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Thread: Commodore drive 1541(C) with magnetic head open: how it can work...

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    Default Commodore drive 1541(C) with magnetic head open: how it can work...

    A typical failure of a 1541 drive with newtronics mitsumi mechanism is a magnetic head that has 1 of two R-W windings open.

    In this condition if you try to read a disk the drive won't read it, and if you try to write into the disc the drive will destroy the magnetic integrity of the disk.....

    If you have a drive with this problem:
    find which side of R-W coils are open (the RED or the BLU one) and connect it (by a little wire) with the central point of the head: it's color is WHITE or it's the biggest wire (shield of the cable).

    NOW THE DRIVE CAN READ!

    REMEMBER: write operation will corrupt the disk.

    The complete "Read & Write" solution for the magnetic head is more complicated and it's currently under development.

    I found not possible (for my abilities) to operate inside the head for fixing the problem, and also modifying the 1541 board could be complicated and not easy to be done for some 1541s owners.

    so my strategy is a simple interface to insert between the head and the board, an interface that emulate a good head at board side, using an "half head" on the other side....

    I have already found a simple solution, but it is not satisfactory enough to be "THE SOLUTION":
    the research has to continue,
    stay tuned ....
    Last edited by MCes; 27th February 2020 at 09:22.

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    I'll be interested to see what you come up with. I have tried this before and it does "work" but it really only makes a poorly-working 1541.

    Most of my heads are not open, but very high (15k) resistance on one side of the coil or the other, so I disconnect that side of the coil from the board and replace with 12ohm resistor to the center tap.

    It can then read, but not that well, especially in the upper tracks at density 0. I think you are only getting half the signal strength, so many errors and misreads...
    Last edited by rcade; 29th February 2020 at 21:16.
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    Quote Originally Posted by rcade View Post
    That is true... That trick only makes a poorly working drive though, I think..
    The "jumper" or the "12 Ohm" that replace the missing half head has to be used only on the missing half head, only for a not-working head.

    Quote Originally Posted by rcade View Post
    Most of my heads are not open, but very high (15k) resistance on one side of the coil or the other, so I disconnect that side of the coil from the board and replace with 12ohm resistor to the center tap.
    You read 15k because on the backside of the head is placed a diode (on the erasing coil) and a 15k resistor on reading coils terminals, so if a reading coil is open you will read the resistor: it's normal.
    I suggest to don't disconnect the (15k) open side of the head, and to put on it a jumper, or a 12...15 Ohm resistor.

    Quote Originally Posted by rcade View Post
    It can then read, but not that well, especially in the upper tracks at density 0. I think you are only getting half the signal strength, so many errors and misreads...
    An reading half coil produce half voltage that feed the analogic chain that end with a voltage level comparator that produce a logic level.
    Normally this level drop (6dB) is not critical so I consider strange that some disk could be loaded or not for this reason, maybe that the physical disk has not an identical alignment to the reading mechanism, so the level is already dropped by this misalignment....
    I'm curious to try it with my oscilloscope .... I have to download a .D64 image of a disk that has this behavior with evidence, could you suggest some titles?

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    Just write out something like the first Activision demo and try it. It will 9/10 get a load error soon into the demo. The higher tracks all get error.
    Any disk really... Also it gets very picky about the media. Some disks it can hardly read at all and others mostly work. This is due to the now low signal strength or focus area.

    Use this attached alignment disk and check the track readability in the alignment test. A jumpered drive won't get 100% reads on all tracks like a good one.

    If I leave the high impedance 15k coil side connected and jumper it, it erases/damages the disk when you read it. This doesn't work for me. I have to disconnect it.

    According to the schematics, it takes both inputs and runs them through a differential amplifier stage, so we are missing half the information, of course.
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    Last edited by rcade; 4th March 2020 at 00:59.
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