Cromemco System Three PerSci 8" Drive Problem ?

Question I have a CS3 with a PerSci dual 8" internal drive that does not recognize/respond when a disk is inserted in drive A, but drive B does respond when a disk is inserted. Drives C and D are empty. The PerSci dual 8" drive is the model with the one belt/on-board controller shared by the two drives. When working correctly, the drive has a mechanical lock that detects the insertion of the disk, engages the read head, and holds the diskette in place. No latch arm needed. There are 4 buttons on the front, one for each drive (A,B,C,D) - If you push the corresponding button and there's a disk in the drive a motor ejects the diskette automatically. Pretty cool for a 1979 micro. Do the diskette detect sensors go bad on these? Next I tried and failed to get a RDOS monitor prompt, although I followed the dip switch settings as instructed in the manual. The drive controller has the RDOS ROM. I did all of the standard things, re-press the IC's, re-seat the cards, etc. The system is in very nice shape, no corrosion or scratches. I feel that I have nailed down the problem to being either the drive, the drive controller, or the terminal. Through a process of elimination I will attempt to replace each component in question (except the dual drive, I will probably put in Tandons instead if I can. I have a head start, the entire system came together in one donation so I was able to record how it was all cabled together, and I can assume it worked once as-was. Also included in the lot was Fortran IV and CDOS software. The printer is a Texas Instruments Omni 800 / 810 RO Terminal. I was told the system cost $15,000 new in 1979. A pretty sweet personal computer. PICS: http://www.vintagecomputer.net/cromemco/system_three/