Restoring a HP9830B calculator.

[Tony Duell]

My guess from glancing at your schematic is that U18 decodes the 'bit image mode' control character and that the extra logic is to toggle between normal character printing and single-line bit image printing.

Anyway, it now seems to work. I would put the printhead PSU back and check the output voltage. And check no enable signals are stuck asserted on the printhead connectors (this is the same as the -A model I suspect, so the articles will be useful). If it all checks out, fit the printhead.