I'm thinking of attempting to convert a NES to be dual PAL/NTSC switchable, and wondering if anyone has tried that before, or has any advice.
Normally, converting a PAL NES to NTSC would involve replacing the CPU, PPU and crystal with ones taken from an NTSC NES.
I'm wondering whether it would be possible to fit an NTSC-frequency crystal in addition to the PAL one, toggling with a switch. The NTSC chips would be mounted on top of the PAL ones, with the corresponding pins soldered together, except for a few important ones. For example the /RST lines would be switched, i.e. in PAL mode the PAL PPU /RST would be high and the NTSC PPU /RST low, and vice versa.
I can foresee several problems with this:
- The switch for the crystals would likely affect the capacitance of the circuit, and hence the clock frequencies. Maybe that could be worked around using variable capacitors?
- Exactly which pins of the CPU and PPU would need to be switched, as opposed to connected together?
- When the (C/P)PU is held in the reset state, does it not drive any of the signal lines? (If it does drive them, this idea is a non-starter.)
- Power consumption. How much power do the chips use when held in the reset state? As an alternative, maybe the Vcc pins could be switched instead, either open or connected to 5V. But in that case, would the "off" chips be damaged by having their signal lines driven?
Overall this would be quite a scary modification to try! A better solution, if this approach is at all possible, would be some kind of add-on board (an updated CopyNES?), which could have additional components, maybe transistors for switching the clock signals etc.