HP-97 Printer Malfunction

[Jonas Sandstedt]

That was a really thorough explanation of the workings, great thanks. I only measured 0.1 V DC over the motor terminals, not knowing it was PWM in the forward direction. I have disconnected the plastic wheel after the motor in the wheel-train, and now the motor just continues to spin in the forward direction – you were right, the motor won’t get started under load, but starts without load. The motor does not stop spinning until the calculator is turned off, and the display shows “Error”. I have now also noticed that the motors makes a very faint start attempt under load, but stops accompanied of the whining sound. And yes, the sound probably comes from the motor, I can hear a similar note, but much fainter, when the motor spins freely without load.