Debugging failure of carriage to move on Calcomp 565 plotter

SYMPTOMS OBSERVED

Operation of the manual carriage control to move the motor left or right had no effect. The same was true when the controller card in the 1130 sent a pulse to shift the carriage right or left. The first coil of the motor was always energized, the other two were not, and thus the motor held its position. The fast step manual switch for right or left carriage also did nothing. 

OVERVIEW OF CIRCUIT

The stepper motor that moves the carriage left or right has three coils, one of which is always energized and the other two off. By changing which of the three is energized, the motor moves one step in the direction of the new energized coil. The three coils are drive by a ring counter that moves between three states in both directions, based on a pulse to shift either to the right or two the left. 

One-shots generate a pulse and debounce the triggers to shift the ring counter left or right. The one-shot can be triggered by the manual control switches on the plotter or by a pulse on the interface from the computer to the plotter. 

The connection of the one-shots for right and left to the ring counter right and left shift inputs is routed through limit switches on the ends of the carriage. If the switch is depressed by the presence of the carrier at that extreme, any pulse to move further in that direction is blocked from the ring counter. 

CHECKING LIMIT SWITCHES

The contacts in the limit switch must be conducting in order for the pulses to get to the ring counter and shift it. I checked the conductivity of the switch using an ohmmeter. The contacts might have tarnished with age, growing a non-conducting layer. This can be fixed with contact cleaner and a burnishing tool. Fortunately, these were in good condition with very low resistance. 

CHECKING ONE SHOT OUTPUTS

The two one-shots, left and right, can be triggered by the manual control for that direction or by the interface input for that direction. The one-shot is a cross coupled set of transistors with an RC constant that holds the second transistor on for a fixed interval then returns the pair to the stable state where only the left transistor is conducting. 

I checked the voltages on the two transistors to verify first that they were set in the stable state with the left one on. I then used the manual control, watching the pulse and the right transistor to see that it turned on, cutting off the left. Finally, I watched the state revert to the left transistor on after the RC interval of about 1500 microseconds. 

The one-shots were producing perfect 1500 us pulses, however the ring counter wasn't moving. There are separate circuits for carriage left and carriage right movements, feeding a shift left and a shift right signal to the ring counter. Both sides worked, both for the manual movement switches and for computer interface commands.

CHECKING RING COUNTER OPERATION

The ring counter was initially sitting with the first stage active, energizing current through coil 1 of the stepper motor. Triggering a shift right pulse from the right one-shot, I watched to see if the transistors in the middle stage activated from the pulse. The middle coil should now be energized, with the other two stages cut off. Similarly, with the ring counter sitting with the first stage active, a left shift pulse from the left one-shot should activate the transistors in the third stage and energize that coil, cutting off the first and second stages. 

The counter never moved, with either type of shift request. On the correctly working drum movement side, the circuit feeding the second coil would snap on crisply and cut off the original coil. On the carriage ring counter, I saw a very short pulse as an attempt to turn on the transistor but nothing changed. 

The circuit uses older germanium transistors; the transistor that did not turn on is a 2N1304 type. To all appearances the transistor is bad, since the diodes and voltage levels around the circuit all seem to be good. However, I am suspicious since the counter does not shift in either direction. It we had a failed transistor for coil 2, that would fail to shift in one direction but going in the other should successfully activate coil 3. 

I will have to study the schematic and look for all the possible single points of failure that would block this from advancing in both directions. If it is one of the germanium transistors, I should be able to find them or a close substitute at worst case. 

ADDITIONAL MISSING PART IDENTIFIED

I began to string the the cables that pull the carriage left and right across the width of the drum, when I realized that I am missing the spring that provides the tension for the cables. There are two, attached the each side of the carriage and then routed through pulleys to the mechanisms inside the case. 

The cables also carry the 24V power to activate the pen solenoid on top of the carriage. Portions of the cable are in a plastic sleeve, providing insulation, with the back portions left as plain wire. The two pulleys on the rear are thus insulated from the machine chassis, used to connect the solenoid voltage to each cable. The two cables not only meet at the carriage, they also meet at the rear of the plotter where they are hooked to a spring (using insulated eyelets on the end). 

Those two eyelets are connected to the ends of a spring, which pulls the cables taut and maintains tension while the pair move the carriage. A round hub on a stepper motor is what moves one of the cables, the spring and other side the carriage moving the other cable in concert. 

Friends at a museum with an 1130 are going to measure the spring, both size and its reaction to a pull. With that information I can find a suitable replacement that will fit into the rear of the plotter and provide the intended tension.