Update on the Studer B67.
I believe I mentioned before that I isolated the problem to the counter board. I don’t know anymore if that’s true, but by probing around it, I realized that the flip-flop comprised of IC3 isn’t flipping or flopping. From all the probing, my understanding of the schematic, and the help of online friends I realized that the tape sensor is in charge of of activating the flip-flop. When the roller turns one way the flip-flop latches in one position, and when it turns the other it latches in the other position.
The manual states that QP-DIR1 and QP-DIR2 are approximately 90 degrees apart, and I realized that depending on the direction of rotation, either signal should be leading. What I was seeing instead is that QP-DIR1 was always leading.
I pulled out the sensor board and powered it with my bench supply. I got 0V at QP-DIR2 (R13), which is weird because I was seeing it at the counter board. So I went probing around the phototransistor (DLQ2). At the collector the voltage was swinging between 4V and 5V, DLQ1 was reading similarly.
DLQ2’s emitter is connected to ground through a trimpot and also to the base of Q5. So I checked the measurement at the base. I was seeing 0V. I tested the trimpot and it was basically shorted to ground. I figured it’s bad so I pulled it out. I tested it outside the circuit and it worked fine. I put it back in, and same thing. I paused and thought for a bit, and realized that it’s possible that the base-emitter junction is short and that’s why the trimpot was reading 0 ohms. I pulled the transistor out and bingo! That junction is short (as well as the collector-base junction). Surprisingly, I have a few BC237Bs on hand, so I put one in.
Since I had the board out, I replaced C2, a 10uF 25V Frako. Since it’ll go short one day, now is a good time to replace it (and I had a replacement).
I also cleaned the board and the weird yellow residue that was on the roller, so now it’s ready for the new rubber ring I got a few months ago.
I installed the sensor back in the machine and now the flip-flop works as intended. With the roller turning CCW IC3 pin 6 is HIGH and pin 3 is LOW. Turning it clockwise makes IC3 pin 6 go LOW and pin 3 HIGH. Good.
However, the problem of just one motor spinning isn’t gone yet. Now what happens is that one motor spins when I pressed PLAY/FF/RW, but turning the roller in the appropriate direction turns on the other motor and turns off the first. So for instance, if the supply is working and I turn the roller clockwise, the take up will start running and the supply will slow down to a stop. Similarly if the take up is running and I turn the roller CCW. In this case the supply will start running and the take up will slow down.
The trimpots (R10 and R19) control the duty cycle! I have to figure out how to adjust them to get 50% duty cycle.