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When I bought this small battery driven printing calculator I was told that didn’t work.  However, it wasn’t expensive and beside some slight discoloration in mint condition, so I bought it anyway.

I took it apart, cleaned the contacts of battery and switches and  ………..  It worked.  As the print was very faint I also cleaned the print head.

However, a few days later it seamed dead.  

Just randomly trying to get some life into it, I discovered that when I manually altered the neutral position of the electromagnetic motor a tenth of a millimeter, it immediately moved the paper roll.  

The “motor”  is not the type of motor which continuously rotates. It does rotate, but only for about 60 degrees each time it is pulse-wise powered. To print a line of digits in a 4x5 matrix 5 pulses are needed followed by some pulses for the line feed. After each pulse a spring takes the motor back to its neutral position.

I found a way to permanently change this neutral position a little bit.  From that moment on it works fine.

Furthermore I replaced the old thermal paper by new paper.  The print is now perfect (see this page)

Paper transport toothed wheel

Electromagnetic drive for paper transport

Keyboard contacts

Voltage step-up capacitor

Transistors being used