HP-97 Printer Malfunction

[Paul Berger (Canada)]

Yes the motor is run electrically in both directions via Q13, Q14 and 4 of the transistors on U1. In the forward direction Q11 conducts through the base of Q9 which is in turn turned on and Q14 is is turned on through R1, connect Vb to one side of the motor and Q9 is the ground path. In the reverse direction it is Q10 that is the ground path and Q12 turns on Q13 through R2. Q8 is turned on to brake the motor by shorting the motor winding. The big difference between forward and reverse motion is in forward the motor speed is controlled by Pulse Width Modulating (PWM) the the signal on the forward line to base of Q11. I suppose a fault in the speed control circuit such that it always thought it was going to fast might could the pluses to be very narrow and not be enough to get the motor started.

It would be helpful to know where you measured this 0.1V. As far as I know the motor is just a PM motor so it would have a wound armature with a commutator and brushes as you turn the armature there may be null spots between the segments of the armature where there will be no continuity between the sides of the motor. Also note that the +5V and ground connections get reversed between forward and reverse. Since the forward signal is PWM measuring it with a meter will not give you a really good indication of the signal level. In between pulses the voltage generated by the coasting motor is monitored to determine how fast the printhead is moving. If the motor is stalled in the forward direction it might hum at the frequency of the PWM signal.

C2 is a 1uF non-polorized capacitor that would be there suppress the electrical noise from the motor, they could go bad, it probably would not hurt to disconnect one end to see if it makes a difference.

When I suggested taking out the idler gear I was not suggesting that there was a mechanical bind in the print mechanism, I am just thinking that the motor may be weakly driven in the forward direction and cannot get started under load.