PDP-8E diagnostic help needed

Hi, I've got a PDP-8E which I've almost got working. Can anyone here help me figure out this remaining problem? As I examine memory, the address lights count up to 01111, then go back to 00000, instead of 10000. I can manually enter an address higher than 1111, but the 10000 and 100000 bits don't stick -- they go low, as soon as I hit the examine switch to step to the next memory location. I can manually load an address 1000000 or 10000000, and hit examine to see 1000001, 1000010, 1000011, etc..., but once I reach 1001111, it's back to 1000000. It was recommended to me that it might be the carry between E52 and E37, or the E38 input multiplexer for bit 7 (on M8300), so last night I socketed and replaced all three of those ICs, but I still see the same symptoms. I have extender boards, so can access M8300 during operation. I measured the carry line between E52 and E37 go low when I reach 1111, but the light for line 10000 doesn't light on the front panel on the next address, and I see the data from memory location 0000, 0001, etc. repeated, displayed as I continue to step through memory locations, as described above. Of course, if someone has a spare M8300 they would be willing to sell me, that's another option. Mark -- Mark G. Thomas (Mark at Misty.com) voice: 215-591-3695 http://mail-cleaner.com/