Older CD-ROM refuses to read media - ideas for repair?

I have an old project laying around that I just spent another two hours on .. It is an 1993 external parallel port attached CD-ROM. A company called "Storage Devices Inc" made it, and IBM resold it. The model number is SCD-683. There are some traces of it if you search the web, but not much interesting. I have the drivers so that is not a problem. The CD-ROM is a 1x SCSI unit that uses a caddy. There is a parallel to SCSI bridge board in the enclosure based on the NCR 53C80 chipset, which was well known back then. The CD-ROM does show up and talk when used on an XT running PC DOS 3.3; it responds to commands like eject and it lets me see its error counts using a utility program. The problem is that it refuses to read any media that I give it. The drive is very clean - I removed the top and had a look inside the mechanism to be certain. It is a caddy loading model so I can't tell for sure that the head has clear access but everything seems to be operating freely. I'm using old pressed aluminum discs (OS/2 Warp) which should be fine in any CD-ROM device. I have tried cleaning the heads with a special CD that has the magic fibers on it, but nothing is helping. (I even reached in gently with a Q-tip and isopropyl alcohol, but that did not change anything.) I'd like to see this thing run - it has a carry handle on the side! Unless I come up with another idea to try the only way that is going to happen is if I swap the drive out. The original drive is an IBM CDRM00101 and the driver is looking for the ID string. So a drive swap with a different SCSI drive will probably require patching the device driver. So, any ideas on what I can do to clean the head further? (It might just be hopelessly out of alignment - I don't know the history of the drive.) Mike