I had one of these old herculus MDA video cards that had artifacts on the screen.. random characters in a few places and some random attributes. That usually means bad video memory. The ram chips were soldered on in this case, so I unsolder all of them. MT (Micron) 4264s.

I was talking to Clay on the phone and he laughed and said ‘All of the micron 4264s go bad!’. Pretty easy to replace, but I was curious which ones were bad and how they were failing.

I grabbed an Arduino and built up a quick DRAM tester. Pretty easy to do given the interface of the DRAMS, and these particular ones have 4ms refresh intervals.

Sure enough, 4 of the 8 work perfectly, and 4 of the 8 have bad bits. You can see in the scope shot a couple of bad bits during a read bank filled with ‘1’s. Most of the bad ones had 400-800 bad bits (out of 64k), and a few had 3-4k.

It was also pretty easy to measure the actual access time, given that these are 120ns chips. It measured about 60ns from CAS fall, which is right on given the spec of 80ns max.