HP-97 Printer Malfunction

[Jonas Sandstedt]

I know that I have not being able to provide any more information in this matter for a while, but have now, at last, been able to get some more electrical information. Getting no more useful data from my DMM, I have now borrowed an oscilloscope to find how the signals really look. When the motor is activated in the forward direction, the FWD signal, pin 5 of XA1P1, shows a square wave with an amplitude of 4V, each pulse having a duration of a little more than one millisecond and the period being a little less than 1.5 millisecond. The collector of Q11 has a DC component of 4V with 1V pulses on top, having a duration of about 0.5 millisecond and the period being a little less than 1.5 millisecond. The collector of Q9 shows the same square wave with a DC component of about 0.3V and the pulses having a total amplitude of 3.3V. The base of Q14 (the other side of R1) shows a similar pattern. When the motor is stalled and the tone is heard, the motor terminals both show a square wave with an amplitude of 3V, each pulse having a duration of a little more than one millisecond and the period being a little less than 1.5 millisecond. When the motor is released, a plastic middle gear being removed, a DC component of 2V is added to the square wave at the red motor terminal (the collector of Q14), the same square wave being present at the other motor terminal. The difference between the motor terminals at this moment is thus a DC voltage of about 2V. The control signal, pin 15 of XA1P1, is a square wave with an amplitude of 6V, each pulse having a duration of a little more than one millisecond and the period being a little less than 1.5 millisecond. The duration of the pulses changes slightly when the motor is released. I can see that the motor does not seem to receive the proper voltage, and it seems as if the problem probably starts at Q11 and/or Q9, but I am thankful for any kind of tips regarding what might be the error here.