The SWTPC 4K memory boards draw 1.5 amps each and you cannot put enough of them in the computer. While FLEX will run in 20K bytes of RAM it works better in 40K bytes. Here is a design for a simple 32K RAM board that draws about 0.15 amps. This board along with two 4K-memory boards will give you 40K bytes and the power supply will not be taxed.
The design uses a 32K by 8 static RAM in a 28-pin package. These RAM chips come in 300 and 600 mil wide dip packages. (Any speed will do, you can't find a 450 ns version.) The data buffer is a 74LS640 or a 74HCT640 octal buffer. The decode is a single 74LS139. There are no dip switches or jumpers; the board is fixed at address range 0000 to 7FFF.
This implementation was built on a 4.5 by 8.5 inch Vector board with holes drilled for the Molex connectors. The pins on Molex connectors are on 0.156 centers but have some movement if your holes are not exactly located. They are held in place by the wires soldered to them. (For the connectors with few or no connections you need about 3 dummy connections for hold it in place.)
This board was build with point-to-point wiring. You will appreciate the simple design after a few hours with your soldering iron.
Close up of front of board(100k JPG)
Close up of back of board (124k JPG)
32k Memory Schematic (JPG 85)
Michael Holley's SWTPC Collection Home Page