demagnetism

[evm1024]

Starting a new thread to explore demagnetization of heads. From fienlly nack in to analog

The screwdriver's magnetism is nothing compared to the much more intense fields set up by the heads in normal recording. And if your compass is not attracted to a magnetic material like a tape head or a demagnetized screwdriver, it won't be able to find which direction is north either. To listen to some people, you'd think tape had to be kept away from motors (pneumatic tape recorders, anyone?) and even out of the earth's magnetic field-- which of course is impossible for practical usage.

You say "50 times." How many would it take to satisfy you? I repaired around a thousand TEACs from customers all over the hemisphere, and found that demagnetizing was never necessary for full performance. (But unlike your avalanche slope, I can't think of any situation where less that full performance from a tape recorder could cost someone their life.) While working at TEAC, I finished up an associate's degree in recording; and although we frequently cleaned the machines and practiced aligning them, the school did not promote demagnetization either IIRC.

Just a few thought come to mind....

The compass needle will indeed be drawn to ferrious materials. However it will not show a preference as to north or south when the material is not magnetized. It is a crude magnetmomoter. (sp)

I can understand your desire to let us know that demagnetizing is rarely needed. And that is a good thing. With that said, what harm comes from excessive demagnetizing?

Do you think that demagnetizing a deck that you do not know the history of is a waste of time or un-needed?

I grew up with the common practice of demagnetizing the tape path every so many hours. I presume that this practice came about in a time when it was actually needed.

This presumes that the tape available had less signal, was more prone to ereasure and so on. Is this true? Or was this the results of horn rimmed glasses engineers (with pocket protecters) going overboard?

It is an interesting discussion.

 

[FALKEN]

I thought you were supposed to demagnetize to protect the alginment tape.

 

[Wil816]

With that said, what harm comes from excessive demagnetizing?

The one thing I've seen that seems like magnetized heads and cuts the performance is when domains from recorded tape get stuck in a pit at the head's gap. Once the head was cleaned however, the apparent problem was corrected, without demagnetizing. A head with pits will continue to have problems though until you lap or replace it. I've wondered though how many of those pits I've seen under the microscope were put there by someone bumping the head with a demagnetizer that no longer had a thin plastic protector over the end. Aside from that, I've never seen any evidence that a demagnetizer ever caused any harm.

I grew up with the common practice of demagnetizing the tape path every so many hours. I presume that this practice came about in a time when it was actually needed.

This presumes that the tape available had less signal, was more prone to erasure and so on. Is this true? Or was this the results of horn-rimmed glasses engineers (with pocket protecters) going overboard?

I'd like to do a search for the answers when I get some time. Please post if you beat me to it. I suspect that if anything is able to magnetize a part in the tape path at all, it'll do it instantly instead of making the remanence accumulate over time (like so many hours of use). I started out demagnetizing too, but observation and the experiment I read about kind of pulled the rug out from what I had been told earlier. Magnetics is kind of a field all its own, and most electronics engineers really don't know a lot about it. I'm far from done learning, and have a lot of catching up to do even in analog since I've mostly been out the field for about 23 years.

A little OT: One of the projects I as an electronics engineer worked on 19 years ago involved the designing of record/play electronics for a cassette tape recorder. This one did not require good music quality-- just voice on one channel and data on the other, plus computer control of the transport-- but the tape-recorder bug (interest) bit me again. A recent need for recording some machinery noise in the field for analysis back at the office prompted me to get my Marantz portable professional cassette refurbished a year ago since it was about $600 cheaper that way than buying one of the new equivalent machines. A Nagra would be much nicer, but the Marantz did the job fine. I'm able to design and build myself a digital one, but there's still something about the analog that attracts me.

 

[Beck]

I advocate demagnetizing on a regular schedule. However, there are some machines in more need than others simply because of the metals used in the tape path. For example, my TSR-8 appears to have all aluminum and austenitic stainless steel guides... no magnetic field is possible on these parts. The heads will still need degaussing. Other machines I own have metals in the tape path that pose a threat to tape if degaussing is neglected.

With all due respect to Wil816 and his extensive background (We appreciate the informed input), I simply don’t trust some methods of determining the presence of a polarized head or tape path.

Magnetometers can be unreliable at these small flux levels. We’re talking nanoWebers here. From a physics standpoint this is an infinitesimal amount of flux. We talk about hot levels and hot tapes like GP9, which runs at 520 nWb/m, but that it is no flux at all in the world of magnetics. A magnetometer may tell you if your tape path is seriously magnetized, but smaller amounts that can cause damage to tape may not register.

This is why I favor J McKnight’s method, which involves taping a high frequency tone, somewhere between 16 and 20 kHz, and then playing the tape back. A magnetized path will erase frequencies at this level and you will see it on your VU meters. With each pass it will erase more, thus each playback will be lower until it can actually drop by several dB. If this should occur with a calibration tape you are going to skew your EQ curve because the high frequency tone will eventually be significantly down from where it should be. Lower frequencies will not be affected as quickly or so drastically. Your 1 kHz tone on an MRL tape will seem perfectly fine, but your 16 kHz could be slowly dying.

Also from a preservation and archiving standpoint degaussing is an important component of routine maintenance. It might take years for a marginally magnetized tape path to audibly alter high frequency content, as most adults can't even hear the frequencies that are being erased. But you really don’t want that to happen at all.

People will have different views on degaussing based on individual experiences and purpose. Frankly many people are afraid of the whole idea. IMO, degassing should be done, but done correctly or not at all. That means making sure the protective plastic is on the degaussing device and best practices are followed.

Metals can become charged from regular contact with magnetic substances like tape, but also from extreme temperatures and impact. Normally, a machine used in a personal studio is going to take a long time to develop problems. This is another valid reason people have opposing views on the subject. I’ve been degaussing since I was a teen, so this all easy for me to say, but I understand people’s fears of screwing things up.

 

[MCI2424]

Starting a new thread to explore demagnetization of heads. From fienlly nack in to analog



Just a few thought come to mind....

The compass needle will indeed be drawn to ferrious materials. However it will not show a preference as to north or south when the material is not magnetized. It is a crude magnetmomoter. (sp)

I can understand your desire to let us know that demagnetizing is rarely needed. And that is a good thing. With that said, what harm comes from excessive demagnetizing?

Do you think that demagnetizing a deck that you do not know the history of is a waste of time or un-needed?

I grew up with the common practice of demagnetizing the tape path every so many hours. I presume that this practice came about in a time when it was actually needed.

This presumes that the tape available had less signal, was more prone to ereasure and so on. Is this true? Or was this the results of horn rimmed glasses engineers (with pocket protecters) going overboard?

It is an interesting discussion.

OK. Demagnatizing is my way of saving a $700 alignment tape from getting f'ed up.

Good enough reason?
I think so.

 

[evm1024]

all it takes is one

OK so we have 2 who voice the thought that important tapes (MRL, maters etc) should have tape paths free of residual magnetism. This may be a useless action or it may just once in a blue moon save a tape from HF losses.

Oh, RE avalanche paths...BTDT. I have been caught in an avalanche. It is worth avoiding.

 

[MCI2424]

OK so we have 2 who voice the thought that important tapes (MRL, maters etc) should have tape paths free of residual magnetism. This may be a useless action or it may just once in a blue moon save a tape from HF losses.

Oh, RE avalanche paths...BTDT. I have been caught in an avalanche. It is worth avoiding.

Actually, MRL tells you to demag before you load an expensive alignment tape. It is akin to using a shock mount on a mic in the studio. Maybe 99% of the time you will be OK but that 1 time.......................

Oh boy.

I, myself, both of us have never had a problem with magnetized heads/guides but I just can't bear the thought of having to buy MRLs for a 2" AND 1/2" machines.

So, I just do it because..................