Good news from the "doubly-unsupported"
MB4,1.
While there still is no solution for the brightness control of the display on this machine, I found a way to get back display backlight power-off while semi-sleeping (utilizing NoSleep in v1.5.1) or when the lid is closed.
Unfortunately, this is a tiny hardware modification that I´ll document later today. It can be done with medium soldering skills and fits nicely into the machine so everything can be assembled back to the original closed state. Total time to do this: Around 1h, with parts costing below €1,-
So here we go with the circuit for the modification.
The BOM consists of
1x 74HCT04 hex inverter logic IC
2x 1kOhm resistors (any wired non-SMT type will do)
The idea behind the circuit is to use the SMC-generated LED PWM signal that ramps from off (floating) to 100% on (3.3V) whenever either the lid is closed, during startup or when macOS tries to enter sleep mode (which is prevented by the NoSleep extension because we could never wake up after that). This signal could be used directly as a driver for the PWM LCD input, only it has a reverse logic for that. Screen backlight is on when the signal is 3.3V and off at 0V.
So we need to invert it at least 1x (but in case of optional additional R-C timing that might have been we do it 3x here).
Pls. excuse the free-hand drawing of the circuit (and Euro style for Rs and IC gates) - I did not like to fire up an ECAD system on my weekend for this li´l circuit ;-)
I skip all steps needed to get to the right place on the PCB, so we dive right in:
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