PDP-11/05 Control (M7261) won't deposit

Working on a PDP-11/05. The front panel appears to work, I can step through and examine addresses in memory, and I can run the CPU and halt it. Deposit, however, does NOT work. When the deposit switch is flipped, the LED's show the correct value, but examining the address again shows no change to memory. I know that this board is the problem, having swapped in a module from a working computer. I can, with the good board, toggle a test program into core. Then, shut it off, switch in the bad board, and the program (still in core) runs, and I can examine all areas of memory and see it correctly. I've only begun to glance at the schematics for the control board, but I would assume there is some kind of write buffer that's not working correctly. I'm going to continue to try to figure this out, but without a card extender, it's very difficult to troubleshoot. Anyone have any ideas on where to start? Failing that, does anyone have an extra M7261 available? :) -Ian