MSXmakers support

Hello,
I think that U45 is used in many parts of the circuit. Sometimes a chip seems to be the culprit just because it enables a part of the circuit and this is currently the case, because basically this is a simple nand gate and sometimes it is used just as a buffer.
It's on the RESET circuit, acts on the 82c55 chip select (ppi_cs) and its result ends up in the slot selection GAL.
I think the problem could be here.
We can lose our whole life replacing chips because they are not 74HCT or overheating the solders and the problem may be right under our noses, what we have not changed or tested yet.
As I have commented before, I have received many error reports due to bad sockets in the GAL or false contact too, so I advise you to take out that specific GAL and check it on a breadboard to test its operation a bit, to see if works as expected.
The 82c55 is also a critical chip, so checking at what state the reset or PPI_CS signal is with a logic probe can give a clue if the Omega is hanged in permanent RESET or PPI never selected.
You can repeat the HALT test but with all the smoke test 4 chips inserted, it should also work as expected, and maybe it will, I'm not sure, but if it's a RESET problem it won't.
About the U23 chip and about the LS/HC controversy:
I believe there are many reputable shops at the US, although you may not find them at JAMECO or Anchor due to the current shortage. I don't recommend you buy at ebay because everyone sells anything, but all my tests with UTsource were good, and it's the only vendor that can offer all the chips at once.

Using all LS chips should work, and even with all HC chips too (according to Sergey Kiselev's comments).
It does not mean that you will get the same results as me at each smoke test, in fact you may insert all the chips at once and (maybe) it will work without problems.
My suggestion is you try with as many chips as possible according to the original specifications and, in those that cannot be found, replacing them in this order:
74AHCT can be replaced by 74HCT.
74F can be replaced by 74LS.
74HCT can be replaced by 74LS.
Only if there is no other option would I replace a 74HCT with a 74HC, and to know that this works I should keep track of the entire circuit by checking how many CMOS inputs the TTL outputs go to.

If nothing this works for you, sharing a high resolution picture of your motherboard would help us to have new ideas.

It's not a good idea to change the chip type without knowing the fan-INs/OUTs and a few other specifications.

Additional results of my testing. still getting "Error: Memory not found" from Smoking Test 3 through all Chips being populated.
With all chips populated, these are the scoped results
ADM U56 - Pin 15 - 5V. Pin 16 - 0V
U2 IC - Pin 6 (PPI_CS) - 4.6V (my friend said this should be 0V based on his understanding of the quick reading of the schematics)
All address lines on U12 (Pin 3 - 9) have continuity and frequency pulses to Z80 CPU
GAL chips re-programmed again and re-verified again. All valid. No change in error screen.

U2 IC - Pin 6 (PPI_CS) - No frequency reading on the scope.
U12 - Pin 14 (PPI_CS) - Same as U2 results. No frequency reading on scope

My friend is asking what actually activates the PPI_CS. Do you know?
Does this help you narrow down where the issue could be?
Does it seem to narrow it down that the 82c55 could be bad?

Just for reference, all of my ICs I've bought for replacement came from Mouser or Digikey. They are very reliable here. I just can't get the 82c55, VDP, and VRAM locally that I'm aware of.

On revision 1.4 board, there are RAM jumpers. Is there a default jumper that needs to be set on them or leaving them of defaults to 512k RAM from the board?
Just trying to eliminate everything we possibly can.

For the DC12v adapter, how do you wire the switch? Middle pin to middle via, but how do you differentiate the pin 1 to pin 3 to the DC board? Never wired a 3 terminal switch before.