Finally got the system processor (TSB) back up to snuff with no parity problems. Looks like the upper 8K of core suddenly got bit 12 stuck on. That's easy enough to fix, barring a core plane failure there's only about 5 discrete components per bit on the card. More exciting though, I got a 7906 drive up and working on the system via a 13037 rackmount box. Woohoo! The good news is the fixed platter (head 2) formats fine and passes diagnostics. The removable platter (heads 0 & 1) I'm not so lucky. I am not sure if the problem is the removable media, or the upper two heads. I'll grab another piece of media shortly and find out. This means that shortly I should be able to supply anyone who wants one - a 7906 disc cartridge with a complete set of HP diagnostics! I really need to boot up DOS so I can dump the diagnostics library to bootable mag tape too. Jay West