I was in the middle of writing a progress post and then I went to probe around IC3 in the transport and realized that IC3, was put in the other way around. That is, pin 1 is where pin 8 should be, etc. I flipped it around and it is now fixed! Here’s a more detailed explanation of what I’ve done in the past few weeks:
First a recap of the problem: Upon turning the machine ON, only one motor will be responsive to the transport controls. For instance, pressing PLAY would make the take up motor spin but not the supply. Then if I spun the roller on the right clockwise, the take up motor would turn off and the supply would turn on and start spinning. Spin the roller counterclockwise and now the take up would run and the supply motor would shut down. Even weirder was that disconnecting the J5 connector from the pre-divider board caused the machine to operate correctly.
So when I picked up troubleshooting the machine again I decided to go over the counter’s schematic to try understand better what’s happening with QP-DIR1 and QP-DIR2. To be honest, I’ve done that before, but this time I also graphed the waveforms along the way from where the signals come into the board, and their way to becoming Y2-FORW and Y2-REVS. I also keep these drawings with the manual for future reference. That didn’t tell me much other than that when the roller on the right is spinning clockwise, Y2-FORW goes HIGH and when it’s spinning counterclockwise it’s going LOW and Y2-REVS goes HIGH. I also realized (might have before as well) that the pre-divider carries these signals FROM the counter TO the transport.
Advice on the tapeheads.net reel-to-reel forum directed me to the tension sensors, so I started learning the circuit comprised of ICs 4, 1, 6, and 2. That made sense to me because that circuit is responsible for generating the pulsating signal that controls the motors. My understanding of how IC4 is used is that it is some kind of a comparator. So depending on YAN-TT1 and YAN-TT2 it’ll go positive or negative. That’s how the lower half of IC4 (YAN-TT2) was acting and that made sense. However, the voltages from YAN-TT1 weren’t enough to cause the comparator to work the right way. Instead it went from +12V to something like -2V. (By the way, that’s still how it is!) I thought that since this voltage is dependent on the displacing of the tension arm, that maybe its mechanical settings are out of wack.
So I took out the left sensor and adjusted it per the manual. I put it back in the machine and now both left and right sensors were locking in place. This one made me scratch my head, but it led me to start figuring out what’s controlling the solenoids of the two sensors. This stuff isn’t mentioned in the manual, so it took a bunch of probing and continuity tests to realize that the solenoids are fed the unregulated +24V and the respective outputs of IC3 (SN75462). I mentioned that to a friend and he said that IC3 goes bad often, so I put in an order for replacements. Meanwhile I went to check the voltages at the inputs and outputs of IC3 to see if it’s working right and that’s when I realized that it was put on backwards. I flipped it and now not only are the sensors not locked, but BOTH motors are responsive to the transport controls!
However, the sensors should lock when the machine is stopped. A quick check of the voltages in and out of IC3 showed that it’s not functioning right – it’s a NAND gate but when both inputs voltages are high (IC7 pin 7 is HIGH meaning the machine is stopped, and then a signal derived from pin 7 also HIGH) I get +24V at its output, but it should be 0V. It’s a good thing I ordered some SN75462 so I’ll drop a replacement in and see what’s up.