MCU logic problem on 112 MKII

Discussion in 'TASCAM DIY Repairs and Mods' started by steve908, Aug 6, 2021.

  1. steve908

    steve908 New Member

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    I'm facing an interesting problem with a 112 MKII that appears to have a misbehaving reel motor...
    I own several 122 MKIII's, but just for fun, I purchased a non-working 112 MKII to fix up because its design is like a simplified 122. Turns out it had a broken gear C, so I replaced that and applied new grease and oil everywhere in the transport mechanism that needed it. New belt too. I reinstalled the transport system in the unit, and everything was working great: ff, rw, play, pause, all solid. Listened to an album all the way through. I did not try the recording function yet.

    With the cover still off the unit, I was preparing to do a calibration while listening to tape inside the unit - just some random type II tape with music on it. I stopped the tape, rewound it a bit, and then pressed play. But, the play head engaged, the tape didn't move, and then the play head disengaged about a second later. Looking inside the unit, when I press play, I can see that the reel motor is trying to turn the supply reel instead of the take-up reel! This change in behavior came out of the blue. I did not touch any of the internal connections or parts before it suddenly happened.

    I dug into the circuit analysis more, and took a good look at the motor driver that controls the reel motor. Here's a schematic where I added labels to the pins on U7: [​IMG]
    The way I understand it to work is that feeding a logic high signal to IN1 (pin 5) tells the driver to spin the motor in the forward direction, and feeding a logic high signal to IN2 (pin 6) tells the driver to spin the motor in the reverse direction. That's why the FF signal from the MCU directly connects to Pin 4 and the REW signal from the MCU directly connects to Pin 6. The MCU (U1) is not seen in this schematic snippet, but it's just above. VR (pin 4) is a reference voltage that determines the speed/torque of the motor. When the deck is not in play mode, the PLAY signals from the MCU is at 0V and Q11 transistor is off. This puts the VR pin at 6.8V because of the 6.8V zener diode D15. When the deck is in play or record modes, the PLAY signal goes to 5V logic high, Q11 turns on, and the voltage on RV is changed to 3V because the 3V zener D14 is now active.

    In theory, when the deck in is play mode, the PLAY signal needs to be at 5V as well as the FF signal. The PLAY signal only changes the motor torque. It's the FF signal that makes the motor move in a forward direction. When probing out the logic signals while pressing the play button, I see the PLAY and REW signals both at 5V while the FF is at 0V. This seems like the obvious reason why the motor is spinning in the wrong direction. But, the question is why!
  2. steve908

    steve908 New Member

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    I tired the record function, and it doesn't seem to work. Well, the record light is not illuminating even for the second while the play head is engaged. Not seeing the record mode engaging gives me a crazy idea... I think this Tascam deck suddenly thinks it's a 112R MKII. It might think it's set to play and record in the reverse direction right now. And, maybe that's why the supply reel was trying to act like a take-up reel. Also, it may not be letting it record because there's no REC SAFE switch for the reverse direction installed in non-R 112. [​IMG]
    I checked the HW model sensing configuration, and it looks good on the PCB. R50 is open with no resistor placed, R51 is placed, W200 is open, and W201 has a jumper placed. I wonder if there is any other thing that could make this 112 think it's a 112R, if that is indeed what is happening. Does it seem possible?
  3. SkywaveTDR

    SkywaveTDR Well-Known Member

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    What's with all the very complicated diagrams? The unit was tricked into the reverse mode by a handling of the transport and the way to fix it is very simple. Get a dB25 hook it into the remote socket and issue a forward play command and the deck is fixed. I use a tact switch soldered on a male db25 that I use to fix this strange behavior. There is a guy on E bay selling a board and circuit for over $100 for this and it is a ridiculous solution. Why not use a Cray supercomputer to do it while you are at it. Pin 1 and 24 come to mind but check the manual.
    The reason this happens is that the same chip is used in the 112R Mk II. You just tricked it into the reverse mode as far as the reel motor drive was concerned. Yes I have seen them come in and had the experience myself a few times. The dB25 used to be available at radio Shack but now they might have to be obtained elsewhere. When you have spent any time in Broadcasting- I was only there for 14 years, you accumulate a lot of this stuff. I probably have 100 of them.
  4. SkywaveTDR

    SkywaveTDR Well-Known Member

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    The remote socket is on next to last page of schematics- It won't let me upload a file with a picture of it here for some reason.
  5. steve908

    steve908 New Member

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    Thanks for the advice! I was wondering with all your experience on these if you have ever seen this problem before. Turns out, it's something familiar that you've dealt with. Guess I'm not surprised. The deck is at my office right now, so I'm going try to issue that forward play command on Monday. I'm not sure if I'll be able to find a DB25 connect around there, but there's a chance if I can if I dig through some older engineering equipment.

    I was only walking through the schematic diagrams because I was backtracking the issue from the reel motor and back. I just wanted to make sure there were no failed components in the driver circuit, which then I found it boiled down to the logic coming out of the MCU.

    It's interesting that the MCU in the 112 can still attempt to perform functions only meant for the 112R. I would have thought the model-indicating resistor straps at R50 and R51 that control the state of pins 7 and 8 on MCU would allow the firmware to disable certain functions based on hardware model. Perhaps just lazy firmware coding?

    If I can't find a DB25 connector, it looks like I can just short pin 1 of the connector to ground, and that would generate the signal at the inverter gate, Q4.
  6. steve908

    steve908 New Member

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    I really appreciate the help, Skywave. The forward play command was a success. I couldn't find a dB25 cable, but my DMM probes fit perfectly into the connector pins, so I just used them to short pin 1 (~PLAY_IN) to pin 24 (GND).
    [​IMG]

    I realize now that all the buttons and input signals on the 112MKII/112RMKII/122MKII are multiplexed together, so it's possible to trick it into registering commands that may not even have a dedicated button.

    Again, thanks for the advice. It would have taken me much longer to figure this out, otherwise.
  7. SkywaveTDR

    SkywaveTDR Well-Known Member

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    That is all you need to do I found out and some guy is selling a circuit to fix this.