Ok Guys, here's the progress so far:
https://www.youtube.com/watch?v=D3pSjQLOlrc
Now, I have this puzzling issue:
1) Booting with the test swtich ON: I can exercise all inputs and trigger all sounds, it doesn't appear to have any stuck input after the 74LS244 input buffers.
2) All CPUs were originally NEC upD780C even on the place where the silkscreen called for a uPD780C-1, however all CPU get 3.070 MHz clock (can someone explain me how the slower 2.5MHz CPUs can still work with 3MHz clock?)
I've replaced the D780C with a Zilog Z-80A, but issue remains.
3) Issue: the game starts with "CREDITS FF" and goes directly on the press 1 or 2 players. At this point, every possible input (even toggling the service switch) will start a 2 players game and the galaga ship has stuck fire, stuck right movement (until it stops at the right end of the field) and there's a continous "coin inserted" series of sounds. The game plays until both P1 and P2 are killed and goes back on the start P1/P2.
In this situation, no inputs are responsive in any way, until a new game can be started with any input again.
4) Sometimes when I'm looking around with the scope probe, things get normal, the ship can be controlled and fired and the coin inserted sound sequence stops. I haven't found a particular zone of the CPU PCB where it does this, one time I was looking around on the clock dividers, another time I was looking at the data bits of the additional Z80 this bootleg uses in place of the custom chips and another time I was looking at signals on the 4 MHz-required CPU.
I've tried rocking the daughterboard in its socket and all CPUs and ROMs in their sockets, with no effect.
I'll be inspecting for bad solder joints anyway.
What's most puzzling is the service mode seems really ok, all dip switches toggles are reported too.
I started reconstructing the circuit into Kicad since I haven't found a suitable schematic for this PCB. The one gallag schematic I've found on the net isn't accurate at all (not mentioning the terrible quality), showing chips in different locations. Even the edge connector pinout is different to what I've found.
Any expert of this board around to exchange thoughts?
Thanks!
Frank IZ8DWF