As I explained previously, 4KB of memory must be available to the PPU instead of 2KB. The memory chip (U4) must be replaced by a bigger one.
The pinout uesd by different memory chips is usually similar. This allows us
to solder the bigger chip where the small one was, with a minimal number of wires
and bent pins. The additional address line A11 must be connected to PPU pin #27. If
you use a memory chip bigger than 4K, connect all the remaining address pins (A12 and higher)
to VCC or ground.
On an old 486 motherboard, I found an As7c256 32K
sram chip. To make sure I did not make mistakes, I drew a small diagram to compare
the pinout of the 2K memory chip (LH5216) with this new chip.