2. Building The DIY RAM Board
The DIY RAM board is simple - it contains 2MB of SGRAM, as well as an optinal PLCC-32 socket for modern flash/EEPROM BIOS chip.
- PCB (4-layer, 1.6mm) - Gerber File
Final PCB is slightly different from this photo thanks to various improvements
- Top pin Header: 20x4 Pin Header, Female, 2mm pitched. Plastic part has a height of 6.34mm
- Bottom pin header: 20x4 Pin Header, Male, 2mm pitched.
I bought mine from AliExpress. These are 4x30 so I had to trim them to 4x20. For the female connector, pluck out the pins on the 21th column with a pair of pliers and cut the plastic on that column with a sharp blade or side cutter.
It is also possible to use 4 strips of 20-pin, 2mm-pitched, single row, male pin headers for the bottom connector, if you can't obtain the 4-row version. However the same can't be said for the female pin header since the rows won't fit next to each other.
- [2x] 128K x 32bit x 2 Banks 100MHz SGRAM.
- I chose the KM4132G271BQ-10. There are other choices such as the MB81G83222-010.
- [16x] 0.1uF, 6.3V SMD ceramic capacitor. Imperial 0603 size.
- PLCC-32 socket, through hole version, post pitch 2.54mm - Link
- PLCC-32 flash chip. Minimum 512KB. I chose SST39VF010 since it's currently most available and affordable. You can go all the way up to the 4MB version of this series.
For the male pin header, you need to make sure the pin 46 on row 3, column 6 is snipped on the mating side. Otherwise the connector won't be able to go in. You can go in with a pair of side cutter, or use a syringe need to rock it back and forth and fatigue it.
For the PLCC-32 socket, check the bottom of the socket. if there is any support sticking out that makes the socket body sit taller, cut it flush with a side cutter, since the socket has a good chance of interfering too much with some SMD components on the vide card itself.
Pay attention to the silkscreen and solder components on the correct side of the PCB.
All connectors and socket are on the [Inside] of the board.
You can start by soldering on the RAM chips and the SMD capacitors around them. These RAM chips are drag-solder friendly, so it wouldn't hurt to take this as a chance to practice.
Inserting the pin header connectors into the connectors on the video card. Then lay the PCB onto the pins. Solder the corners of the pin headers while pressing down on it to make sure the PCB is snug. Then proceed to solder all the rest of the pins. Due to the fine pitch of the pin header, it's possible to make bridges. Make sure you double check the work before testing.
Do NOT attempt to remove the board from the video card without soldering a lot of the pins, as you might end up pulling the few soldered pins out of the female connector. It'll be a hassle to reseat the pulled pins back in.
Remove the add-on board with the soldered pin header connectors, flip it over and seat the PLCC-32 socket into the [Inside] in the correct orientation. Install the add-on board again with the socket in place, press it down and make sure the board is as level as possible, then solder in the PLCC socket.
Solder two wires to the bottom pin header and two points on the video card as shown above. The add-on board also has a helper indicator just in case.
You must remove or disable the original BIOS chip before you can use your own custom BIOS chip.
If removing the original BIOS chip is undesirable or impossible - I suggest cutting the OE# line to the original chip on the video card and solder a jumper wire from the OE# pin to the Vcc pin on the original chip itself to make sure it doesn't output anything.
If the computer refuses to boot after installing the board, or showing strange glitches on the screen, that means there are connection issues.
Double triple check all the solder joints on the RAM chips, as well as the pin connectors. Make sure all pins are soldered and no bridge exists.
Spray contact cleaner into the female connector on the video card and the add-on board.