Is tape head degaussing a myth after all?
- Thread starter soviet123
- Start date
lonewhitefly
Active member
I've definitely experienced a problem where I was getting a horrible signal to tape, then de-magged and the problem was resolved. My memory of the event was that cleaning alone had not resolved it. I remember thinking de-magging was a subtle thing, and couldn't cause this type of problem (barely getting signal to tape) ... but then I demagged, and was shocked that this fixed it. Granted, this was the only time it was extremely noticeable. I don't demag very often though. My general experience is that I don't notice too much after doing so, but it might be a little cleaner sounding afterwards (of course, I'm usually cleaning and de-magging in the same session).
In my experience, the importance of cleaning heads and demagging (and how often you do it) is dependent on the machine and the tape. I remember always having to clean the 80-8 head ... it would get so dirty so fast ... but on a Scully 280, I rarely need to because gunk doesn't build up very easily. Ampex decks seem to be somewhere between the two. But that's just my personal experience.
I have a hard time believing that demagging is just made-up stuff that a bunch of people decided is necessary when it's not ... there are probably rare instances when heads can get magnetized, and it's a standard practice to prevent these kinds of problems, just in case. That's what common sense would tell me.
In my experience, the importance of cleaning heads and demagging (and how often you do it) is dependent on the machine and the tape. I remember always having to clean the 80-8 head ... it would get so dirty so fast ... but on a Scully 280, I rarely need to because gunk doesn't build up very easily. Ampex decks seem to be somewhere between the two. But that's just my personal experience.
I have a hard time believing that demagging is just made-up stuff that a bunch of people decided is necessary when it's not ... there are probably rare instances when heads can get magnetized, and it's a standard practice to prevent these kinds of problems, just in case. That's what common sense would tell me.
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Nobody has said that here.
Does common sense tell you to put air in your tires just in case they are low, but not to check the pressure before you pump more in, or check the pressure after you've pumped more in?
Does it make common sense to just add air once a week, just to be "safe"? Or could adding more air cause a problem, just as easily as adding more are solves a problelm? How would you know which situation you have? Is there any instrument you might use to do a before and after test? Is there any point in doing that?
Is this parallel making any sense?
Is this a post where I can only speak in questions?
rob aylestone
Moderator
My old Ferrograph had a Ferrograph branded damagnetiser. I doubt they had marketing gimmicks in the 70s.
"They even include their own special branded bumper jack, custom designed to fit their car!"
No marketing gimmicks in the 1970s? You must be kidding. Though I hardly think a branded demangetizer is a marketing gimmick. More like a clue. "Wow, their machine is so poorly designed that it needs demagnetizing often enough for them to include the tool."
Kind of like your new car coming with a can of transmission leak sealer...with the manufacturer's brand name on it...you know, just in case.
Thanks. I apologize for the attempt.
Lt. Bob
Spread the Daf!
lol ..... you don't get sarcasm either do ya'?
Lemme be clear ...... I'm sure there's plenty of evidence that demagging isn't necessary.
In fact, for the sake of honesty I'll have to admit that I've never really run into problems that were solved by demagging.
I've questioned the need myself thru the years.
The idea that the super low levels of magnetism on tape could magnetize everything it touched always seemed a little suspect.
So while I still demag on occasion IF I EVEN USE MY TAPE MACHINES ANYMORE, it's more because I've worked with these machines for so long that it's kinda a habit more than anything else.
And it's part of the process that I enjoy doing even though it's probably a waste of my time.
But I have absolutely as much experience in the industry as you do.
And so do some others here.
So talking down to us as if somehow you're THE expert and none of us know anything is crap.
If you want to be a jerk about it knock yourself out; but it's not necessary and certainly counterproductive.
Lemme be clear ...... I'm sure there's plenty of evidence that demagging isn't necessary.
In fact, for the sake of honesty I'll have to admit that I've never really run into problems that were solved by demagging.
I've questioned the need myself thru the years.
The idea that the super low levels of magnetism on tape could magnetize everything it touched always seemed a little suspect.
So while I still demag on occasion IF I EVEN USE MY TAPE MACHINES ANYMORE, it's more because I've worked with these machines for so long that it's kinda a habit more than anything else.
And it's part of the process that I enjoy doing even though it's probably a waste of my time.
But I have absolutely as much experience in the industry as you do.
And so do some others here.
So talking down to us as if somehow you're THE expert and none of us know anything is crap.
If you want to be a jerk about it knock yourself out; but it's not necessary and certainly counterproductive.
famous beagle
Well-known member
Let's fight about it some more. I think it's fair to say that there are two camps here:
1. They want to demag on a regular basis because they feel it can only help (assuming done correctly), it's part of the routine, and it's what the manual says to do.
2. They want to question the need for demagging regularly because they don't feel it's necessary on newer(ish) machines.
Those positions have been firmly established, and lots of math has been presented. I don't see anyone changing their opinions.
1. They want to demag on a regular basis because they feel it can only help (assuming done correctly), it's part of the routine, and it's what the manual says to do.
2. They want to question the need for demagging regularly because they don't feel it's necessary on newer(ish) machines.
Those positions have been firmly established, and lots of math has been presented. I don't see anyone changing their opinions.
famous beagle
Well-known member
Or maybe more like your new car coming with a spare tire and a lug wrench?
famous beagle
Well-known member
You got me!
I apologize to the group.
I did spend a month researching this, end to end, with specialized instrumentation, followed up by interviewing the industry experts. If anyone else has also done that, they haven't said so. Just thought there may be some benefit in sharing the findings. Perhaps not.
Thanks, I really don't want to be a jerk. I've made my points, and been rebuffed. Certainly, that's counterproductive from my side.
Oh...wait...were you talking to me?
Just a thought...might add group 3 (with one member):
3. They want to demag only when necessary, as indicated by analysis and symptoms, then determine the root cause in an effort to eliminate or reduce that necessity.
I actually did, in another post, but again here's how it happens as I understand it.
Ferrous material has a natural magnetic state to which it will return if left uninfluenced by external fields. Sometimes that natural state is polarized, sometimes less so, and sometimes it is neutral to the point where it can't reliably be measured. If a material has a natural magnetic state of polarization, we call that a permanent magnet. How readily a material will accept a new polarization is a function of it's coercivity. How much it retains polarization is a function of its retentivity. Some materials have an ability to return to their natural polarization fairly quickly, even after their fields have been temporarily neutralized by degaussing. This can be seen in some (older) tape machine parts, where there's no magnetizing field present, yet the parts "accumulate" their own field. So one possibility is that the screwdriver's natural state is polarized, and it is to that state it will slowly return even after an attempt to demagnetize. I found many materials would do this when I had access to the Gaussmeter with hall effect probe.
If you're certain your screwdriver was not initially magnetized, one possibility is that it has very low coercivity, and can be easily magnetized by external fields. There is a pretty large .5 gauss field just about everywhere on earth. And much stronger fields around other metal parts. It could have taken on a polarization from an external field. Even in the steel office building where I did my testing, I found strong DC fields in various placed. AC fields were, of course, everywhere. One thing to know about AC fields is, we can't always assume they are capable of demagnetizing, as distortion of the magnetic waveform in one polarity or the other effectively produces a DC component which can obviously magnetize something. Distortion of the magnetic waveform is caused by various means, some of them also being a distorted electrical waveform, but that's one way to get a polarizing field out of an otherwise depolarizing system. But in short, there could have been an external magnetizing field present near the screwdriver, and it's material was easily magnetized.
Anyway, it pretty much has to be one or the other.
miroslav
Cosmic Cowboy
Yes.
It's about the metal, and the magnetic fields involved.
Tape is made to be easily magnetized...to allow it's particles to easily respond to the very small magnetic field coming from the heads which are directly in contact with the tape.
The tape recorded parts are made NOT to be easily magnetized by the magnetic field coming from the heads.
I bet if you lifted the tape even 1/8" off the heads...nothing would happen to it (just my guess).
That's why the demags (the good ones...the small ones are a waste of time) are made to emit much stronger magnetic fields, in order to overcome the reluctance of the recorder's metal if it's going to have any effect. Also why you have to watch how you demag when you do, because you can leave much greater magnetism on the hard to magnetize metals, than what's coming from the heads...and then you'll have that much harder a time removing it. IOW...the metal is not easily magnetized by small fields from the heads...but with a strong field, it's possible and at the same time equally harder to remove once there.
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You need to read my stuff a bit more carefully. That's not what I said at all.
Forgive me being picky here...tape is actually not made to be easily magnetized, or it would be easily erased by stray fields. It's actually fairly difficult to get it recorded, which is what that monster bias signal is all about. Yes, it's easier to magnetize than a lot of materials, but it has to be at least sort of resistant or it won't work as a storage medium.
Correct! 1/8" will completely prevent recording, though some erasure may occur from the erase head. A micron or two will actually inhibit recording quite nicely, which is why we have to keep heads clean.
Yup. The field on a head is concentrated in the head gap, less than 10 microns wide, and outside that area the field is impossible to even measure accurately. On a properly operating recorder all fields on all heads are AC, which are actually demagnetizing fields. No magnetic field form a head could ever magnetize any parts other than the tape or the head itself, if the machine is defective.
However, the field from a demagnetizer is a monster, both in cross-section and strength. Just a bit of non-symmetrical field, or a switching transient can present a pretty rediculous pulse in one polarity, which is a nice magnetizer.
miroslav
Cosmic Cowboy
I was indirectly talking about tape VS the rest of the transport metals. The tape is the only thing really affected by the heads.
That's what I meant by easy to magnetize....maybe my wording implied the wrong thing.