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Thread: Where can I get a tape head demagnetizer?

  1. #21
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    Quote Originally Posted by Beck View Post
    Demagnetizing your tape path is absolutely critical. All tape machines need this procedure as part of routine maintenance. Every time you play a tape on magnetized heads some of the signal on the tape gets erased, especially high frequencies.

    TWO DEMAGNETIZERS THAT WILL GET THE JOB DONE RIGHT:

    Han-D-Mag (Best with 1/4" and up)
    http://blevinsaudioexchange.com/handmag.html

    TEAC E-3 (Best with cassette and up)
    Tascam E3 Head Demagnetizer
    (not in stock but there's a nice photo)

    Demagnetizers use alternating magnetic fields to create a neutral state on the tape path and heads. The demagnetizer must be strong enough to overcome the magnetized tape head and other metal in the tape path. The above are two of the best. The Teac E-3 is getting hard to find new.

    Check eBay.

    http://cgi.ebay.es/ws/eBayISAPI.dll?...tem=7514247729

    http://cgi.ebay.es/ws/eBayISAPI.dll?...tem=7514281807

    Cheepy models like those once made by Pickwick and sold by Radio Shack or the Sony HE-4 are relatively light in weight and too weak to neutralize the magnetic charge on the metal being treated.

    MODELS TO AVOID:

    Sony Head Demagnetizer HE-4
    Nakamichi Head Demagnetizer DM-10 and no-name brands of the same model number
    Radios Shack models 44-211 and 44-207
    MCM Tape Head Demagnetizer Model 720880

    …or anything about the size and weight of a pencil-style soldering iron.

    -Tim

    Why would the Sony Head Demagnetizer HE-4 be any different from the Teac E-3. They are both about the same size. The Teac E-3 has a small flimsy tip at the end. It looks like an old telescoping whip antenna. How would that be any stronger, magnetically or physically than the Sony HE-4? For example the Sony HE-4 is listed as the head demagnetizer tool in the BVW-75 BetacamSP Maintenance Manual. The HE-4 has a light at the end of the tip and I think it's perfect to get into small areas such as demagnetizing rotary drum heads for 8mm and 6mm tape paths such as Hi-8, Digital8, DVCAM and MiniDV camcorders. The Sony HE-4 can pick up a screw therefore I do think it's strong enough to overcome a magnetized tape head.












  2. #22
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    First, what I'm about to write will be viewed by many as controversial, by others as wrong, by still others as hopefully insightful. But I have no doubt that, because what I'll say flies in the face of "common wisdom", "standard practices" and even many maintenance manuals, there will be those that just ignore all of this. That's fine, do what makes you feel good, but there is at least some science behind my statements below.

    Several decades ago I did some in-depth research into the effectiveness, and necessity of demagnetizing recorder parts as part of routine maintenance. I won't bore with details, but part of the research involved renting a lab-grade hall-effect probe gaussmeter in order to measure DC magnetic fields, analyzing their effect on the recording process, and their effect on pre-recorded test signals. In addition to the gaussmeter, a swept spectrum analyzer was used, as was a low distortion sine generator, and several strong permanent magnets as DC field sources.

    In short, what I found is this:

    1. Operating a tape recorder does not cause anything in the tape path to become magnetized. Not even a tiny bit.
    2. All parts including heads have made from materials with a natural neutral magnetic state, to which they tend to return even if deliberately magnetized.
    3. The magnetic flux required to affect a recorded tape is much higher than anything anywhere around the tape recorder, except in the erase and record head gaps. The effect of a steady-state magnetic field (a DC field) on recorded tape is first noticed as partial erasure of high frequencies, and that occurred with a field somewhere above 200 Gauss. Tape has very high coercivity by design, meaning you need a high field strength to cross the threshold above which the tape will become magnetized. That's what the bias oscillator does. By being many times hotter than the actual audio signal on the head, the bias forces the signal over the coercivity threshold into the more "linear" range of the tape. But that means it's harder to erase tape than you might think. Typical guide and head residuals were in the zero to 20 gauss range, far below tape coercivity levels.

    4. Erase and record heads are self-demagnetized by the bias oscillator, which provides an AC field in the gap that is thousands of times higher than what a de-magnetizer can induce.

    5. The field found on recorded tape is incapable of magnetizing anything. It is minuscule, and when the tape moves, it becomes an AC field, which would demagnetize if it had any effect.

    6. The effect of a DC field near or in a record head produces two measurable results: First, a significant elevation of even-order harmonic distortion, and second, an elevation in low frequency noise (a sort of gravelly sounding noise). But again, the field has to be fairly high for this to happen. You need a spectrum analyzer to differentiate even harmonics from the normal odd harmonics created in the recording process.

    7. If tape machine parts become magnetized, there is a defect in manufacture (the material doesn't have a natural, neutral magnetic state), or a defect in the tape machine, either due to a failure or design flaw. Asymmetrical bias waveforms will present a DC component, any DC on a head due to a circuit defect (bad blocking cap, for example). Demagnetizing may temporarily remove the residual, but if it creeps back, something is "broken", and needs repair or replacement.

    8. Lastly, casual use of a demagnetizing device, even if the classic proper techniques are observed (slow removal, switching of when several feet away), sometimes resulted in an increased magnetic state rather than a decrease. There's no way to tell if you are demagnetizing, or magnetizing, without further testing for the effects of magnetism. Once again, these are HF erasure of a recorded tape, or increase in noise and even-order harmonic distortion during record. Yes, you need instrumentation to do this.

    During the research, I also found that attempts to deliberately magnetize a tape head with a strong permanent magnet were successful, but over time, the head returned to a neutral state by itself. It took 24 to 48 hours, but then all the effects of magnetism vanished. The same was true of guide parts, though some could not be magnetized being non-ferrous.

    In summary, routine demagnetization is unnecessary, and could actually make things worse. If you plan to demagnetize, you also need to be able to measure the effects of residual magnetism to see if you've made it better or worse, or if it was needed at all.

    Have fun,

    Jim

  3. #23
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    interesting. Lots of speculation about this and not the first time I have heard that demagnetizing is unnecessary. When I first bought one of my multitrack decks, the previous owner did not own a demag unit and used it for several years before he passed it on to me.

    I used the machine a handful of times before I obtained a head demagnetizer and actually demagged the heads and tape path. I noticed that with NR off, the tape hiss signal would 'fluctuate' and not remain steady when recording even with no input signal. After I demagged the heads I did not have this problem again even recording on the same tape and haven't had the problem since on that or any of my machines. I also have recordings to prove it.

    Now that could be because of something else but hey it's no big deal to just demag the tape machine every once in a while.

  4. #24
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    You've basically confirmed my conclusion. You had "measured" a symptom, demagnetized, and "measured" the result. If the problem had returned, it would have been prudent to track down which path element had accumulated the magnetism, and address the cause.

    I didn't mean to imply that residual harmful magnetism can never happen, just that is the result of an anomaly, and indicative of a correctable problem. Normal use does not create the issue, and routine demagnetization is not required or desired.

    Perhaps I should emphasize, the effects of residual magnetism are what needs to be monitored. It is not possible to measure the residual magnetism in tape path elements, other than the very large ones, without a very small Hall-effect probe. The classic hand-held "magnetometer" is not useful, though often marketed for that purpose. In no case is it possible to accurately measure the field on a tape head, as it is concentrated at the gap which is far to small for even a Hall-effect probe. That's why we need to concentrate our observations on the effects of a residual field rather than the field itself. Also, the symptom that changes observably with the lowest field strength is the recorded even harmonic distortion, which is clearly increased with even a field of 80 to 90 gauss, which is still a lot, but much less than the over 200 gauss required to erase high frequencies. Unfortunately, recorded even harmonic distortion only indicates a field at the record head, nothing else. The second most sensitive symptom is the recorded noise floor, which should only be 1dB or so higher than the noise floor of virgin tape. However, since bias asymmetry and erase current asymmetry have similar symptoms, they need to be ruled out as well.

    If you record a 15Khz tone and play the tape over and over, say, half a dozen times, and don't observe a significant loss in level, you don't have a problem. Significant loss would be more than 1.5dB overall, long term average, ignoring short term dropouts. There is normally some HF loss with repeated plays, as some tapes are not calendared as well as others, which can be seen by very short period dropouts. This can be ignored, we're watching for long term average level loss that keeps getting worse with each pass. After several hundred passes, there will always be noticeable degradation in HF stability, which is why test tapes are only good for so many plays, then should be replaced.

    Incidentally, there's a post up the thread a ways that references demagnetizing video formats, particularly digital video tape formats. Video tape has an even higher coercivity, and tape made for digital use is higher yet, meaning, if there are measurable effects, you have a really big problem that should not simply be addressed with a demagnetizer. Digital tape formats are most immune to DC fields, as they operate with very high coercivity and linear response is not a concern. This mode of operation is required for minimum bit-error rates, and is highly resistant to HF erasure as a symptom, and not at all affected by an increase in harmonic distortion since they operate by recording at or near to the saturation level. Anyone who has tried to bulk-erase video tapes would notice that it's harder to accomplish than bulk erasing audio tapes.

    Now, if we could eliminate the problem of HF loss with repeated plays, and make a recording system immune to magnetic fields, without noise increase and high levels of harmonic distortion, and mechanical speed variations, we'd have a much more stable and reliable recording method. Oh, sorry, that would be digital recording.... See, you guys just re-discovering analog tape have a different view than those of us that started there, then moved to digital. We always were fighting for lower noise, lower distortion, less flutter, better tape path guidance, and a list of other issues with analog. Digital solved all of them, and introduced a new list of problems, which have now been mostly solved. We kept trying to make the recording be indistinguishable from the raw console output, and it was always clear which was which if you were running tape. Digital fooled us far more often. We were trying to closely replicate the original. Today, it's not about matching to the console, it's about creating a particular "sound" based upon a set of historic defects. Not bad, just a different goal. Analog tape is a massive and complex "mask" on the original that, in some cases, gets you closer to that goal.

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    Here is a chart from a Service Manual of a MiniDV HDV camera, showing the periodic inspection list. This includes cleaning and degaussing the rotary drum.



    MiniDV HDV (High definition MiniDV) has dropouts and errors. Moving from Standard Definition to High Definition has made it worse where the tape had to be re-designed for better quality to improve dropouts and error rates. The higher end tapes are now made with advanced metal evaporated II process (smaller particles) and dual layer. This is to reduce errors and dropouts.





    Visual dropouts or glitches do happen on digital tape and it does ruin a shoot. Next time you watch TV and see a video glitch, it may not be a transmission hit, but a tape hit in recording.



    By these newer technologies you have a higher output with lower noise. The improved CNR (carrier to noise ratio) reduces the errors. Can demagnetizing the head after 500 hours of use help error rate? If others notice a reduced hiss of the noise floor on analog tape then I think so. If one is not using the most expensive digital master tape, then I think demagnetizing will have a bigger impact.

    Magnetic Tape Developments for HDV Recording with MPEG-2 Compression from the SMPTE Motion Imaging Journal, May/June 2006, is an interesting read. Link below.

    http://bssc.sel.sony.com/DigitalMast...TE_reprint.pdf

  6. #26
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    Quote Originally Posted by T.Perpendicular View Post
    Here is a chart from a Service Manual of a MiniDV HDV camera, showing the periodic inspection list. This includes cleaning and degaussing the rotary drum.
    It actually understandable that a service manual would recommend this, the manual is no place for controversy. But the concept of routine degaussing has its roots deep in history when low residual ferrous materials were difficult to make, and recorder electronics were far less sophisticated.

    Consider the type of field a demagnetizer creates: an AC field with a zero average polarization. Consider what we have a tape machine that could present a polarized steady-state field, or an AC field with strong DC component, required for magnetization. In analog audio recorders, the strongest field present is the bias and erase field, again, an AC field with zero average polarization. We have the actual audio signal, an AC field with zero average polarization. And the field presented by recorded tape, same thing, an AC field with zero average polarization. All of these would demagnetize, not magnetize.

    In a video deck, the carrier is an AC field, as is the erase field, again a degaussing mechanism in and of itself. Magnetic materials used in tape heads and guides in any magnetic recorder must, by design and necessity, have a neutral magnetic state with low retentivity. This means that even if you try to deliberately magnetize one of these components, after a period of time it will revert to a neutral state. If not, they would need demagnetizing every few minutes!

    The point there is that routine degaussing is unnecessary, and because it can actually increase (temporarily) residual magnetism, even if the user is "doing it right", it is ill advised without monitoring the indicating effects of residual magnetism to see if you have helped or hurt with your efforts. I'm not saying "don't do it ever", I'm saying "don't do it blind" because it's probably not necessary, and you could be making things worse in the short term.

    The article on video tape formulas for HDV was interesting, but please don't extract from it that degaussing will improve your CNR. What should have been apparent from the article was that HDV, and DV tape is very different from audio tape in formulation and required characteristics, and less apparent but still included (in Table 1) was the fact that to improve CNR, you have to increase coercivity, which makes the tape even less susceptible to the effects of low level residual magnetism by making a tape that requires higher flux levels to record on it.

    Video and digital tape is not used to record linear, analog signals. In fact, to reduce dropout and improve CNR, it is formulated to be deliberately used in a highly non-linear region. There is simply no way, apart from a catastrophic failure, that video tape path components could have high enough residual fields to affect CNR significantly. Surface imperfections and head contamination are far more significant issues, by many orders of magnitude.

    Quote Originally Posted by T.Perpendicular View Post
    MiniDV HDV (High definition MiniDV) has dropouts and errors. Moving from Standard Definition to High Definition has made it worse where the tape had to be re-designed for better quality to improve dropouts and error rates. The higher end tapes are now made with advanced metal evaporated II process (smaller particles) and dual layer. This is to reduce errors and dropouts.
    Very true, but in reducing dropout and error rates by using advanced formulas, the coercivity was increased making the tape less sensitive to magnetism.

    Quote Originally Posted by T.Perpendicular View Post
    Visual dropouts or glitches do happen on digital tape and it does ruin a shoot. Next time you watch TV and see a video glitch, it may not be a transmission hit, but a tape hit in recording.
    Again, very true. But don't confuse dropouts or glitches with the effects of residual magnetism. They are not at all the same mechanisms, and only distantly related.

    Quote Originally Posted by T.Perpendicular View Post

    By these newer technologies you have a higher output with lower noise. The improved CNR (carrier to noise ratio) reduces the errors. Can demagnetizing the head after 500 hours of use help error rate? If others notice a reduced hiss of the noise floor on analog tape then I think so. If one is not using the most expensive digital master tape, then I think demagnetizing will have a bigger impact.
    You seem to assume that usage time has a proportional relationship with residual magnetism. I hope I've laid that to rest in the above comments, but if not, I'll say that residual magnetism in any magnetic recorder is an anomaly caused by defects in materials, design flaws, or electronic failures (the latter being the most probable). It's not a result of use, wear, or time.

    You really cannot compare the improvement on noise in analog tape by demagnetizing with reduced errors in a digital video system. The mechanisms are very different, as are the energy levels involved. If you did introduce a very strong DC magnetic field into a digital video system to the extent that it raised the noise floor and thus the error rate, you would be producing a lot of very short term errors that would most likely at some level exceed the system's ability to error correct or error conceal, and the signal would completely collapse. Tape error correction is based primarily on the statistical probability of errors in size and frequency that tape produces. A decrease in CNR would be unexpected by this system, and over-run it's capabilities fairly quickly. Fortunately, with the extremely high coercivity levels of digital video tape (that is the point of it), it would be impossible without the use of a strong permanent magnet, to introduce enough DC field to raise the noise floor.

    Noise caused by a DC field in analog tape is an entirely different mechanism, because the recording method is entirely different. Analog tape depends on a more or less linear relationship between the recorded magnetic pattern and the analog signal. Because tape is highly non-linear, high levels of AC bias signals are used to push the low audio signals through the non-linear range where the tape doesn't want to accept a new magnetic state. An extraneous DC field increases noise by introducing gross asymmetry to the composite record signal. The noise that results is largely the analog of variations in the tape's specific coercivity, or non-linearity levels, reproduced as a fluctuating DC component. It's character is of predominantly low frequency noise, with elevated hiss only in extreme cases. (Hiss in recording does come from other sources).

    Video recording doesn't concern itself with overcoming the non-linearity of the tape, but rather takes advantage of it to extend CNR. The video signal (or data) is recorded with an RF carrier at a high enough level to "force" the tape to accept a new magnetic state. It doesn't matter if this signal is linear or not, because to recover it all we need is its frequency. The analog tape noise mechanism simply doesn't exist in digital or video systems.

    Hey, all I was trying to do was shed some light on the analog tape routine demagnetizing myth. I didn't mean this to be a term paper.

    If you get nothing else from all of this, please get this: for analog recorders, if you insist on demagnetizing, please check for the results with proper instrumentation. If you don't, you can just as easily make it worse. With instrumentation, you'll probably find it's not required. In rare cases, you might find you have a serious problem to fix. For video recorders, frankly, I'd keep that degausser out of the machine. One slip, and you could introduce a problem you that you'll have no ability to analyze (I don't have a bit-error rate counting device in my tool kit, do you?).

    Cheers,

    Jim

  7. #27
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    I'm getting an E-3 with a Tascam recorder I bought on Ebay. I guess the guy didn't know he could sell it separately and make a killing.
    Quote Originally Posted by Steenamaroo View Post
    One day I'll make it into somebody's signature line.

  8. #28
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    Quote Originally Posted by jant View Post
    I am trying to find somewhere that sells inexpensive tape head/degaussers. Radio Shack used to carry them but now only stock the cassette tape demagnetizers. I need one for my space echo. thanks, James

    Look on Ebay for the TEAC/Tascam E-3 Demagnetizer.
    It has a skinnier design which would be great
    for getting into the small Head area of your Tape Echo.
    Don't forget at least 95% Isopropyl Alcohol.
    Good Luck.

  9. #29
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    After being around reel to reel recorders all my life,even as far back as everyone going into the studio and getting it right all at once(before multitracks came along)I have always demagged the heads on my reel to reels.The Shack demagger always did a great job for me.
    Most importantly is to turn the demagger on about 18" away from each head,moving slowly toward the head as well as guides,and using a slow forward movement.As you reach the head or guide slowly circulate the demagger in a circular motion on the head directly on the head(or guide) then,remove the demagger slowly until your about 18 to 24 inches away from the head before cutting the demagger off.Do this on each head and metal guides.Using a good head cleaner and a good rubber cleaner will keep your machine in tip top shape.
    Do not use head cleaner on your capstan pinch roller as the alcohol will dry the rubber out.then invest in a good test tape to align your heads or check their alignment.Being in radio for many years,I always had access to the stations engineer and I would take the machine
    to the station and get him to align my heads as this can be very tricky.If you follow these steps and your heads are worn,a good test tape will determine if they need replacement or not.Hope this helps you out

  10. #30
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    I've had a bunch of old RtoRs over the years. Even had and used a wire recorder for a while. I have definitely noticed a difference on playback, but not very much on others. It's hard to say because there are too many variables to the situation.

    Some actually sounded better after demag. On some, didn't notice any difference at all. But, I never had any specialized, sensitive equipment to measure magnetism.

    "Your ears are better judges....and a hell of a lot cheaper."
    [SIGPIC][/SIGPIC]Instant Karma tried to get me... and succeeded

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