After my first terrible attempt
at installing the Hi-def NES mod, I eventually managed to source another NTSC PPU.
And contrary to my worries, a jumper cable took care of the only via, and once I prepped the motherboard with the sockets, the standard (non-HDMI modded) NTSC NES had been restored to life.
Keen on continuing the project, I proceeded with the steps... and everything went smoothly. Except with the finished install, I get neither HDMI nor composite nor sound from this NES anymore. (though the sound is not really expected to work without a working PPU...)
The behaviour is quite odd really... if I leave the NES unpowered for a while, then power it on, I will get a mostly black screen with occasional "hickups", indicating some video syncing could be taking place irregularly. And the LED on the Hi-Def board does not light up at all. After trying this for a while and powering it off, there will be less "hickups" until I power it on again.
Additionally, at one point, I managed to get more of a composite signal - white horizontal stripes and a buzz - along with the LED being lit in orange, by touching the plastic package of the PPU while it was running. However, I have not managed to reproduce this improved(?) behaviour again.
My PPU is not getting hot at all... but curiously, the CPU is getting rather warm quickly... although not terribly. And I have checked there's no short circuits between Vcc and GND at least.
I'm tempted to remove the interposers and try it with the socketed CPU/PPU again. But I'm unsure whether this will work, or could potentially even damage the board, now that the Q1 resistor is replaced with a bridge, and how to even get the system correctly powered with the interposers removed, without undoing the work of replacing the voltage regulation with the Hi-Def board.
So can anyone give some clues on how to go about debugging this incomplete (and hopefully not busted...) mod? Specifically, I'm wondering:
1) How should the LED on the Hi-Def board behave in a successful installation? What does a non-lit LED mean? And what does an orange-lit one mean?
2) Is there some way to test part of the installation, by keeping the voltage regulation of the Hi-Def board but removing either the CPU or the PPU interposer?
3) Anything else worth looking into?...