Gents, I FIXED IT!!!!!
It wasn't the PSU. It wasn't a component at all... it was a broken (!!) trace to ground in the network of the dual-slope integrator. Picture attached.
This damage occurred when/after I slid off the circuit from the case. I don't remember hitting anything.
No sign of the fallen trace on my desk. Now I hope no more traces will simply fall with gravity! For posterity, I've taken photographs of the board how it looks now
Meanwhile I've even found out that Mouser stocks a neon bulb to replace the broken "1". A modern version of NE-2E.
Now the clock project can begin. Thank you all for your support!
Paolo