Setting the clock up properly made the clockchip loose its setting, ie produce random time. Was it the stepper making too much electrical noise/power dips? Measuuring it revelaed that the ArduinoPro was a 3,3V version ie the clockchip didnt get the 5v it needed.

New 5V versions of the ArduinoMiniPro, and some new smaller stepper (the ubiquitous 28BYJ-48) ordered.

The software acted strange - but the new stepper has gear and thus 2000 steps/revolution so there was (another) overflow in intermediate calculations. Easily fixed by changing the constant 2000 to 2000L ie. the long datatype, and the compiler thus made all integer calculations in long. Still something was not quite matching - how many steps are there for a full rotation?

Turns out it may be 2048 or 2047.something depending on the precise manuafacture. (google for "48byj-48 gear" and see all the ugly details). I opened my stepper, counted the teeth and was happy to notice mine is exactly 2048 step/revolution.

Another lucky break is the output shaft is the same size so I just need to drill new mount fittings.

Early tests with the new steper shows it much smoother/quieter.

Had to supress for the minute hand to nudge the hour hand every minute (as the stepper resolutin now is so high) so it only does it on even 5 minutes.

Another visual improvement will be for hour hand not be 100% accuratly positioned, but scaled closer to hour digit so it easier to see what it points to. It is hard to see at 25 to 35 minutes past where it is right in between.

For now I take the 5V external (USB) supply jumpwire it to Vcc and the Clock-VCC, bypassing the 3.3V regulator, but of course will be done proper on the next one.

Also need the button mounted for (re)setting the clock and clockchip - and write some code.