Magnetic Maniac Manages Mangled Memory

Ahh, floppy disks. Few things carry nostalgia quite like a floppy — either 3 1⁄2 or 5 1⁄4, depending on which generation of hacker you happen to be. (And yes, we hear you grey-beards, 8-inch floppies were definitely a thing.) The real goodies aren’t the floppies themselves, but what they carried, like Wolfenstein 3d, Commander Keen, DOS, or any number of other classics from the past. Unfortunately a bunch of floppy disks these aren’t carrying anything anymore, as bit rot eventually catches up with them. Even worse, on some trashed floppies, a format operation fails, too. Surely, these floppies are destined for the trash, right?

Well, hold on. [AnotherMaker] discovered something that might breathe a little more life into those dead disks — magnets! To be specific, he’s using a Degausser, namely the Realistic Bulk Tape Eraser, though enough time with a strong magnet would probably work, too. Thoroughly treat the disk, pop it back into the vintage machine, and there’s a decent chance it happily formats. Now all that’s left is to figure out why.

Is this an alignment problem, where multiple drives have written in slightly different places, and the read heads are picking up these errant areas even after the write head starts to format? Or maybe there’s a spot in the disk that is going bad, and the stronger magnetic field is required to reset the floppy’s field. Let us know your guess, or if you know the answer, fill us in!

29 thoughts on “Magnetic Maniac Manages Mangled Memory

  1. In the days of cassettes it was known that a static field demagnetizing a tape was a way to make the tape noisier by 10db than if an AC field erased it. Worst of all were jam boxes that had a pivoted magnet for an erase head. Just don’t record a tape on them, or play your favorite mixtape. All those magnetic moments on the media are set at max instead of neutral when this is done. Those magnetic pass-thru erasers were made for bookies and dealers in the days of landline phones and answering machines.

  2. I have always (used to) bulk erase disks. I worked in engineering at a local FM/AM radio station and gave new life to a bulk eraser that was shattered on the floor by wrapping the left over transformer in thin white foam shipping wrap. I brought it home to my shop and use(d) it to bulk erase cassettes and VHS tapes as well as floppy disks. I didn’t think the magnetic particles went bad but but lost the magnetic signal over time. The tapes and floppy medium gets brittle as they get older though. Magnets can be affected by dropping them on the floor even.

    1. I just went to a new dentist after I did no longer feel confortable going to my old dentist because (amongst others) she still used floppy disks. last visit was 5 years ago.

      And doctors, lawyers and notaries in the Netherlands regularly use fax machines because a fax counts as a signed document wheras an e-mail doesn’t, and reputably is “safer”.

    1. Decently sure the disk gets a new serial when formatting. If any, there’s nothing special with how the serial is stored on-disk: it’s simply a field in the FAT, meaning that it’s on a location on disk indistinguishable from any other location.

  3. Why? That’s easy: Every drive will have a slightly different offset on the track’s position. If the disk has been written in a few different disk drives then there will be residual data that your drive can’t fully erase. This “hack” probably won’t make any difference on disks that have only been used in one disk drive.

  4. I designed a lot of equipment to test and duplicate floppy disks for MEMCON, Mountain Computer, TRACE, Data Services International and 3M. I could do a 4 hour seminar on the subject – but who would care? But for those who care a little bit: The tracks on a floppy disk (typically 40 or 80 per side) had a DC erased guard-band between them. Signals were written using MFM (for “double-density”) or GCR (early APPLE) encoding. In most cases, the recording and playback head was a center-tapped coil and each half was alternately fed from a (basically) constant-current source controlled by the test signal fed to a D-type flip-flop. Testing involved writing a non-encoded continuous square wave signal at the highest frequency present in the encoding scheme on that type of disk and after recording, the same “head” was used to compare the analog amplitude (usually using analog comparators) of each signal peak (both positive and negative) referenced to the average of all the peaks of that track (or the previous one). A variable threshold was necessary because the inner tracks have a smaller diameter and are therefore shorter and have lower amplitude. In some cases, the (almost sine wave) output of the read-write head was not simply checked for amplitude but also fed to a peak detector and digital “margin-window” circuit to insure that each peak coming from the head was in exactly the right place in the time domain. A peak that was too low in amplitude (usually below 60% of the average on that track) or was shifted more than 10% from its ideal location indicated that there was either a “hole” or thin spot on the coating, or that there was contamination or the oxide forming the coating or there was a “clump” in the oxide slurry that had been used to coat the disk. In any of those cases, the disk would be rejected. After this “Missing Bit” or (in some cases) “Shifted Bit” test, the disk (disc?) was DC erased and read again by the same head. If the coating was uniform, there was a constant DC bias on the disk and the signal from the head would be zero. But if there was a defect, the magnetization level would vary, which would induce a voltage in the head. If this voltage was more than 20% of the normally written level, the “Extra Bit” error would be flagged and the disk would be rejected. The testing and duplication equipment used heavy-duty floppy disk drives. But since the same equipment was used to both write and read the result, any mis-alignment with the ideal track location would not be flagged, and potentially, the wrong area of the disk would be tested (or written, in the case of duplicators and formatters), so it was very important that all floppy drives used in this process be in perfect alignment. After the testing process, bulk disks were fed through a big degausser – an AC eraser to remove the (magnetic) DC bias that the disk had after testing.
    For pre-formatted and duplicated disks, a similar process was used, except no analog testing was done, instead, a track was written and then read back through a slightly degraded read chain and a margin window circuit that insured all the bits were in the right place according to the encoding rules. If the disk drive could read its own track, the copy was deemed to be good.

    1. I’d like to see a hack to record analog audio onto floppy disks. Start with an ordinary 80 track (5.25 or 3.5) drive. Most drives can hold 2 to 4 more tracks than the standard 40 or 80. Old 35 track 5.25 drives seem to be incapable of being driven past that limit. There’s an old Windows program called Copy Star that enabled a bunch of different floppy disk tricks like extra or fewer tracks, changing the number of sectors, and changing interleave for faster reading and writing. I don’t recall if it could use different numbers of sectors on different tracks.

      The next level of analog flopping audio would be switching out the head stepper motor for one that does finer stepping.

    1. I tried to convert a couple of 720K disks to 1.44M. Didn’t work, wouldn’t format properly because the lower coercivity coating used on 720K couldn’t hold the stronger magnetic field used with 1.44M. Same issue with attempting to format a 360K to 1.2M.

  5. It was “rubbish” because he used it improperly. At least watch the video all the way to the end first. It makes a very small area of magnetism, and by putting it right at the center of a tape cassette, he erased half of each spool, with predictable results.

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