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Joined: Aug 2005
As far as I can tell, the only 'feedback' from the printer to the rest of the machine is the out-of-paper sensor. Problems with the printhead or solenoid driver can't be detected automatically.
In whcih case, I'd suggest investigating the paper sensor. There's an LED and phototransistor on the little PCB on top of the printer. These connect to a circuit built round one section of an LM339 comparator (U11 in my notes, but this is not the offical HP designation) on the keyboard/display/printer interface. The output of that comparator goes to a shift register that's connected to the keyboard/display/printer controller chip (the big 40 pin on on that PCB).
Since you imply the keyboard and display are fine, I'd not suspect the controller chip or the shift register at this stage.
But I would check the paper out circuit. Trace the connection from the yellow wire to the paper out sensor to the -ve input of the comparator (IC11c in my notes) so you find the right chip. Then look at the output of this comparator with a logic probe (or similar) when you insert or remove paper from the printer. If it doesn't change state (or if it's not a valid TTL voltage level), debug this circuit. It's not complicated.