Need Dynamic Ram Expert Help

I recently purchased a 1995 vintage piece of test equipment that utilized either 1M or 4M dynamic ram modules (70nS) of the old 30pin SIMM configuration. The embedded processor board that used this ram is based on a Motorola 68020 controller and designed to read and write data to the DRAM using a 16bit wide bus. The board is designed to accommodate two banks of dram. Each bank will consist of two 8 bit simm drams. I?m sure many of you remember this typical motherboard configuration of this era. I am attempting to determine why I cannot get 4M memory to work is this unit(I?ve tried three types). There is a dip switch which you select either 1M or 4M memory, but the ram self test fails miserably when using

4M modules. The 1M modules work just fine. I?ve studied the R/W timing signals (RAS/CAS, etc) as well as refresh timing and all looks to be OK, with lots of margin. What I am not sure of is how to determine the signal integrity, such as overshoot, ringing, etc... One thing that I did was to determine if my power supply was holding up during the R/W process, understanding that there is considerable power requirements for dram during R/W. Using a scope and testing the power supply while triggering off of the R/W lines, I could see no sagging or spikes occurring. I also thought initially that there could be some logic issues when the switch being set to the 4M position. So, what I did was to use some insulating tape to tape up the A10 PCB contact on the 4M simms and pull up the A10 to +5 on the simm to commit this address line. Then set the switch to the 1M position and see if the dram would then work as a 1M part. No luck, still failed the memory test. I could use some help understanding the difference(s) in the dram (other than size). If anyone would like to help out, I would certainly appreciate it as I am quite lost at the moment. Thanks Jim