getting to know your read/write heads

getting to know your read/write heads

Postby mcanulty » Sun Jan 16, 2011 9:51 pm

This is a long time coming, I've been in the process of finishing an MFA program at Mills and it sucked up all of my time last term. While I've still got a few days I dusted off some folders and made some images for the floppy audio tutorial. Thought I'd get a start here and then move it over to more organized locations later.

Here's an image I found in a patent for an r/w head:

Image
It's from US Patent 4,251,842 - Magnetic Recording and Reproducing Device

That gives you the basic idea of what's going on, (ignoring 7 and 5 for now) there's a coil for inducing a magnetic field and a bit of metal for directing that field across a gap which will then organize the magnetic material on whatever media it's traversing. And it works both ways, the magnetic media can induce an alternating current in the metal which gets translated to the coil and back out to where we can hear it.

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Now, if you look closely at your own floppy drive, you should be able to locate the head.

1. Here's a random drive I just happen to have lying around (I have a lot of them for some reason :)

Image

2. Each drive and drive manufacturer was different, so the covers and cases are all secured differently. Here's that Mitsumi with the cover removed:

Image

3. The two head arms are designed to squeeze the doublesided 3.5 inch diskette and sandwich it between the read/write heads. Two separate ribbon cables come out of the arms, one for each head.

Image

4. Pardon the blurriness here, but having removed one of the ribbon cables you can see that there are more than two connections per head, in fact there are five different contact points.

Image


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So why five contact points? This is so that the heads could do something called "tunnel erase", basically trimming the edges of a data write in order to keep the tracks from bleeding into each other on the disk and corrupting the data.

With a graphic I found on page 647 of Scott Mueller's "Upgrading and Repairing PCs"
http://books.google.com/books?id=E1p2FDL7P5QC&lpg=PA646&ots=M1tlzb70fC&pg=PA647#v=onepage&q&f=false

you can picture the head looking like something like this:

Image

Personally, I'm pretty sure that this bleeding across tracks is partly what's responsible for the cool sound that happens when you use the system to write audio, so needless to say we don't really have to worry about it too much. However, to get more familiar with the head, let's take a look at those 5 connections.

Here's a cool graphic I found in another patent:

Image
from European Patent Application #82104726.3

Notice how one of the connections doesn't seem to be going to anything? That's actually connected to the shield, which can come in handy later. In this diagram it's the middle connector, that won't necessarily be the case in your floppy drive.

In order to figure out what's what, you should take out your multimeter and start probing the leads. It can be a pain to probe the leads on the tiny flex cable, so you can try a trick that I figured out at some point which was to tape the cable down. That way you're not too likely to rip it and it will help a ton when you want to probe it or solder it later.

This is a photo of one of the half height TEAC drives that I did my project with, those things were tiny!

Image

When you take out your multimeter to probe the connections, you will find one that has a very large resistance from the others (that's the shield line) but you may be surprised to find that all of the other lines are virtually shorted to each other. In reality they have a very small resistance, but they are all connected.

Trying to figure out what was going on with the head made a lot more sense when I discovered this diagram in a Commodore 64 manual:

Image

Back when it was worth trying to fix floppy drives they provided this diagram and gave the proper resistance for each coil and how they should look when you were probing each section. I went ahead and labeled these since you'll have to figure out what part is which on your own.

A couple of useful observations: The resistance between A and C should be the same as the resistance between A->B and B->C added together. or:
A->C = (A->B) + (B->C)
also:
A->D = (A->B) + (B->D)
and:
C->D = (C->B) + (B->D)

I made a little chart for each head I was working on and then labeled each connection so that I would know what it should look like. It doesn't have to be pretty, this is how mine looked:

Image

Positioning a probe on line 1 (as drawn on the page), I measured and recorded the resistance between 1 and each of the other non-shield lines. Then I did the same with 2 and 3. I drew out the diagram found in the commodore manual and did the algebra by intuition. The two smallest figures conveniently add up to the resistance between 1 and 2, which leaves only 4 standing on its own. If memory serves each floppy drive I discovered followed this pinout, but it's fun to draw the little diagrams and it makes you feel good.

Once you've got it all figured out, you can either pick two and solder them up, or solder up all 5 and pick later. Don't forget to wire up the shield as well. If you have trouble soldering regular wires to the small connectors, you can try magnet wire. That worked really well for me:

Image

I taped them all down and then soldered them one by one.

Okay I have to run for now, but more to come soon. Let me know if anyone has any questions!
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Re: getting to know your read/write heads

Postby mcanulty » Mon Jan 17, 2011 3:02 am

updated the post with tunnel erase data. next topic to write up is digital control I think.
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Re: getting to know your read/write heads

Postby StatiK » Mon Jan 17, 2011 11:07 am

wonderful ifnormation. I must admit im impressed with the level of research you have done. But this has given me the motivation to locate and rip apart a floppy, looking forward to more info!
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Re: getting to know your read/write heads

Postby aelginfluensa » Fri Mar 11, 2011 2:20 am

Hi

Thanks a lot for the great info! I never understood which output/input was to the tape heads before.. Circuit girl's video is very inspiring but not very easy to follow..

Another thing I haven't figured out: how do I power up the motor? Can I use a battery or do I have to dig into another computer to get cables?
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Re: getting to know your read/write heads

Postby mcanulty » Tue Mar 15, 2011 10:12 pm

The motor can be powered with the same power as the rest of the drive. A battery probably wouldn't last very long, I would use some kind of power supply. I personally used an ATX power, if you want to make it general purpose I used a tutorial like this one:
http://www.wikihow.com/Convert-a-Computer-ATX-Power-Supply-to-a-Lab-Power-Supply

But you don't have to do anything to it necessarily. Just turn it on and plug in the drive.

Let me know if that helps!
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Re: getting to know your read/write heads

Postby lazart » Fri May 13, 2011 2:25 pm

hi,
found this fantastic project, that's probably be the best analog delay ever :) nothing new for two months?
I'm wondering how you managed it to read and write, or play and record sound at the same time? did you just use the r/w-heads on both sides? is the signal getting to the other side of the disk?
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Re: getting to know your read/write heads

Postby mcanulty » Sun May 15, 2011 7:51 pm

Hi lazart! I just finished an MFA in electronic music at Mills College on Friday, and the past two terms have had me a lot busier than I had imagined. But I'm getting my breath back and always am hoping to be more active about posting more material.

I had issues using both read heads, so for a continuous delay I had to use multiple drives, that was the really hard bit for me actually, trying to get the analog stuff to work right when I was a little inexperienced in that area. But I learned a lot and it worked!

When trying to use both sides, I found that the opposite read head would just electromagnetically couple with the other write head, so I was blasting the audio into the other head which made it kind of useless. It might be possible if you can write at low enough volumes, but on the other hand it might just be a physical limitation of the system.
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Re: getting to know your read/write heads

Postby lazart » Wed May 18, 2011 4:18 pm

Hi mcanulty,
Hope you'll get back a lot of breath soon :)
I thought about using the part of the head for tunnel erase to write and erase with an attached magnet, but maybe they'll also couple.
There also seems no way to get one head a bit away from the other. I got only one floppy drive that might be broken, can't get it to work but maybe I'm doing something wrong with the controlling.
Found this link http://www.epanorama.net/circuits/diskstepper.html which may be useful for someone here?
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Re: getting to know your read/write heads

Postby aelginfluensa » Sat Jun 04, 2011 1:12 pm

More patrol..

I did some soldering on one of the head connectors today, probably the hardest solding I ever did.. But I think I got them pretty good after a while. To which connections shall I connect the cables from the tape deck? I tried a few different combinations but all I heard was humming. But I don't what's on the floppy I tried with either..
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Re: getting to know your read/write heads

Postby mcanulty » Sun Jun 05, 2011 8:58 pm

Hey, glad to hear you're making progress! I experimented a bunch and found they all work pretty much the same. Just pick one coil and write to it using an amplifier. Basically drive the coil like it's a speaker. Start with a low volume, but on the bright side, if you burn it out you have other coils that you can use! After you've written some audio, listen back to it. The data on the disk won't sound like very much, it's not really written loudly enough to be heard over the hum of the drive anyway. But once you've written audio onto the diskette, you should be able to hear it with the lines coming from the tape recorder.

Dan
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