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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 16th Aug 2019, 4:45 pm   #1
hillmanie
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Smile Demagnetising

Been doing some research on demagnetising, both on tape heads and bulk tape. The latter is what's driving as I've bought a quantity of recorded tapes off eBay, none of which seem to have shed dust.
The head types are cheap enough but still do not conform to my strict budgetary control rules as compared to a DIY solution. Noted a DIY version using shellac coated wire from a scrap AC motor, the wire being re-wound a nail! Further research came up with using a permanent magnet for bulk erasing. The videos showed the magnet being thoroughly swept over the spool and slowly withdrawn. Am not sure if commonly available magnets would be strong enough to bulk erase though. One Youtubber rigged up a small magnet in a holder mounted close to and in contact with the tape whilst running in Play mode.
Has the community any suggestions or comments?
Thanks
Tony
PS
Count of old eBay 'fixer uppers' now standing at 5. Work in progress encouraging enough
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Old 16th Aug 2019, 5:33 pm   #2
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Default Re: Demagnetising

I'd be wary of using any type of permanent magnet. The thing to aim for is a gradually decreasing, alternating magnetic field.

For example, a commercial, mains-powered demagnetiser I used came with instructions to slowly move the reel of tape around immediately above it, then gradually move the reel further away whilst continuing to move it in circles. Stop when you can no longer reach the demagnetiser's on switch whilst holding the reel!

The demagnetiser was just a flat-topped metal box with a hefty electromagnet inside. ISTR the lights flickered when it was switched on.
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Old 16th Aug 2019, 5:50 pm   #3
hillmanie
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Default Re: Demagnetising

Thanks for that. Yes, I've seen these on Youtube, Radio Shack types. Made a loud buzzing noise with a short duty cycle. Still around too but not in budget parameters.
Tony
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Old 16th Aug 2019, 5:50 pm   #4
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Default Re: Demagnetising

I don't know if it, or anything similar, is still available, but I have a Matsushita (Panasonic) electronic head demagnetiser built into a standard cassette shell. This gadget, which runs from a single coin cell-it's been so long since I needed to change the cell, I can't remember which number cell the demag. uses, was supplied to the firm for whom I used to work about 30 years ago. You simply insert it in the player or recorder, press play, and, after about 10-20 seconds hear a loud beep. Job done.
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Old 16th Aug 2019, 5:59 pm   #5
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Default Re: Demagnetising

A permanent magnet will erase tape if it is passed over the poles (some tape recorders in fact used them) but the tape is left 'noisy'. Better would be an AC field which is gradually reduced to zero.

Whether a powerful magnet is strong enough to bulk erase a reel of tape, I don't know. There is a reasonable chance that a 1" diameter neodymium-iron will penetrate right through 1/4" of thickness of a reel. One way to find out is try it!
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Old 16th Aug 2019, 8:07 pm   #6
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Default Re: Demagnetising

The magnet off an old clapped out 12" loudspeaker would be pretty strong example of a permanent magnet.
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Old 16th Aug 2019, 8:16 pm   #7
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Default Re: Demagnetising

Quote:
Originally Posted by rontech View Post
The magnet off an old clapped out 12" loudspeaker would be pretty strong example of a permanent magnet.
Or the magnet from a dead microwave-oven magnetron. But as others have noted, you really need an alternating magnetic-field which is initially high but dies away to zero over a period.

[In the past I was responsible for 'confidential destruction' of used data-storage media: for this we used one of the tables-with-a-dirty-great-mains-powered-electromagnet-under-it erasers - followed by putting the tapes/cartridges/hard-drives through a machine whose toothed rollers reduced them to 'swarf'!]
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Old 16th Aug 2019, 8:31 pm   #8
hillmanie
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Default Re: Demagnetising

[In the past I was responsible for 'confidential destruction' of used data-storage media: for this we used one of the tables-with-a-dirty-great-mains-powered-electromagnet-under-it erasers - followed by putting the tapes/cartridges/hard-drives through a machine whose toothed rollers reduced them to 'swarf'!][/QUOTE]

Were you satisfied that this was enough to ensure all the data was irrecoverable !!!

T
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Old 16th Aug 2019, 10:47 pm   #9
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Default Re: Demagnetising

I've got one of those Radio Shack hand held erasers. Inside, it is basically an standard E-core transformer with one end of the core missing. It works well for bulk erasure, but care has to be taken to move it in a proper sweeping motion in order not to leave any segment of the reel partly unerased, which results in the original recording leaking through, often accompanied by swishing sounds.

(In my case this is made more difficult because the eraser is designed for 120V operation, and my step-down transformer isn't really up to the job, so the eraser runs at 90V or so. (I've been thinking about designing some form of capacitive dropper but I'm a bit worried in case the whole contraption gets into resonance.))

At any rate I'm wondering if it wouldn't be possible to modify a transformer by taking apart the laminations and rearranging them to get the required open end.
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Old 16th Aug 2019, 11:08 pm   #10
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Default Re: Demagnetising

Reminds me of a story told to me by a colleague who used to work for Customs & Excise in the 1970's. VAT data used to be loaded on computer tape at the Southend office and a messenger took them to London by train. Problems were sometimes experienced with data corruption, eventually narrowed down to tapes transported by a particular female messenger. When an investigator accompanied her on one of her trips, it transpired that she always travelled in one of the "Women Only" compartments that trains used to have then, putting the tapes in the luggage rack above her head. On that line, the "Women only" compartments were next to the guards van. On top of the guards van was the pantograph, and, only a few inches from the luggage rack via the roof, were the contact breakers of the switch gear that effected the change between 25kV and 6.25kV operation. There were several such changes between Southend and London, so lots of strong electromagnetic pulses to cause data corruption.

I did once attempt to corrupt data on some 3 1/2" floppies (both 720k and 1.44k types) using a variety of permanent magnets and a home-made electromagnet briefly energised form a 12V car battery. I was unable to cause any errors whatsoever, even by opening the protective cover and stroking the magnets over the disk's surfaces.

Extremely strong permanent magnets used to be sold by some boat chandlers on the canal network, useful for fishing for lost windlasses etc. I don't know if they are still available as the canal authorities have recently taken a dim view of magnetic fishing in their canals.
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Old 16th Aug 2019, 11:40 pm   #11
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Default Re: Demagnetising

Has anyone tried to use an induction 'hot' plate. These must have strong fields and high frequencies.
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Old 17th Aug 2019, 1:17 am   #12
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Default Re: Demagnetising

Indeed: on our last visit to Warners, there was a notice at the hot buffet saying that anyone who had a pacemaker should not get closer than 2 feet from the induction hotplates. That same week, newspapers reported on a politician who was saying that everyone should switch to induction hotplates to save energy, following which someone from a pacemaker organisation wrote in to say that official medical advice was that they were definitely dangerous for pacemaker users.
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Old 17th Aug 2019, 9:24 am   #13
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Default Re: Demagnetising

Perhaps the induction hotplate would heat the oxide up beyond the Curie point and thus totally erase the content - might be tricky to use afterwards though.
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Old 17th Aug 2019, 9:31 am   #14
hillmanie
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Default Re: Demagnetising

[QUOTE=emeritus;1169036]Reminds me of a story told to me by a colleague who used to work for Customs & Excise in the 1970's.

Bringing it all back! Spent a lot of time in that building and related in Victoria Ave, '71-'74. The pre-VAT hardware was like something from the movies. You should have seen that roomful of stand up tape drives. . Note that the hardware was 1960s, no disk storage. ICL, I believe the maker (probably by licence from one of the big US players). Great canteen on the 13th floor of Alexandra House
TT
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Old 17th Aug 2019, 10:31 am   #15
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Default Re: Demagnetising

Re post no. 5:

Indeed - the Grundig TK1 (3" inch spools, portable transistor tape-recorder, circa 1964) used a permanent magnet to erase, and yes, the recordings were somewhat noisy, although the TK1 used AC bias on the record/relay head for actual recordings which would have reduced the background noise contribution from the erase process.
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Old 17th Aug 2019, 10:41 am   #16
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Default Re: Demagnetising

Wasn't there a way of designing a permanent magnet erase head so that the tape was first fully magnetized with the north pole of a magnet, and then fully demagnetized using a following south pole, the cancellation not being fully perfect in practice, but still better than a single north or south pole which although providing complete erasure would also leave the tape in a fully magnetized (DC) condition.
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Old 17th Aug 2019, 11:46 am   #17
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Default Re: Demagnetising

Yes, the Soundmirror tape recorder does just that. The magnet has two pole-pieces, one from each pole, like the prongs of a fork but interleaved, so as the tape passes over them it sees first a North pole, then a South pole, then a North, etc. I suppose the final couple of 'prongs' could have been of narrower section so they didn't carry as much flux, so as to leave the tape with less residual magnetism, but they didn't.

The erase head disappears through a hole in the deck when not in 'record' mode.

In actual use, the high level of recording AC bias does reduce noise. Pulling a length of tape past the erase head alone, and then replaying, shows it is very noisy. But recording nothing and then replaying is much quieter, showing the further partial erasure of the bias.
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Old 17th Aug 2019, 12:50 pm   #18
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Default Re: Demagnetising

The induction hob idea has been tried on other forums and apparently the hob will not turn on without something metal on it.
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