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Can anyone steer me in the right direction to correct this behavior?

At the beginning of the strip the battery was nearly dead, but the printer seemed to work fine. So I charged the batteries overnight. When I tried again, I get the solid lines across diagonally as the paper advances then across again. It doesn't stop until I turn off the calculator.

If I plug the calculator BACK IN, it works correctly. See correct prints midway.

You should stop using the calculator until the issue is fixed. It looks like two segments of the print head are permanently heated which will most certainly kill them over time.

It seems to be a driver issue, not a print head issue. May be an intermittent short circuit on the board.

Ah, duly noted.

Is there anything in the two separate power supply circuits that would lead to this?

I've no detailed info about the printer mechanism but here are knowledgeable people on the forum which will help you. My answer was just an interpretation of the what the picture shows.

Power Supply Schematic

Can someone help explain this circuit to me? I am a ME by training, not an EE. I am guessing that something unique to the battery circuit has failed and allowed the voltage or current to differ from the intended design, thus causing the odd behavior only when the battery is fully charged or NOT hooked to the charger.

Edited: 31 Aug 2011, 7:06 a.m.

I doubt this strange behavior is caused by the power supply circuit.
The thermal elements of the printer head take a lot of current
and i expect they will be supplied directly from the battery.
I agree with Marcus that this behavior is caused by the printer head driver circuit.

Well I don't agree about the intermittent description as I seem to able to reliably recreate the behavior.

I am going to measure some of the voltage drops with and without the power supply and see if I discover anything.

when I plug it in with the battery the printer is noticeably faster and higher pitched when it prints.

Does that help anyone help me?