After some silence I can report SUCCESS on that issue! YEAH! Happy I am: After first repairing the TopMax programmer, digging out my old GALEP3 prgrammer and building a very special adapter socket for the DS1553, I was able to have a detailed analysis:
(1) The DS1643 and DS1553 chips can stop the oscillator to save some energy when stored away. (2) After attaching a new battery to very old chips (i.e. if battery voltage goes below ~0.9V) the chips automatically revert to the "oscillator stopped" condition, even if new battery is connected. (3) So with new DS1553 and old one with new battery connected, the clock is stopped! (4) The OpenBoot firmware, before powering the screen and running self test, seems to rely on a running clock, i.e. it is not coming up at all if the oscillator is stopped! (5) To me that is a bug in the startup code of the UltraBook IIi's firmware - it does obviously not start the oscillator by itself! After activating the oscillator and clearing my NV-RAM the UltraBook IIi is coming up into the OpenBoot Firmware again!!! Lesson for others: If your battery voltage drops below 2V, you will see the "NVRAM invalid message", but the machine will boot/work. But after some more years when the battery voltage gets low enough for the oscillator to stop you will run into trouble! Next step will be trying the same on my UltraBook (DS1643 instead of DS1553 on the UltraBook IIi). But need to Xray that one first before attaching a new battery.
