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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 7th Oct 2021, 8:08 pm   #1
Edward Huggins
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Default Tape head demagnetisation

I have been using R2R and cassette units for over 60 years now. In that time I've always cleaned heads with IPA and the well-regarded ALLSOP cased cleaner for cassettes. But I've never attempted to demagnetise a head on any unit. I have been nervous about using a Wand type or a Cassette cased type for fear of accidentally inducing stray magnetism by misuse. What should I look out for in using a Wand type system, especiaily the rate of withdrawal away from the heads? I've noticed that some cassette cased types use a crude rotating magnet and others have quite dense PCB based circuity. Indeed, can demagnetisers actually make things worse? Please help this Old Codger out with an Idiot's Guide!
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Old 7th Oct 2021, 10:30 pm   #2
Ted Kendall
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Default Re: Tape head demagnetisation

The aim is a gradual increase in flux to saturate the head, guide or shaft, followed by a gradual decrease to virtually nothing, leaving the target in a degaussed state. Compared to the 50 Hz field rate, any comfortable speed of approach or withdrawal is gradual. The major hazard is the risk of scratching the head - actual contact is not necessary, and it is advisable to put a sleeve over the bare metal shaft of such as the early Ferrograph defluxer.

So, taking care to clear the immediate area of recorded tapes, switch the defluxer on at arm's length from the machine. Bring smoothly up to the head, pause a beat and then withdraw to arm's length and switch off. You may want to treat each head, shaft and lifter separately. Also, it is polite to ears, amplifiers and meters alike to do this with the equipment switched off
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Old 8th Oct 2021, 5:38 am   #3
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Default Re: Tape head demagnetisation

The bit about driving to saturation is very important.

Ferromagnetic materials have a well-known B-H curve, going into saturation at the ends and having hysteresis between the upwards and downwards directions of change.

The demagnetiser has to be strong enough to drive the target item round its hysteresis loop. The non-linear saturation effect centres-up the magnetisation. As you slowly move the demagnetiser away, the hysteresis loop being circumnavigated at 50Hz gets smaller and smaller until you're left with a part without remaining permanent magnetisation.

So the magnetic 'overdrive' does the demagnetisation. And the gentle withdrawal is how you get out of overdrive without leaving some magnetisation from abruptly stopping the demagnetiser. Get the demagnetiser well away from all important things before turning it off.

A bit of tape or sleeving over a bare metal pole piece of a demagnetiser allows you to touch your heads and tape guides, capstan etc without scratching. This allows you to know that you got close, to a defined distance. Just waving it about nearby is a bit uncertain.

David
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Old 8th Oct 2021, 5:28 pm   #4
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Default Re: Tape head demagnetisation

I have taken the rough edges off mine with emery cloth and use a bit of heatshrink when doing the head faces.
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Old 8th Oct 2021, 6:13 pm   #5
Ted Kendall
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Default Re: Tape head demagnetisation

Incidentally, it is as well to note that Revox specifically advise against using a deguasser on their cassette machines as it can cause premature demagnetisation of the capstan motor rotors. To be fair, I have never had magging-up trouble with these transports.
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Old 9th Oct 2021, 10:25 am   #6
Edward Huggins
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Default Re: Tape head demagnetisation

Thank you all for the detailed replies received so far. I will approach the use of a Wand with due caution. Can anyone offer any more of the issues (if any) in using a cassette cased degauser? Obviously my Nak 2 is getting on now, and my Philips Black Tulip even more so, but don't want to risk anything.
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Old 9th Oct 2021, 2:06 pm   #7
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Default Re: Tape head demagnetisation

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Originally Posted by Edward Huggins View Post
Thank you all for the detailed replies received so far. I will approach the use of a Wand with due caution. Can anyone offer any more of the issues (if any) in using a cassette cased degauser? Obviously my Nak 2 is getting on now, and my Philips Black Tulip even more so, but don't want to risk anything.
Classic Naks have lovely precise head adjustments but the mounting has some fragile plastic parts. Be careful not to accidentally knock the head as you bring the demagnetiser tip onto the head face.
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Old 10th Oct 2021, 4:12 pm   #8
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Default Re: Tape head demagnetisation

Hi, there's a train of thought that says head demagnetising is only necessary on three head machines, as with a two - head machine the record/play head is often (though not always) connected in opposing phase during record and playback modes, so is subject to a "self demagnetising" effect. Opinions on this do differ, however.

The advice about gradually decreasing the magnetic field during the process of demagnetising is important, otherwise you just end up with a head that is more magnetised than when you began

Regards

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Old 10th Oct 2021, 4:50 pm   #9
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Default Re: Tape head demagnetisation

The bias field at a record or record/replay head doesn't drive things into saturation (for obvious reasons) it just goes a little less than half-way in order to gloss over the central kink. So it isn't an effective degausser.

The erase head does get clobbered.

Erase oscillators in tape machines need to be designed to diminish steadily in amplitude and not stop abruptly. On some good machines you'll find a small reservoir, but the oscillator also needs to work down to low levels before oscillation stops. This isn't obvious, nor does it get mentioned in most texts on tape recorders.

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Old 10th Oct 2021, 5:23 pm   #10
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Default Re: Tape head demagnetisation

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but the oscillator also needs to work down to low levels before oscillation stops. This isn't obvious, nor does it get mentioned in most texts on tape recorders.

David
Very interesting point - obvious now you point it out, but it hadn't occurred to me before!

Mike
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Old 10th Oct 2021, 5:37 pm   #11
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Default Re: Tape head demagnetisation

Percy gives it a mention in his book.

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Old 10th Oct 2021, 5:57 pm   #12
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Default Re: Tape head demagnetisation

A reason not to muck around with track selection switches while recording. Can leave an erase head magnetised.

David
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Old 10th Oct 2021, 7:46 pm   #13
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Default Re: Tape head demagnetisation

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A reason not to muck around with track selection switches while recording. Can leave an erase head magnetised.

David
Definitely - Twin Track Leevers Rich machines supplied to the BBC had a big red label to this effect by the track selection switch.
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Old 11th Oct 2021, 1:43 pm   #14
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Default Re: Tape head demagnetisation

Mightn't a cap across the erase head winding forming a damped resonant circuit be enough to cause erase signal to drop off gradually enough?
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Old 11th Oct 2021, 2:00 pm   #15
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Default Re: Tape head demagnetisation

Often there's a capacitor across the oscillators HT that remains in circuit to provide the oscillator run down when it's HT feed is disconnected, Ferrograph for instance, 16uF in the Series 2, 8uF in the Series 5.

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Old 11th Oct 2021, 2:46 pm   #16
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Default Re: Tape head demagnetisation

It's difficult to get oscillators to run down to very low levels, so a two-pronged attack is used.

A capacitor holds the power supply up and stops things falling too quickly, and an LC oscillator running quite high Q is used to store AC energy and slow the decay once the active devices in the oscillator drop out. Good erase oscillators use something like a pot core or IFT and couple the erase head windings to it. If one channel isn't in use, there is a dummy head circuit for keeping operation normal. bias is done with high voltage into high value resistors to approximate a current source. Moderate erase oscillators use the erase head, but keeping DC out isn't always good.

Some people tried R-C multivibrators.... no stored AC energy, no high voltages. Problems!

David
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Old 11th Oct 2021, 4:10 pm   #17
barrymagrec
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Default Re: Tape head demagnetisation

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Originally Posted by Radio Wrangler View Post
It's difficult to get oscillators to run down to very low levels, so a two-pronged attack is used.

A capacitor holds the power supply up and stops things falling too quickly, and an LC oscillator running quite high Q is used to store AC energy and slow the decay once the active devices in the oscillator drop out. Good erase oscillators use something like a pot core or IFT and couple the erase head windings to it. If one channel isn't in use, there is a dummy head circuit for keeping operation normal. bias is done with high voltage into high value resistors to approximate a current source. Moderate erase oscillators use the erase head, but keeping DC out isn't always good.

Some people tried R-C multivibrators.... no stored AC energy, no high voltages. Problems!

David
Pretty much how Leevers did it. Pair of ECC82s in parallel as a push pull oscillator using a Mullard pot core as a tuned O/P transformer. 8uf cap on the HT to give a controlled decay. The erase head was coupled as a series tuned circuit for maximum erase current. Running at 95kHz with a double gapped Ferrite erase head it would erase a peak level tone into the noise (better than -80dB)
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