It is a known fact, that some digital signals are interfering with the video output signals inside the VIC-II chip of the Commodore C64. This results in a typical vertical stripe pattern in the monitor output of the C64. The main culprits are AEC and PHI0 (VIC-II pin 16 and 17).
The LumaFix64 addresses these issues by inverting AEC and PHI0, then attenuating them with potentiometers and then the signals are coupled into the S/LUM signal (VIC-II pin 15) via a small 47pf capacitor. The potentiometers can be adjusted to minimize the interferences of the signals mentioned above.
Start by removing your VIC-II chip from your Commodore 64 and insert in into the socket of the LumaFix64.
Socket the LumaFix64 into the VIC-II socket on your Commodore 64.
Dial both potentiometers counter clockwise. Turn your Commodore 64 on and slowly dial both potentiometers clockwise until you achieve a clean image.