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Thread: Stepper motor problems with Panasonic JU-475 floppy drives

  1. #1
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    Default Stepper motor problems with Panasonic JU-475 floppy drives

    I took the FluxEngine to VCFe Zurich the other day --- only to discover that both my 5.25" floppy drives had failed, in the same way: grating noises while stepping the head, in that very characteristic manner which sounds precisely like a stepper motor missing steps. When single stepping you can actually see it failing to move occasionally (drive goes chug, head jiggles but doesn't move).

    Luckily, someone lent me a replacement drive for the demonstration, and it worked absolutely fine (a surprising amount of interest! Lots of people asked if they could buy one), so I'm pretty confident that it's a drive problem and not a controller problem. But it's very odd that both drives died at mostly the same time.

    I've just torn down one drive as far as I dare without damaging the head alignment and I don't see anything wrong; rotating the worm gear by hand moves the head carriage and it doesn't feel stiff or scratchy. This suggests a stepper motor failure? Do they fail? I did on spec replace some of the caps on the drive board, with no effect.

    Anyone recognise this behaviour? Any suggestions for repairing them? 5.25" drives are increasingly harder to find. I've just seen a new-old-stock drive on eBay for £200...

  2. #2

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    Quote Originally Posted by hjalfi View Post
    I took the FluxEngine to VCFe Zurich the other day --- only to discover that both my 5.25" floppy drives had failed, in the same way: grating noises while stepping the head, in that very characteristic manner which sounds precisely like a stepper motor missing steps. When single stepping you can actually see it failing to move occasionally (drive goes chug, head jiggles but doesn't move).

    Luckily, someone lent me a replacement drive for the demonstration, and it worked absolutely fine (a surprising amount of interest! Lots of people asked if they could buy one), so I'm pretty confident that it's a drive problem and not a controller problem. But it's very odd that both drives died at mostly the same time.

    I've just torn down one drive as far as I dare without damaging the head alignment and I don't see anything wrong; rotating the worm gear by hand moves the head carriage and it doesn't feel stiff or scratchy. This suggests a stepper motor failure? Do they fail? I did on spec replace some of the caps on the drive board, with no effect.

    Anyone recognise this behaviour? Any suggestions for repairing them? 5.25" drives are increasingly harder to find. I've just seen a new-old-stock drive on eBay for £200...
    I have a JU-475 with the exact same symptoms

  3. #3
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    I have experienced something similar on occasion, I believe... Is the noise like a loud chattering or stutter-stepping? I don't think the drive itself is at fault but think perhaps it is a communication issue between drive and controller.

    Check termination on your drive - the last drive in the chain needs to be terminated, even if it's the only one - and try substituting another controller (or motherboard, if your controller is built-in; this is one reason why VLB motherboards are sometimes valued, as you are free to swap out the controller since there usually is none on board.)

    -CH-

  4. #4
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    Check the power getting to the drives. Try an alternate power supply. Since both started doing this at the same time, this seems likely.

    I have some drives that will intermittently behave similar to what you describe. And each time fiddling with the power connector "solved" the problem. I'd guess the +12v is not getting enough power so the drives are still running but not stepping right. The alternate drive that worked may not have used +12v or not needed as much power. Switching power supplies fail very slowly over time putting out slightly less and less power (replacing electrolytic capacitor's in the the supply often remedy that).

    One other possibility, I have seen output transistors that operate stepper motors fail on hard drives, but I don't believe I have seen that problem on floppy drives.

  5. #5
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    I've seen the stepper drivers fail on some Tandon drives. Don't have enough experience with the Panasonic ones, though.

  6. #6
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    Different make-model of drive, but supports suggestions of some earlier posts:

    * The thread at [here] is about a YD-380 drive 'making very strange sounds' due to lack a terminator.

    * In one of my IBM 5170's is a PSU, which, when insufficiently loaded, produces DC voltages that are way off spec. How I discovered that was by investigation of the 5170's YD-380 drive making 'chattering/clunking' sounds.

  7. #7
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    I've tried it with and without terminators, in both positions in the cable, and with and without a 3.5" drive in the other slot. It behaves the same both ways. I have not tried it with a different controller because I don't have one.

    I hadn't thought of power --- I'm using a repurposed external drive bay (for an optical drive, and did you know that a full height bay is not twice the size of two half height bays? I do, now). My digital voltmeter reads 11.88V for the 12V supply and 5.07V for the 5V supply. I don't have a datasheet for this drive, but that seems well within spec. There's no wobbling that I can observe when seeks happen.

    The output driver is an HA13421P dual bridge driver. It's surface mount and I don't want to touch it.

    It is possible that there's something wrong with the way I'm driving it --- I did build this controller board myself. The datasheet I have (for a different drive) says that the step pulses should be 1us minimum, 3us between leading edges, and the code ends up being ludicrously simple: https://github.com/davidgiven/fluxen...sn/main.c#L141 I don't see anything immediately obvious wrong there...

  8. #8
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    hjalfi,
    My TRS-80 Model 1 had a stepper problem as you described. I figured it was the TI Stepper Driver.
    But it turned out to be a Cold Solder Joint in the actual Motor PCB. It took a while to track down
    the problem. It was an Aerocomp Flippy Drive, that was a heavily modified MPI drive.

    Your problem could be similar.

    Larry

  9. #9
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    Does the drive behave on a standard "tweener" floppy controller? In my own code, I've found that most floppy drives are edge-sensitive. The 765 uses a pulse width of 6 µsec.

  10. #10
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    6us for the step pulse width? That's... bigger than I thought. And a closer look at the datasheet shows that the 1us figure is the minimum width. I also notice that the drive actually steps on the trailing edge of the pulse, which I wasn't expecting. Sadly, changing the pulse width doesn't help, but spotting something I hadn't noticed in the data sheet is always useful.

    Unfortunately I don't actually have another PC to test on. All my PCs are laptops, with non-standard floppy drive connectors.

    I spent some time teaching the FluxEngine to read the signal line voltages using a couple of ADCs. I discovered a weird thing: I originally used REDWC for testing with, as it was relatively harmless and could be flipped on demand, until I found out that on my 3.5" drive it's not connected to anything! So if I set it to logic 1, which as it's a open drain pin caused it to float high, it read about 0.8V. I'm now using SIDE, which is connected to something.

    Here's what I see with a 3.5" drive first on the chain and a terminated 5.25" drive last. It's exactly what I expect. Output voltages (board to drive) float high at 5V; the PSoC's not capable of driving all the way to ground against the drive's pullup resistor, so the low level is 0.5V. Input voltages (drive to board) float high at the controller board's lower supply voltage of 4.8V; when the drives are both deselected, they're correctly letting the line float.

    If I remove the termination resistor from the 5.25" drive, the output low voltage drops to 0.08V. Which is also exactly what I expect to see.

    So I don't think there's a termination issue.

    Code:
    Output voltages:
      Both drives deselected
          Logic 1 / 0:  0.53V / 5.00V
      Drive 0 selected
          Logic 1 / 0:  0.54V / 5.00V
      Drive 1 selected
          Logic 1 / 0:  0.54V / 5.00V
      Drive 0 running
          Logic 1 / 0:  0.53V / 5.00V
      Drive 1 running
          Logic 1 / 0:  0.54V / 5.00V
    Input voltages:
      Both drives deselected
          Logic 1 / 0:  4.86V / 4.95V
      Drive 0 selected
          Logic 1 / 0:  0.01V / 4.85V
      Drive 1 selected
          Logic 1 / 0:  0.00V / 4.85V
      Drive 0 running
          Logic 1 / 0:  0.01V / 4.85V
      Drive 1 running
          Logic 1 / 0:  0.00V / 4.86V
    Last edited by hjalfi; December 10th, 2019 at 03:18 PM.

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