PDP-8/e M8310 Help

Hello folks, I have a PDP-8/e where I've tested all the boards in a friend's PDP-8/e and they are all generally ok with the exception of the M8310 which did not work in his machine. I started to do some very rudimentary analysis of the what isn't working. I did the following. 7100 CLA clear AC 7040 CMA (complement the AC) 7040 CMA 7040 CMA 7040 CMA The complementing worked fine. I set the switch to AC and single stepped through the little program and watched the all 12 lights alternate on and then off. So far so good. I then did this: 7100 CLA 7001 IAC (increment AC) 7001 IAC 7001 IAC 7001 IAC And nothing happened, no incrementing of AC took place. So I may be onto something here. I then looked at the link bit which I believe is viewed by setting the switch to STATUS and looking at bit 0 "LINK" I did the following program: 7100 CLA (Clear Link) 7020 CML (complement Link) 7020 CML 7020 CML And I found that the link was not alternating. The lamp was lit at all time unlike the same test with the CMA. I then read in the Maintenance Manual Volume 1 on page 4-7, Item 11 which gives the following symptom "CPMA, MB, PC, or AC do not increment" And the likely cause of "CAR IN L is always high to the adder. It should be generated for ..." and presents a series of events that should cause the CAR IN L to assert. Now two questions. 1) It appears that CPMA, MB and PC all do increment but the AC does not. Does this help me to narrow down which of the events a. through f. listed might be my problem? 2) Is the link bits inability to complement help narrow down the issue? 3) Might anyone know what the fault is? 4) Are there any other steps you might suggest I take to further narrow down the fault? Thanks Eugene