Here we go:
1. CE on RTC goes high for a very short while after power on, and then goes low. After turning power off it goes high and then slowly goes to zero.
2. 74ALS74AP:
- pin 1 and 4 are high all the time
- pin 2 - high for a short period of time and then stays low
- pin 3 - 24MHz, Vpp 3.5V
- pin 2 is not connected with pin 9, at least there's no continuity between them
- pin 5 is also high for a while, but then goes down to ~2V, which I observed earlier.

So what's next? Should I unsolder the sucker and try a new one?

Archon wrote on 2023-05-08, 19:58:

Here we go: 1. CE on RTC goes high for a very short while after power on, and then goes low. After turning power off it goes hig […]
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Here we go:
1. CE on RTC goes high for a very short while after power on, and then goes low. After turning power off it goes high and then slowly goes to zero.
2. 74ALS74AP:
- pin 1 and 4 are high all the time
- pin 2 - high for a short period of time and then stays low
- pin 3 - 24MHz, Vpp 3.5V
- pin 2 is not connected with pin 9, at least there's no continuity between them
- pin 5 is also high for a while, but then goes down to ~2V, which I observed earlier.

So what's next? Should I unsolder the sucker and try a new one?

Pin 2 behaves as expected. Pin 5 does not. This can mean one of two things: Either something else is sourcing way too much current, so the 74ALS74 is unable to pull the output low enough, or the 74ALS is broken. To find out whether something is sourcing too much current, put your meter into 200mA current measurement mode, red wire to 74ALS74 pin 5 (the "2V" pin), and black wire to GND.

If the current is below 5mA, the 74ALS74 should be able to sink that current, and likely is broken, so replace it.

If the current is above 10mA, some other chip is broken and pulls the line high strongly. In that case, looking for the culprit can be tedious, because there are so many chips connected to that signal. First try removing all socketed pins that connect to 74ALS74, pin 5, and see whether the voltage goes down.

My money is on the '74 output being damaged. What I would do is connect a 100 ohm resistor to that pin, the other end of the resistor to a wire with alligator clip. Power the system up, wait 2s, attach the clip to GND. This should pull the line below 2V to a valid TTL low state. Now this will result in pretty noisy transition to L but it should lift the reset on ISA and mobo chips - try it a few times if it doesn't work right away. It's way easier to test this way, before removing any chips from the mobo.

If it doesn't work then check the voltage on the pin with the 100 ohm resistor connected to GND. If it's still way above 0.8V then something is pulling the line high (this doesn't mean the '74 is fine, it's output can be now damaged anyway, but it would mean it's not just the '74 that is causing this).

- 100 Ohm resistor made the signal dropped a bit to 1.98V. No changes in behaviour as I observed it with POST card and RST led was on all the time (IRDY is off).
- multimeter connected to pin 5 showed 38mA 😐

...but, following your suggestions, I've started removing ICs, that are socketed and bingo - removing PAL chip (16L8APC) made the RESET signal disappeared and IRDY lighted up. So... what's next? PALs are programmable so is there any chance to get that one working or it's just a dead end?

Welp, that's the worst possible case you got there. A standard TTL chip can be replaced, even one of the chipset LSI ICs (if there are any) might be sourced with some luck. But a PAL with unknown content? Nope.

Did this mobo work at all? Or did you get it in this state? If the PAL chips are socketed then perhaps someone removed them and then put back into wrong sockets. Or, perhaps, just put some random PAL in place of a missing one to sell the mobo as complete (no missing chips).
See the if the mobo runs without that PAL. Does the POST card show any codes? The next thing to try is to bend the offending pin on the PAL out a bit so that it doesn't make contact when the chip is socketed. That's assuming it is a correct PAL but has a single fault on that particular pin (acting as output rather then input). If none of that helps you can try swapping PALs around. Other than that... I don't see much hope for the mobo, sorry.

Deunan wrote on 2023-05-09, 21:18:

Did this mobo work at all? Or did you get it in this state? If the PAL chips are socketed then perhaps someone removed them and then put back into wrong sockets. Or, perhaps, just put some random PAL in place of a missing one to sell the mobo as complete (no missing chips).

I see two PALs with stickers still on them. Those stickers contain the component designator (U117 and U129) as well as the board name "SAM/12". Those PALs should not be mixed up. On the other hand, the PALs at U122 and U132 have their sticker torn off. They might be swapped without that being obvious. But if the PAL that pulls RESET high ist the one you mentioned here:

Archon wrote on 2023-05-07, 20:43:

- 16L8APC - (PAL chip by AMD) pin 11 (input);

that PAL is broken. As you wrote yourself, pin 11 is a dedicated input pin. It must not source 38mA ever. Pins 12-19 are I/O pins. It depends on the programming of the PAL whether they can source current or whether they act as high-impedance input. A PAL with the wrong program might output 38mA on Pin 12 in a circuit that expects pin 12 to act as input, but pin 11 isn't chosable as input. There is one critical connection you should check: Ground on pin 10. If that connection is broken and the PAL has no ground at all, it might start behaving funny, including sending the ground current through an input pin. 38mA operating current through Vcc and GND on a fast PAL are completely normal supply currents, you just have to make sure that current stays on the supply pins. GND current will not be diverted into input pins, as long as the input pins are not 0.5V below GND. If the GND connection is working as intended, no digital input pin is ever 0.5V below ground (i.e. at -0.5V), so the ground current will not be misdirect into other pins.

Guys - it works!
Once I removed the PAL, I saw that the socket is slightly corroded so I gave it a little cleaning and voila! Motherboard is now alive and gives me a picture.... but here comes another problem that once the graphic card is initialized and displays its stuff (aka `Cirrus Logic 512KB` and such) it suddenly hangs and I see no activity on the memory (no TTL signal there). Anyway, that's another problem that I'll try to solve myself, but - as usual - any advices are appreciated.

Thank you guys for your effort and help. I'm amazed how skilled you are!

Is that corrosion from a battery leak or some rust deposition (barn find that was stored under something that rusted)? You might need to fully clean the mobo first - do not be afraid to wash it with water and soap, use soft brush only for any scrubbing. The important thing is to properly rinse it and let it dry - which might take a few days in normal humidity at room temp. So do not rush it.

Contact metal pads were not as shiny as they should be imho, not rusted or affected by battery leakage since there's no single place over the board that has this specific greenish corrosion.

I had cleaned it a bit before first boot but, as you suggested, it should be completly washed. I'll do it this weekend I hope. Thanks.

Heheh I tried some other graphic card and it fully works now!
Such a big investigation and such a dumb cause. I'm sorry for that, because I should washed the board completly in the first place. Anyway thank to you guys I found the culprit.

Archon wrote on 2023-05-11, 13:21:

Once I removed the PAL, I saw that the socket is slightly corroded so I gave it a little cleaning and voila!

Wow! This means that my idea about a missing GND connection of that PAL (no contact on pin 10) could actually be the root cause of your issue. Thanks for reporting back.