The engineers who designed and produced these machines know far more about it than I do. I follow their recommendations. Somehow I think they wernt responding to a myth.
And I just went and dragged out the manuals for final generation pro tape machines from 3 different manufacturers that state "Clean and De-magnetize daily..."
What you are actually saying is:
1. The engineers wrote the manual (not true)
2. The engineers designed a machine with ferrous material with a highly polarized natural magnetic state (not true)
3. The engineers designed a machine with electronics that would magnetized heads in normal use (also not true)
4. The engineers, as smart as you may say they are, are so inept as to design a machine that needs daily repair to compensate for poor design. (sadly, sometimes true, mostly not. Perhaps the MR16 is one where 4 is true)
I've already explained why the manuals say what they do. I've already explained how to determine the need for demagnetizing.
All magnetic materials have the property of coercivity (how much external field strength it takes to reorient it's magnetic state), and retentivity (how much magnetism remains in the material after the magnetizing field is removed. AC fields cannot magnetize anything unless they are asymmetrical. That means the field on a recorded tape, and the bias and erase signals, as well as the audio component.
I've already noted several times that
I'm not saying "Don't demagnetize", I'm saying, if you must do it, do it with testing before and after. Before you start, look for an elevated noise floor with virgin tape, look for even-order harmonic distortion in recorded signals. Look for a gravely sounding noise component in recorded signals. Look for high frequency erasure (play 5 second loop of 15KHz tone, and watch for gradual lowering of signal. 25 plays and no change, you're good). Then demag, and test all those things again looking for a change, either better or worse. You will find that sometimes you make things worse, sometimes you make things better. That should tell you that just randomly demagnetizing is a crap shoot.
Years ago when I researched residual magnetism in tape recorders, one of the experiments I did was to test the effectiveness of standard degaussing methods. I had a ferrous guide component from an older machine (I think it might have been from a 3M/Mincom). Using a Bell Gaussmeter and hall-effect probe, I measured it's magnetic state, found it to be almost immeasurable. Then I attempted to magnetize it using a rather strong permanent magnet. Getting that piece to accept magnetism wasn't easy. I found I could not magnetize it by passing a pole of the magnet across the guide perpendicularly. Not at all. I had to use the old science class brushing technique, brushing the magnet in a single direction repeatedly in line with the guide. I never did get the thing up to 15 gauss, but I took on something like 8 or 10 gauss. Then I tried to demagnetize it with a tape head degausser. I used my best technique, but my first attempt did very little, reducing the field strength by a gauss or so. My second try actually increased the field strength, and only after several tries, measuring with the gauss meter each time, was I able to neutralize the guide again. That tells me that no matter how good you think you are at demagnetizing, you can actually make it worse just about as often as make it better. You need to measure the result to know. For a ferrous guide, you could use a gauss meter, but not for heads. The field is concentrated in the gap, and no magnetic measurement device, not even my tiny hall-effect probe, can get into that field. So you're left with measuring the effects of the field, as mentioned above.
BTW, I did magnetize the guide and leave it on the bench over night. The next morning, it was almost demagnetized. Had elves invaded my shop and degaussed it over night, or did it return to its natural magnetic state? I'm guessing the latter, and that was probably why 3M chose the material.
Taking a look at a few tape machines, the Studer B67, A80, A810, A820 (I stopped dealing with them after that) don't have a single ferrous component in the tape path except for the heads. Some machines may have steel guides, but I'll bet not a whole lot. Apart from heads, what would you demagnetize?
Consider heads for a second. The erase head has a huge ultrasonic AC signal applied to it when operating, which is stronger than the field at the tip of a head degausser, and that erase current is demagnetizing the head all the time. If the erase current is ramped up and down, as it should for punch-in/out, there's no single polarity transient to magnetize it. The same is true of the record head, the bias must be free of any DC component, or it will have lousy noise performance and high even-order distortion. If the bias is ramped up and down, again for punch-in/out, the head is self-demagnetizing. The play head should have no voltage applied at all, so there's no way it should become magnetized by the electronics, unless something is broken. Moving tape presents an AC field, and an very weak one at that, to all tape path components. All AC fields demagnetize, they do not magnetize.
So, why would a head or a guide become magnetized? Only two reasons. One, it's a manufacturing defect, two, there's been an electrical failure and a DC component has been applied to the head, magnetizing it.
Ever seen an error in an equipment manual? I have, and not just a few, many. Ever see warnings and cautions in manuals that are clearly there to cover the manufacturer's legal tookus? The manual may suggest daily degaussing, but a responsible tape operator would determind the need, then the effect of degaussing, especially since the very act can end up magnetizing instead.
All magnetic materials have the properties of coercivity and retentivity. I haven't studied tape formula enough to know if one tracks the other, but they seem to. However, it seems some forum members also have these properties, only they appear to have very high coercivity, and very low retentivity.
And finally, I don't care if one guy insists on randomly magnetizing and demagnetizing his heads and guides every day, with his demagnetizer. Go ahead, knock yourself out. I'll just make sure not to give that guy any of my masters to work with.