Electone FX-20 - custom memory board (not working)

The Electone saves on memory cards which are a 2kb SRAM with backup battery. I had some 512kb SRAMs lying around, so I decided to make a custom memory unit that would allow the user to select one of 256 "banks" (upper address bits) and show the organ the other address and data pins. The memory is backed up with a supercapacitor instead of a battery, so every time the organ is turned on it recharges. With the exception of the supercapacitor, all the other parts used here were spare parts or salvaged, I didn't buy anything!

It was a great idea in theory, but not so much in practice--the bank-selection circuit works, but the organ shows an error when trying to read or write. Guess I'll troubleshoot it later.

The two digits are incremented separately with the buttons on the right and show the number in hex on the LED displays. The button on the left resets both. It was the simplest scheme I could come up with using only a handful of logic chips.