Diagnosed failure point of Calcomp 565 plotter carriage stepper circuit

SINGLE POINT OF FAILURE MUST BE SHORTED TRANSISTOR

The symptoms were that the ring counter was not advancing in either direction. That implied a single failed component must cause this since different parts were involved in moving based on the direction. 

The ring counter has three stages, each driving one coil of a stepper motor. The design is such that only one of the three stages has its output conducting at any time. A request to shift the ring counter comes in through two paths, one to shift to the right and the other to shift to the left. 

If we imagine that the ring counter is currently operating with the first stage active, then a shift right pulse will be passed only to the trigger transistor of the second stage. A shift left pulse is passed only to the trigger transistor of the stage 3. 

The one stage that is conducting permits the shift pulse to be passed to the next stage. When a pulse turns on the next stage, the voltage at the conducting transistor forces the other two stages to turn off. 

I verified that the output transistor for stage 1 was conducting and that it was passing the shift right pulse to the trigger transistor of stage 2. However, stage 2 did not switch on, there was only a very short blip. Further, since it did not switch on, stage 1 was not forced to turn off. That might look as if it could be caused by a bad trigger transistor for stage 2, except that the same failure occurs with a shift left pulse which involves a completely different stage and trigger transistor. 

If, however, the germanium PNP output transistor of stage 1, a 2n392, had failed in a short circuit then it would effectively be conducting at all times. Initially it would direct the shift pulses to stage 2 or stage 3 trigger transistors, but would continue to conduct which would immediately force those other stages back off. 

REMOVED THE SUSPECT TRANSISTOR - STEPPER NOW MOVING PARTIALLY

I pulled the transistor off the board. That would allow the trigger to move the ring counter to stage 2 or stage 3 depending on whether we selected Carriage Right or Carriage Left, but it would not be able to move back to stage 1. It was enough to verify that a permanently conducting (effectively shorted) transistor was causing the symptoms.

TRIED TO MOVE SAME TRANSISTOR TYPE FROM PEN UP/DOWN CIRCUIT - ALSO BAD

The flip flop circuit that drives the pen solenoid is triggered by Pen Up and Pen Down commands, which should alternate turning the solenoid on and off. I get good trigger signals when the switch is moved but their is no change in the output transistor.

Since it was the same type of transistor (2N392), I pulled it to see if it was good and might temporarily give me a working carriage stepper. Alas, it too was stuck on, both in the pen solenoid circuit and therefore for stage 1 of the carriage ring counter when I moved the transistor there. Ordered another 2N392 from eBay.