This thread is so old, the schematics do not load anymore, so I am attaching the schematic that I found in another thread. If there is a more recent schematic, please re-post it to this thread for others.
The mod only does DC restoration for the Y luma signal, but the mod does not also add DC restoration to the Y-R, Y-B, Y-G chroma signals that are attached to the CRT's G1 grids. So the described mod would improve luminosity consistency, that is, the black levels and white levels would be consistent, but chrominance consistency will still vary depending on the amount of average red, green, and blue in the scene. Since Y-R, Y-B, Y-G are independent signals that are AC coupled to the CRT's G1 grids, the level of color saturation for each color will depend on the average amount of red, green, and blue in the scene.
Has anybody tried DC restoration for the Y-R, Y-G, Y-B chroma signals that the GE Porta Color sends to the CRT's G1 grids?