maybe its just the image quality, but I found at least one sus spot.
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upper is from nils, THANKS NILS'S HELP!
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if you can confirm that the solder joints are ok, can you do some continuity test on the power trail, data bus, address bus and control lines (from IC pins to the expansion socket pins).
Then you can check whether the reset operation is activated by the RS button, the RESET pin on processor needs go from high to low, this can check the 556 (U4) chip.
After, you will find some inspiration in the 74LS145 myth just discovered (
https://groups.google.com/g/pal6502/c/KPfvKGdgOcQ), each one of these two chips can cause the on-board LED display won't light up, one (U1) is to drive the LEDs, another (U5) is address decoding.
Thanks,