Although the PS2 is generally my console of choice for casual gaming, the original PlayStation 1 still has its uses. It still has use for multiplayer gaming thanks to its built in Serial I/O port. And with the recent release of the PSIO, the Parallel port can be used to make your system completely solid state.
This is a good thing too, as many of these consoles are almost 20 years old - which is a long time for a disc based console. Disc drives don't last forever, and for most of the PS1's I come across the cd drive is usually the first thing that has gone bad.
Replacement PS1 laser assemblys are fairly plentiful but they don't have the quality or longevity of the OEM part. Still, it is a cheap and easy fix to breath life into an old PS1 you may have lying around.
If you want to keep your console original, you will have little choice but to salvage another CD unit from an existing console. There are about 29 different revisions of the original Playstation console, and 25 for the original model and 4 of a complete redesign known as the "PS One". Throughout these revisions Sony changed the laser assembly about 6 times.
The Lasers are numbered as follows:
KSM-440BAM: SCPH-100x - 103x PSOne
KSM-440ADM: SCPH500x - SCPH700x series
KSM-440AEM: SCPH750x - SCPH900x series
KSM-440ACM: Replaces AAM/ABM
KSM-440AAM / KSM-440ABM: 100x - 300x (Rare, mostly only found on launch models)
Of these models, the ADM/AEM units have longer cables and are interchangeable, the others have short cables. So if you want to use a BAM laser in a original model PS1 you will have to wire an extension or cut/mod the metal casing.
Beware of anyone claiming that they have an official Sony replacement they can sell you. Most of them just clones. The best way to tell a clone is to compare what your buying to an original drive - official Sony parts have smaller prints on the underside of the drive - most of the clones just have a few block letters/text.
Flipping over the console you'll notice that Sony keeps things simple - 6 screws and the top cover lifts off. Once that's done all that is left is to loosen the the 2 cables and swap in the new laser.
Once that's done, the top cover can be lifted off. Simply lift the drive off the metal pegs and loosen the ribbon/power cables.