nesdev.org

so, I followed this guide:
http://www.dbwbp.com/index.php/13-elect ... -snes-cart

to upgrade the SRAM of a 1A1M-20 board from 16 to 64kb, and I'm getting a WEIRD problem
Anything that gets written to the SRAM is offset by 4.

I fill zeroes, I get 8kb of solid 04s.
I fill FF, it comes out as FF, because that's as high as it goes.

What might cause that?
Could the capacitor next to the SRAM have been damaged when I mounted it on the reverse of the board to fit the larger SRAM?

As a random guess, you may have a bad solder joint on the D2 pin. Or maybe damaged the RAM.

Another possibility is that you broke one of the solder pads when removing the RAM and installing the new RAM. maybe one of the pads isn't making contact with the trace...

Definitely check the D2 pin. Check it for continuity with the D2 contact on the cart edge, then check it with continuity (or resistance) to Vcc. My guess is it's either floating or somehow pulled up to Vcc. Another thing to test would be to do a sequential write test where you write 0x00 to address 0x00, 0x01 to address 0x01, etc. up to 0xFF, then repeat that until you've filled the whole chip. That's a good way to check not only the data lines, but the address lines as well. If D2 is pulled high, you should see 0x04 0x05 0x06 0x07 0x04 0x05 0x06 0x07 0x0C 0x0D 0x0E 0x0F 0x0C 0x0D 0x0E 0x0F and so on.

I desoldered the chip and found the trace was lifted and hanging on by a bare bit of solder mask.
damn these fragile single-sided boards.