Optical pickups
Optical pickups are the moving part of CD modules and the weak link of CD systems.
They consist of a laser diode (often wrongfully convicted of failure) and driver, fixed and coil-actuated optics, and a photodiode array.
Increasing laser power may make CD drives work again by making the light shine through dirt and contaminants on optics, but it actually uses up the diode faster (can even kill it in a matter of minutes). A real solution is to clean all optical elements in the beam path, which involves taking the pickup apart.
Adjusting the pots located on the CD module can help in some cases, even if those settings shouldn't drift over time.
Some might find more convenient to replace the whole pickup. They can still be found on eBay for around $30. Some sellers re-package and advertise the pickups as exclusive, original replacement parts for consoles, at ridiculous prices.
NeoGeo CD Front loader
Original reference: KSS240 (A)
Compatible models: The Mega CD's one
NeoGeo CD Top loader
Original reference: H8147AF
Compatible models:
NeoGeo CDZ
Original reference: H8151AF
Compatible models: