LED Module 1602 cannot be dimmed

[Crypto71]

Hi,

nearly finished conversion of an old electric organ into a MIDI-device.

My problem now is:

I am using a LED 1602 module. The core display something readable (needs further testing as I do not know yet if all my DINs etc. work properly), but I cannot use P1 of the core module to dim or brighten the LED module. Simply nothing happens. I can set the contrast though. It works well.

What am I missing here? What could be wrong?

Thx for any hint,

Crypto.