voltage for the keypad (measured across R24,27 or 30). Accordingly, my 3.85V setting at TP14 reduced the keypad strobe pulses to ~3.12V (from an original 4.2V). Here is a screen shot from my oscilloscope (after modification):

It turns out that this new lower voltage does not cause any trouble. The reason for this is that the 6500 microcontroller is based on TTL logic. TTL logic accepts HIGH signals down to 2.7V. Turning R15 fully low pulls the strobe below 2V, and the keyboard consequently stops working. This demonstrates that there is enough safety margin if the strobe is at 3.12V, i.e. no further modifications are necessary.