HP-97 Printer Malfunction

[Jonas Sandstedt]

I have recently bought a HP97 calculator; it was in restored condition with re-built battery pack and new magnetic card driving wheel. When I received the calculator, it worked fine and printed perfectly, but the card reader would sometimes not read, and the seller told me he had had problems connecting the flat cable to the reader after repairing the drive wheel. I carefully disassembled the calculator (as described in several Internet places) and adjusted the flat cable. While doing this, the black wire connected to the printer feed button got loose, unnoticed by me. I re-assembled the calculator, and started it. Having removed the print paper, the printer tried to feed paper, but the printer head now moved erratically, first quickly and then very slowly when returning to the "home" position. I turned off the calculator, found the loose wire and re-soldered it. Now, the printer again tried to feed paper when the caclulator was started, but now seemed to move normally until it suddenly stopped in the "home" position. Since then, the printer head has not moved as it should anymore. I have manually fed paper into the printer by turning the motor forwards and backwards, but this has not changed the printer malfunction. When there is a printer action demanded, such as paper feed, nothing happens, but a faint whining sound is heard and the display reads "Error". When the calculator is turned off and on again, the sound is gone, and the calculator, including the card reader, works fine until a printer action is demanded. Then the above happens, and while the whining sound is heard, I have manually moved the print head by turning the motor by hand. It moves freely (the motor then not being energized), and the print head is moved 1-2 millimeters before the motor is turned on and the print head is quickly returned to the "home" position, and then the whining sound stops. So the motor works, and the print head is always returned when moved slightly from the “home” position. On the printer board, the transistors Q13 and Q14 measure OK, as well as the resistors R1 and R2. Pin 1 of U5 also measures OK (these should be checked according to the service manual). Now I turn to this forum for help, not knowing how to continue.