Studer B67 Mk II problem fixed*!

(*almost)

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.

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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.

I haven’t updated in a while!

Here’s what was going on. First, I’m a little busy studying, so that takes priority over the Studer (and all other projects for that matter). I also started re-arranging my work space, so now there’s more storage and hopefully less clutter when I’m done, but that made my desk even fuller for a few days. Lastly, and most interesting is that my friend and I narrowed down the problem.

I looked at the block diagram (p. 240 of the PDF manual) and noticed that signal from the right sensor board goes to the pre-divider, and since part of the problem is that spinning the right roller causes the supply motor to stop running, I figured I’d look around the pre-divider board. My friend suggested I replace IC3 on the counter board, and also to try replacing all the ICs on the pre-divider. I did replace all the ICs on the pre-divider, and that solved the smaller part of the problem – spinning the roller by hand causes the supply motor to stop working. However, I don’t know which IC did it.

Today I’m going to recap the counter board and change that IC and see what happens. I hope that will be it. Voltages of everything everywhere test fine, so the counter seems to be the last stop.

Also contributing to the delay was that I had to order new ICs, and by accident some of the ICs I bought were SMD, so I had to get new ones.

Ok, so the capstan always spins if there’s no tape loaded unless all 3 speed buttons are depressed. I depressed them and the capstan stops spinning.

I keep trying to isolate the problem:

Swapped transistors. The problem persists (take up motor is spinning).

Swapped motor control boards. The problem persists.

Installed head stack (I don’t know, maybe because the tape sensors aren’t “functioning”?). The problem persists.

I also noticed, and don’t know if that’s much of anything, that when I press play, the supply motor just spins fast clockwise. Same thing when I press rewind or fast-forward. Not sure if that’s anything, yet. It also makes sense to focus on the take up right now.

Maybe the motor’s run capacitor shorted?