This question is for those of us who had cassette decks and used them alot. Did you own and use a head demagnetizer? Have you noticed a difference or is this one of those mythical audiophool ideas?
No that is not mythical.
Tape heads and guides do get magnetized over time. This not only causes poor high frequency response, but also tends to erase the high frequencies on the tapes.
Now machines vary in their propensity for this. Machines with single record/playback heads are less prone to it than the more expensive three head machines with separate record and playback heads.
The reason is that the machine is supposed to be designed with some degree of auto demagnetization. This is done through the tape bias. A critical function during record is the AC bias signal to the head. This actually needs setting for different brands of tapes. It is best to set up a machine (this takes instruments on most decks) for one brand and brand type, of each of the different categories. Some very expensive machines had auto bias, or a means of easily setting bias and EQ, as they had built in instrumentation, like my pro TEAC machine.
Now the record bias is a high frequency AC signal and it tends to demagnetize the head to which it is applied. However on a lot of machines especially the cheaper ones, this signal would die too quickly or even instantly, leaving some magnetization on the head. On a good machine the bias should die slowly.
So yes, machines need regular head and guide cleaning with isopropyl alcohol and regular demagnetization.
Tape machines need lots of maintenance. That is one of their problems. This is especially true now, as tapes are aging and the binders failing. This results in excess oxide shedding and more fouling of heads and guides.
Now you need to demagnetize properly, or you make things worse. You start the demagnetizer away from the machine, and slowly bring it up to each individual part to be demagnetized, hold it close, but NOT touching for a few seconds, and then slowly withdraw it keeping the demag unit on, so the field dies away slowly and not leave the part/head with more magnetization.
Doing it correctly is very important.
The aging tape problem is huge now and getting worse with time. The fouling issue is only one part of the problem, the other is increased head and guide wear, as the binders which are failing are also the lubricant.
I have a tape baking system here for restoring binders, at least temporarily, to get a digital archive copy.
Tape machines are trouble, and not for the faint of heart. They get out of adjustment. Azimuth alignment of heads is crucial. Even very small azimuth errors indeed cause serious loss of high frequencies and degrade S/N. The high end NAKS had a complex auto alignment system. Otherwise to do the adjustments takes alignment tapes and instrumentation. The other critical adjustments are tape tension across the heads, bias and equalization adjustments.
Back in former times individuals who owned expensive machines like Revox took their machines into the dealers regularly for these complex adjustments. I remember years ago, Hi-Fi Sound in the Twin Cities had a long bench of techs, who did nothing else except tune up tape machines, mainly Revox as they were a dealer. I have had tape machines since my teens, and do my own maintenance. It is a highly skilled endeavor.
