While debugging the PDP-12 at the RICM we found a mistake on sheet TC12-0-LTR in the PDP-12 schematics. The pins on the data cable between the TU56 and the TC12 labeled BT2 and FT2 should be BL2 and FL2. DEC wired the TC12 backplane per the incorrect schematic and grounded pin F06T2 instead of F06L2. The W032 on the end of the data cable has nothing connected to pin T, so this leaves the TC12 end of the Triax cable shield for data track 3 ungrounded. Data track 3 is the one on which we are having a noise problem, so this is a promising discovery. The 1969 and 1972 PDP-12 Engineering Drawings both have the same revision sheet TC12-0-LTR and the same error. Maybe all PDP-12s have this wiring error? -- Michael Thompson