I decided to upgrade my c128 VDC to 64k. To test the RAM I wrote a Basic program that cycles writing 0 to 255 through all 64k. This was fine, but when I changed it to write the address high byte (stretch 0 to 255 across the 64k), I found location 0-16383 had taken on the same values as 16384-32767, and 32768-49151 were the same as 49152-65535. It sees the 16k Banks 0 & 1 as the same addresses, and Banks 2 & 3 as the same. I can write any two bytes, 16k apart, and the value will duplicate either direction.
At first this said to me that "A14" was stuck either on or off. I tested for continuity from each chip's pin of U23 & U25 back to the VDC, and for a short between any two pins of each IC. All tests passed. That's when I remembered RAM doesn't use 16 address lines. So now I'm stumped. Can the VDC malfunction so that A6 is stuck on or off with either CAS or RAS but not the other? Or can the problem still be somewhere else and not the VDC? Any ideas? I have a good multimeter and can borrow a scope but don't know what kind of signals to look for.