dec /34a issue

I just brought up my /34a and apparently it's sick. It has one DD11-PK. Configuration is as follows: 1 - M8266 (A-F) 2 - M8265 (A-F) 3 - M9312 (A-B), M7859 (C-F) 4 - M7891 (A-F) 5 - Grant (D) 6 - Grant (D) 7 - Grant (D) 8 - Grant (D) 9 - M9302 (A-B), Grant (D) What works: Storing & retreiving various patterns from ram via the front panel works fine in all cases. Looping on CLR PC loops as expected Looping on BR . loops as expected Trap catcher works (first pass halts at 1030 filling ram, then a BR . loops as expected). The memory address test program fails though. It halts at 246 indicating a memory addressing error. R1 points to 422. Examining memory via the front panel shows the following: 420 420 422 177355 424 177353 426 177351 430 177347 So it looks to me like it is able to store 420 in 420, but nothing after that. I would normally think there is a problem with the memory board (M7891). However, I have replaced that board with 2 others, and all 3 boards fail at the same address AND with the same values. I find that likely to rule out the memory board as really being bad. In addition, I can deposit and examine values to locations 420 through 430 via the front panel just fine. It's my understanding that the KY11-LB puts data in memory via the unibus, so I would think this makes it somewhat unlikely to be a backplane issue. Is it a strong likelyhood that the problem is the cpu set itself then, as that's what would be writing the values to memory during the address test? And as I type this, I just noticed something interesting. The numbers stored in ram at 422 through 430 are the right numbers, just inverted logic. More specifically if you invert all the bits in 177355 you get 422, if you invert all the bits in 177353 you get 424, inverting 177351 gives 426 (all the latter values being what I'd expect). I'm guessing there's a dead inverter on the cpu set somewhere perhaps? But if that's the case, why does 420 get set to 420 correctly?? Any advice is most appreciated :) Jay West