PDP-11/45: RK11 V - Checksum issues

Decided to use the disk utilities provided in PDP11GUI to write a fresh RT-11 pack. PDP11GUI successfully assembled and downloaded a driver, then took a little under 2 hours to download the pack image over the console serial line and write the pack without indicating any errors.

The resulting pack fared no better at boot than any of my existing legacy packs, however. I attempted to verify the pack via PDP11GUI, and noticed that the controller was indicating checksum errors for every sector on cylinders 64-127 and 192-202 (that is, whenever bit 11 of the RKDA was set). Tried this on several other packs, including the RKDP pack which had previously booted, and found that these errors were returned for these cylinders on all packs, so definitely a controller or drive issue, and not specific to any particular pack.

Verified that bit 11 of the RKDA could be read and written normally, and that when so addressed the RK05 drive would mechanically seek to the correct cylinders. Programmed some format-mode reads (these just return sector headers) manually via the front panel, and verified that the sector headers were being returned correctly from disk for the affected cylinders.

Tried a few flipchip swaps to see if the affected bit would move: B18 and B19 (adder; sheet RK11-C-14; no change), A21 and B21 (RKDB; sheet RK11-C-10; no change).

Programmed some all-mode reads (these return preamble, header, data, and postamble) manually via the front panel. These showed that during reads of affected cylinders, bit 11 was stuck always on. When reading unaffected cylinders, bit 11 turned on and off normally. So it seemed bit 11 was "leaking" from RKDA to RKDB.

Seeing this, tried a few more swaps: A15 and B15 (internal bus, sheet RK11-C-20, no change), A23 and B23 (RKDB data path; sheet RK11-C-21; bingo!). Stuck bit went away on this last swap. So it looks like a failed 7401, E2 on the M149 in slot A23. Pulled, socketed, and put some 7401 on order at Jameco where I can pick them up tomorrow on my way in to work. Getting closer!