Power supply ok, nothing happening

I have assembled both boards to the point of testing the control board before attaching the LCD. I had to replace the two TL074 as I had them in the wrong way round (they had a dot at the wrong end! And, to be fair, an indent at the right one). When I powered it up the first time, connected to the control board, with the ICs in the wrong way, nothing happened except that the cornermost voltage regulator got exceedingly hot.

I tried with the TL074s in the right way, nothing doing obviously, so I replaced both of them. The power supply tests fine now, the correct voltages are at each test point, and it is outputting +5v to the control board. However there is still no joy with the control board, no LEDs light up. Is there any likely candidate for this failure, or could it be any number of things?

Thanks,
Joe

So you connected the digital board via the Stacking connector and have no Power on the Output Expansion? Then the whole Digital board has no power - look for bad solder Joints around the Stacking connectors. Resolder everything until you get the Voltage measured ant the Output Expander Port.

No, nothing doing on those two pins. No voltage at all.

Have you measured the Voltage on the Digital Board - try measuring on the OutPut Expander Ports +5V and GND Pin…

post some pictures of both boards if you can ?