Lens cleaning and replacing

[fratres]

Having problems with read errors with the 400C laser of my V7, I tried to clean the lenses with glass cleaner, q-tips and alcohol. Most guides only give hints about the upper lens, but as the second lens (fixed in the sled beneath the upper lens between the coils) also gets dirty easily, I tried to clean it as well. After the cleaning the laser even worked worse, so I suppose I did something wrong, maybe pressed too hard. Taking a closer look at the lower lens I got the impression that it used to be coated (a darkpurple gloss at the sides and on the bottom side) and that this coat had come off in parts in the middle, leaving a yellow-purple mosaic on top of the lens. Suppose that´s the reason why the laser doesn´t work. Is that lens coated, are there any hints on how to clean this lens safely, and is this lens replacable without replacing the whole laser unit? It can easily be lifted and taken out after removing the sled with the coils, but can it also be replaced?