Oh well, I managed to fix it! 😁 Just writing here brought me good luck!!!

I thought I saw the data line "changing" when I touched an electrolytic cap. I just replaced all three of them - some looked in bad shape but they all measured fine.

And... it works now!!!

Surprisingly, those I/O lines still look wrong. Meh! 😀

Thanks for your help! 😉

When I read the tittle the first thing I thought was "check the main chip for bad soldering , it's a common issue on those old boards" =]

Great repair job!

ahah fantastic! I was puzzled to see so many legs just moving freely when I touched them. But that wasn't enough. I have a feeling that the caps are fine - at least I don't see anything wrong with them, not even ESR - and the new solder job did the trick!

Well, I now know what to do on my next "faulty" board I just bought! 😀

And I've just fixed mine. Black screen. i re-soldered the GPU and replaced the capacitors but that didn't help. I re-soldered all the chips. nothing.
I soldered the sockets for the ram and replaced the memory chips but that didn't help either.
I checked the ROM and it was ok, but I programmed a new one just in case. This did not change anything either.
On the oscilloscope I could only see activity at first second time. 40Mhz clock signal present.
Today I soldered out the 74xxx chips to check on the tester. All chips good. I soldered the sockets and put the chips back in. The card started up. Probably something must have been out of contact at or around them, as I used a hotair when removing them.

Never give up.

I now have a card with polymer capacitors and MLCC. I think there's another tanal one in there that I'm going to replace with MLCC as well.