Floppy Disk Halp

[MrMacPlus]

I read that demagnetising a 1440k floppy will essentially make it more compatible with 800k drives.

To clear up the persistent confusion and superstition about 800K vs. 1.44MB media, here's the correct story: There is about a ten percent difference in the magnetization thresholds (called "coercivity") for the two media, with the 800K stuff having the lower value. If you want to get technical, 800K media have a nominal coercivity of 650 oersteds, versus 720 - 730 oersteds for 1.4MB media. So, 800K drives may find it difficult to write on 1.44MB media. However, ten percent is not a large difference, and in fact, is about the same as normal variations within a batch from a given manufacturing run. Plus, coercivity varies with temperature, too. So, the two media are not as wholly incompatible as lore has it.
However, if a 1.4MB disk has ever been written on by a 1.4MB drive (and this includes formatting), an 800K drive's weaker write fields may not be strong enough to reliably over-write the existing data, and you'll have flaky behavior (particularly if you're unlucky enough to have a drive with write currents at the low end of the spec, trying to write on a floppy with coercivity at the high end of spec). But if the floppy is virgin, you'll rarely see any problems at all. You can "re-virginize" floppies if you have a good demagnetizer handy.

I read that from the pickle's low end mac faq. I was able to scramble a floppy with a good old supermagnet taken from a dead HD, but I don't know if that is suffecient, as that is "magnetising" not demagnetising. Also, before I get "yelled" at, I don't have access to 720/800k disks, except for 2 that I found, it obviously would be a better value to use the 20+ HD floppies if I could, instead of buying more expensive and harder to find 720/800k disks. I don't have a demagnetizer around and calling the obvious places in town didn't give results either. One method I also tried was to put the floppy in front of a monitor (and a tv later) and turn it off, then on, thus degaussing the CRT, no result :?: .

 

[tomlee59]

As the author of that particular tidbit, I can say this: A proper demagnetization requires the generation of a decaying AC magnetic field -- the idea is to reverse the magnetization of the domains in the media, while simultaneously diminishing their strength. This way, the media's magnetization symmetrically approaches zero. Your instincts are accurate: Your procedure basically magnetizes the floppy in one direction, and so you will most likely be disappointed by the results.

All that said, the easiest thing by far is simply to find 720K PC floppy disks (I'm not yelling at you -- just stating an observation; perhaps things are more difficult where you are, but the 'net is everywhere). They are the same media as 800K Mac floppies (only the formatting differs). Many office supply stores either still have some sitting on their shelves, or will order them for you. There are many online sources (still) as well. Using HD floppies in DD mode involves more fuss and bother (and risk) than it's worth, except in emergencies.

 

[MrMacPlus]

So, when I rubbed the magnet over both sides of the disk I was effectively erasing the data by writing 1s all over the disk? Would building an electromagnet be a way to get a negative field to wipe the disk to all 0s? I really would like to get these demagnetized, if possible, but I'm not really magnet-saavy. I understand I need to get the magnetic field on the disk to 0 gauss. That's all.

 

[tomlee59]

Imagine that you are "unit bar magnet" living on the surface of the floppy, and lying in the plane of the disk. One end of this magnet is North, and the other is South. To demagnetize you, we want to flip you completely back and forth, initially with some large amplitude, but gradually weakening with time. Your procedure with the permanent magnet did not achieve this result.

If you are absolutely determined to make a time-consuming/educational project out of this, you can build your own demagnetizer. The basic ingredients are an AC power source (isolated from the mains for safety), and an electromagnet. For the latter, you can wind your own, modify a transformer, or simply purchase one. You need enough ampere-turns to induce, say, a peak flux density of a kilogauss or so (easy enough) at the surface of the disk. Wave the thing over the surface, and gradually withdraw the magnet while continuing to do so. Once you get a couple of feet away, turn off the power to the electromagnet.

There's no need to repeat the procedure with the disk flipped over. The disk is so thin that the field will be about the same on both sides.

 

[MrMacPlus]

The AC power source would be an AC battery? I barely understand this, but I assume that DC would send only one polarity all the way through, just like my HD magnet? So I would need to make an electromagnet out of an AC battery, then wave that over the disk, and that would be that? In my opinion, going to buy 720k floppies would be more time consuming and troublesome, really. The only reason I need floppies is because I can't seem to get my zip disks to be bootable, and the 40MB hard drive, well, is old, and not trustable. A few bootable floppies to be there in case of emergency is kind of needed.

 

[tomlee59]

The result is what you want -- an alternating magnetic field. Since you cannot purchase an "AC battery" I don't know what you mean. All batteries are DC, but conversion circuits can produce AC from DC, of course (these boxes are called "power inverters" if a relatively low DC voltage is used to produce a mains-level AC output).

I don't mean to be insulting, but the fact that these basic sorts of things are unclear to you is a strong indicator that this project is simply not for you. There are too many ways in which you can fail, or in the worst case, even injure yourself.

I urge you simply to purchase some 720K PC floppies from an online vendor. A 10-minute Google search should turn up a suitable -- and reasonably-priced -- source. Or go to Radio Shack and see if they still sell AC-powered bulk erasers for cassette tapes. Those would do nicely.

 

[Mac128]

For the latter, you can wind your own, modify a transformer, or simply purchase one. You need enough ampere-turns to induce, say, a peak flux density of a kilogauss or so (easy enough) at the surface of the disk. Wave the thing over the surface, and gradually withdraw the magnet while continuing to do so. Once you get a couple of feet away, turn off the power to the electromagnet.

I think you may also need a "flux-capacitor" ... oh wait, that sends the disk back to 1984 and could warp the space time continuum. LOL The things you say Tom!

 

[tomlee59]

Well, at least I left out all mention of chicken parts...

 

[MrMacPlus]

Alright, I get that the current needs to change polarity and gradually get weaker. I might call Staples and see if they can even order 720k disks, on their online store they don't have any. if I could make my Zip disks bootable I wouldn't be bothering. And I didn't get insulted, but I wouldn't nessecarily call that stuff "basic"

:-/

 

[tomlee59]

Here's an example, found in three or so mouse clicks:

http://www.oldsoftware.com/floppy_disks.html#anchor270200

~ $0.60 per disk is not a superb deal, but it ain't bad.

I'm sure you can find even better deals after more mouse clicks.

Btw, where are you located (you may wish to put that info in your profile)? If you're not far from an MLA member, you'd likely be able to get a few disks pretty quickly and cheaply.

 

[Mac128]

Zip disks are bootable. Which Mac are you using?

See here:

http://68kmla.org/forums/viewtopic.php?t=2544

 

[MrMacPlus]

I use Lido 7 as my disk formatting utility, and when I format and copy the sytem folder over and try to boot from it, I get a happy mac for a second and then an almost immediate sad Mac. I can get the code in a little bit if it would even help. I follow the rules posted at some site and none of it even works the way it says. :-/

 

[MrMacPlus]

Now both my external and internal floppy drive is having problems, they refuse to format disks most of the time and writing disks in Disk Copy is futile. I also got the error code, it's 0F001C. Some sort of exception.

 

[MrMacPlus]

I was able to build a DC electromagnet with a C battery. Now what am I supposed to do to make an AC current? Common sense tells me I shouldn't strip the end of an AC adapter and hook those up to the magnet. Should I modify an AC adapter to get it's power from a battery?