[sdiy] OBXa Memory problems

[gm]

I'm currently dealing with an OBXa that was working, but is now 
exhibiting memory problems.

The ADC is working:  all switches & pots can be used to set a patch. 
Holding down Write & selecting a patch location will let the patch play, 
but as soon as the patch is reselected or another patch is selected, all 
patch data goes back to random.

Some givens:  all power rails are clean and correct; all address & data 
lines are present; ROMs, RAM and CPU have been swapped out with no 
change.  This unit is one of the early upgrades, with the piggyback 
memory board, running version XAC0 in 2716 ROMs.

Since all ICs are socketed, I swapped out A117 and A115 and tested the 
memory protect switch -- switch grounds A117 pin 1 as it should when 
engaged, and releases ground when turned off.

I have the feeling I'm missing something stupidly obvious here, and 
would appreciate any pointers as to where I can look next.

Thanks.
~GMM