.... The best test for demonstrating the phenomenon is the one I shared earlier in the thread from Jim McKnight of MRL. And it works.
More questions:
Do you perform the McKnight test every time you degauss?
Do you do a before and after using the McKnight test?
Again, what specific machine?
You seem to be getting the point that I'm saying "never degauss". If I'm leading people to that, I'm sorry, that's not my point at all. My point is never degauss blind, because if there is a problem it seems to fix, you need to find the cause.
There are many machine defects that can cause residual. Asymmetrical bias is one, leaky caps in the record amp is another, and a defective head is yet another. These are real, but can be fixed.
Degaussing blind is like changing your car's spark plugs once a week because "spark plugs foul with use". True, they do foul and carbon up, but that's a symptom of a problem, not a product of normal engine operation. If you drive a 1959 Chevy, it might be necessary for real, but if you drive a modern car, you might sell it before it needs new plugs. Yet, if they did foul often, would you begin changing them routinely, or find out why? And wouldn't you know they fouled by the performance loss?
If you have a vintage machine (which they all are now, right?) then anything is possible. But if you know that because routine degaussing has been traditionally recommended as a cautionary measure, then assume that the reason is that the normal use of a tape machine causes residual magnetism by nature, you are making a connection on your own of two unrelated premises. The connection is not actually stated. Rather, the procedure is recommended because of the consequences of a defect: destroyed masters and test tapes. It's a precaution.
What I'm suggesting is, monitor the symptoms, find the cause and fix it, then
continue monitoring for symptoms. Otherwise, how do you know that once a month/week/year is enough? What if the problem comes back in 24 hrs? What if you induce a temporary field by sloppy degaussing? How would you know for sure you're not making it worse? That's why I'm saying that
blind degaussing is not recommended.
Here's the wake-up call. In my research I found that even though I thought my degaussing technique was perfect, it wasn't. I could easily increase residual without knowing it. I had followed the recommended procedure, slow movements, not turning the degausser on or off near the machine, etc. Monitoring the results taught me how to properly degauss. It was hard to learn, because I had to deliberately magnetize the deck so I had something to degauss. If you have a hard time with not routinely degaussing, just try taking a big permanent magnet to your play head!
Now consider tracking down the cause. Will playing tape magnetize a head or guide? Tape has, by design, a very high coercivity level. That's how strong a field it takes to begin to magnetize tape (record). It takes a very strong field to get past that coercivity level, and that's what bias is about. Pushing you way past the non-linear region, into the levels above which tape will take on a more or less linear representation of the signal you are trying to record. The same high coercivity is responsible for the tape being resistant to stray fields during storage, like the earth's field for one. If the field strength is too low, the tape just resists it, nearly completely. Try recording with no bias; it just doesn't happen. Yet, the actual recorded flux on tape is very very weak, barely inducing a current in the play head which takes a huge amount of amplification to recover. So, lots of field strength to record, and the result is a tiny bit of recorded field.
All magnetic materials have a certain coercivity level below which they ignore an attempt to magnetize, above which they may take on a polarized state. Materials also have retentivity, or the ability to hold a magnetized condition. Tape has very high retentivity, and coercivity, heads have very low retentivity and relatively low coercivity, or distortion would be a problem, and playback wouldn't be possible. But, that means they are hard to magnetize with a condition they hold on to. The field moving tape presents to stationary parts is alternating, which is a degaussing field. The flux on tape is so weak it can't magnetize parts even if it's stationary. So, playing tape won't magnetize anything in the tape path.
Recording involves very high bias currents, again, AC, which tend to degauss. The audio signal mixed in is much lower, but still AC, and therefore degaussing in effect. Erase currents are basically bias, and designed to degauss tape, and will degauss the erase head when it does.
Heads must, to operate properly, have very low retentivity and coercivity. If they didn't, they would be a cause of distortion and non-linearity. And the signals applied to record and erase heads will degauss them. There should be no electrical signal on a play head other than that induced by the moving tape, which is, again, AC, degaussing if it were strong enough.
So, that's why I say there is nothing in the process that should magnetize parts. There are defects, however, than can. DC on heads, asymmetrical bias, head materials with a polarized natural state (these are defective, should be replaced), high switching transients sent through heads (power up/down, mode switching) for the big basic ones. The presence of any of these represents either a serious design flaw (like that of an extremely primitive or low cost machine) or an issue requiring repair.
Let me pose a final question:
What is the cause of residual magnetism in a tape path if it's not a defect in the machine?
Sorry for the long post, there's just no way to talk science without at least some detail.