4. Mods
The card come with 2MB of RAM for the FBI, and 4MB for each of the TMU. This configuration limits the resolution to 640x480. I'm sure this serves the arcade games well, but this is not on par with the PC cards, which have 4MB for the FBI.
Luckily, adding the missing 2MB RAM is as easy as putting the chips on the back of the card. Voodoo 2 runs at 90MHz, so make sure you don't put in anything too slow. I used EliteMT M11B416256A-25J, which runs at 100MHz.
Notes:
- All the missing SMD caps are 0.1uF. I suggest using imperial 0603 ones and solder them towards the outside as much as possible. Since the power is 5V, don't use something with a voltage rating too low. Go for 10V or above. Install these before installing the actual RAM chips.
- Pin 1 (the slanted edge) of the RAM chips should point down, towards the PCI edge. To make the work easier, install the middle two chips first, then install the outside two.
These arcade Q3D cards output a washed-out image. I checked carefully around the DAC area and noticed they modified the reference design, probably to achieve a better outcome with consumer displays instead of the VGA PC monitors.
There are two revisions of the board. One with a diode and a resistor bodged on, one with them built onto the PCB. The diode pulls the DAC pin 46 Vref down to GND, while a 330 Ohm resistor pulls it up to 5V. The Rset resistor on DAC pin 42 Rset also deviates from the reference value of 141 Ohm, to 182 Ohm. All these mods together caused the brightness issue.
The mod is simple. The area in question is the lower right corner of the DAC chip.
- Remove the floating bodged diode
- Remove the 330 Ohm resistor between C8 and FB4
- Change R33 to a value near 141 Ohm
- Remove D90
- Remove R521
- Change R33 to a value near 141 Ohm
The diode that was bodged/built onto the board was a LM385M3-1.2 voltage reference diode. The SMT version was a SOT23 chip with R11 marked on the package
The SMD resistor that was added was 330 Ohm, imperial 0603.
The PC version ofd the Q3D card has an even weirder set of resistor values. DAC pin 42 Rset was set to 301 Ohm instead of 141 Ohm. Then R78, R79, R80, R81, and R82 were all using a 7.5 Ohm resistor, instead of the reference 75 Ohm.
