Bulk tape eraser

Hi DragonForce,
a brief search of the open core AC electromagnet calculation brought nothing to me. I expect that the trial-and-error is the simplest one in this case.
Get a variac instead of a taped transformer. It is useful in many cases. Be aware of the fact that the variac secondary is directly connected with the power lines and keep safety in mind !

BR from Ivan

I thought of that Harry, only I have no idea what value to use.  I'm pretty useless huh lol!

I'd experiment, but mains electricity and a transformer winding that is likely to burst into flames is putting me off somewhat.  It really does get hot, you can smell the enamel burning after 5 seconds or so, and after 10 it starts to smoke.  and as Ivan suggested, feeding the transformer from a lower voltage transformer seems to be the way to go - I'm currently looking on ebay for something with multi-taps so I can experiment safely.

I'm only 51, I'd like to see 52......

Ruud, thanks for explaining how it works, I can see you put a lot of effort into your reply in order to help me.  I appreciate it

So silly question time - is there a "rule of thumb" about how many turns per volt vs current drawn in something like this?  This transformer runs very well and cool when the E I laminations are in place, but remove the I laminations and it's a fire risk!  Now there has to be a way to work out how many turns of (say) 42 SWG ECW it would take around that lump of iron for it to put out a thumping great magnetic field, without self-destructing in the process.  I'll hazard a guess and say it'll probably take many hundreds, if not thousands, since the only experiment I have tried so far is to feed the 20VAC from this transformer into a coil of 500 turns, wound on an old transformer bobbin from 28 SWG ECW.  Loud noise, and smoke - so that was a no-go.

Anything I have found deals with lots of variables, and assumes a "closed" core, both E and I laminations in place, for use as a transformer rather than an AC electromagnet.  I'm almost at the stage where I take a tip from Ivan and get myself a valve output transformer and see what that gives me.  Those things aren't cheap however, and if it's going to cost lots of money, I may as well just go and buy a ready build bulk eraser - I was hoping I could build one with bits I have just hanging around the place.....

Just a couple of comments from me: I used to work i the RAF and all 8 air traffic channels wsere recorded on 2" tape. the rolls were about 20cm Diameter. We used to erase them a month after the were removed and this was done by passing them through a core-less coil with 50Hz energizing the coil.

To cut down the current use a capacitor, not a resistor. Resistors dissipate power, capacitors do not.

BR Harry

I think working with recording tape, the manufacture of cassettes, bulk erasure etc would be my ideal job!  I just love the way they work and how to work with them.

Many thanks for the replies guys, I've learned a lot from this thread.

In the past I worked at a laboratory were they did magnatic tape measurements.
Some of the things that were always measured was the so called 'bias noise'; the noise that is produced after recording the tape with 'silence'. (Only the bias voltage applied to the recording head.)
The 'bias noise' was compared to the noise of the virgin tape (=a tape that never had been recorded.) The bias noise was always higher than the noise of a 'virgin' tape.
An other measurement was the DC noise. This was the noise that a tape produced when it was recorder at a very low frequency, for example 5 Hz. This low frequency is technically close to DC.
When you erase a tape with a permanent magnet, all magnecules are arranged in the same direction. (Anyway: this is what you are trying to do!) But in reality, you don't arrange ALL magnecules, this is the reason for the noise when you use a magnet. When you use an AC bulk eraser, you 'shake' the magnecules in different directions. Bacause this is a repetitive action (AC!) there is a bigger chance that you 'touch' all magnecules. In fact, the noise level of a properly AC bulk erased tape is the same as a 'virgin' tape.

By the way: the best way to adjust the bias voltage on a recording head is to record a very low frequency (<= 20 Hz.) at a fairly high level, and adjust the bias for minimum noise. There will be a sharp 'dip' in the noise. Of course this is the easiest to do with a 3-head tape deck.
Personally, I always use the rumble filter on my amplifier, to just hear the noise and filter out the low frequency.

For what it's worth, the purpose of me wanting to completely erase these tapes is basically laziness on my part.

See, I bought a bulk lot of 200 recycled cassettes, of various lengths.  They were literally pennies each - hence why I bought lots.

I intend to use them on my old 1982 Sinclair Spectrum home microcomputers for archiving software.  Now I want the tape to be as clean as possible because once the program has been recorded, I'd like the rest of the tape to be blank until the end.  Nothing worse than loading in a program and then hearing a load of noise from an old (in this case, the original) recording after.  So, I can either record my data and then erase until the end of the tape (extra wear and tear on my tape decks, takes a lot of extra time etc), or I can simply erase the whole tape in the first place and just use it as a "new" tape.

I have noticed that using the powerful AC magnet, the tape was absolutely silent, no noise whatsoever, and I couldn't tell the difference between reading the leader tape and the actual recording tape, it was that silent.  If I pass that same tape through any of the decks I have that uses a DC erase head, even with the record levels set to zero, a high level of hiss remains.  Hardly conclusive, but it seems to me that an AC powered electromagnet does a better job than a DC powered one.  The drawbacks of using the AC transformer type of electromagnet is that it gets VERY hot, VERY fast - to an extent that it is a dangerous fire hazard.

I have a tape deck that has a permanent magnet in the erase head, it just swings into place when you depress the record key.  It's quite clever to see it working actually.  This too is far from being noise-free, but I can't compare this because the recording level isn't manually settable, so the ALC circuitry just builds up the gain with no signal applied. the resulting amplifier noise being recorded.  I can see how a permanent magnet would destroy the magnetic pattern stored in the tape itself, but I can't see how it would "scramble" it - surely it simply re-arranges the magnetic fields into a uniform pattern, and so leaves the tape magnetised. Wouldn't an AC erase head leave a fluctuating pattern also?  

That's why I chose to use the "bulk erasure" method of an AC magnetic field applied to tape that isn't moving, if that makes sense, it seems to me to be the only really effective way.

I'm getting quite an education here guys   Thanks for that:idea:

Hi,
I remember in one of my previous jobs it was forbidden to erase tapes using permanent magnets due to increased noise. Maybe it had to do with old sorts of tapes or weak HF oscillators in the recorders ?
Nevertheless an AC electromagnet is simple, strong and safe.

BR from Ivan

I don't think I completely agree with you, Ivan.
In the past, some cassette recorders even used a permanent magnet to erase the tape before re-recording.
Also: if erasure by a DC (or other permanent magnetic) field is good enough for broadcasting purposes, there can't be very much wrong with it, IMHO.
If you played a just bulk erased tape, there was a lot of noise, caused by the permanent magnetic field.
But after recording, the noise was completely gone.
Remember that when the tape is recorded, the (AC) erase head will completely 'destroy' any magnetisation.
Further the bias current used in the recording head will also 'reshuffle' all magnetic particles.

Hi Ruud,
the capacitor discharge into a coil produces damped oscillations. This type of eraser is very powerful, but rather unsuitable for hams.
I would never recommend using DC current nor permanent magnets, as the tape is erased, but in saturation after its use. It seems to be better to push the tape into the center of the hysteresis diagram.

BR from Ivan

Years ago, when I worked as a sound engineer at a radio station, we used several bulk erasers. Most of them were based on the 'open core AC principle'.
But we also had a very specila device: a big metal box (80 x 80 x 80 cm) with a slot where you could put in some big (> 30 cm) tapes. You could put in 4 tapes at once if I remember correctly. Then you pushed a BIG red button. At first, nothing happened. But after a few seconds, you could hear a loud BOINGGGG!!!! and the tapes were erased. (Sometimes a program that wasn't broadcasted was accidentally erased this way!) I think during the seconds between the moment you pushed the button and you heard the 'boing', some big capacitors were charged. The 'boing' was probably caused by the discharge of the capacitors in a big coil. I once saw the box opened. Indeed you could see relais, capacitors end a lot of copper wire. So you can also erase tapes with DC current. In fact: you only need a strong magnetic field!

One of the bulk erasers we used. (Not the one I desribed in the text.) In general it was a bad idea to wear a mechanical wrest watch...

Hi,
either use a series resistor, or decrease the voltage using a suitable transformer. The erasing effect will decrease too, of course. Such devices are usually designed for short work only, with long pauses to cool down.

BR from Ivan

Bugger, I've already typed this once, maybe I didn't hit the send button

It works great, only problem is, the coil gets very hot very quickly, I can smell the enamel of the wire burning.  Still, it took less than 10 seconds, and the tape is virgin - completely wiped.  It's so quiet I can't tell the difference between reading the leader tape and the actual tape itself now, so thats a perfect result

Do you suggest a resistor in series with the coil to prevent me from setting fire to my house?  

Ivan, many thanks for the advice, if not for you I'd still be looking on ebay for a commercial unit.

Hi DragonForce,
I am glad I helped you a bit.
The advantage of a valve output Xformer (audio output, TV frame output etc.) is that it usually has an air gap to prevent saturation due to the DC component of anode current. The E laminations are all on one side, the I ones on the other. It is easy to open the core by removing the I ones. You may "enjoy" some bad work when disassembling the core of a mains transformer, turning the E laminations and pushing them back.
A pushbutton switch on the coil may be quite helpful, too.

BR from Ivan

Excellent, many thanks Ivan.  I don't have a valve output transformer, but I do have a nice 15-0-15 @ 5A that will come apart with a bit of persuasion.  You've saved me many hours of boredom, thanks again

Hi DragonForce,
you describe almost the exact way. The only difference is you will probably have to make circles with the transformer to erase all the tape uniformly. If you use an output transformer for valves or a choke with an air gap, you simply remove the block of I laminations. The secondary does not matter provided that it is not shorted.

BR from Ivan

A lot of you know of my activities with early 80's 8 bit computers.  I've been fortunate enough to have acquired a large quantity of brand new, "recycled" cassette tapes from a duplication business somewhere near Manchester, England.  They're good tapes, nothing wrong with them at all - just surplus to requirements.  So, I got them CHEAP!

So now I need to erase them.  Yes I know they get erased when you record new data onto them, but I don't want to sit there and record 10 minutes worth of data, and then have to sit there and wait for the next five minutes or so whilst the recorder erases the rest of the tape til the end.  My equipment will get worn out all the quicker, and since it's 1980's vintage and spare parts (drive belts, tape heads etc) are unobtainable now.

So, I need a bulk tape eraser - that means a nice strong electromagnet!  Does anybody have some rough plans on how to build one?  What frequency is best etc?

I was thinking of getting a decent mains transformer, stripping it down, leaving out the "I" laminations so I have the "E" laminations and the primary winding only, sitting my tape on top of the thing, powering it up, and then lifting the tape off slowly after about 5 seconds.  Repeat til all 200 are done.  Does this sound like a plan?  I don't want to scrap a good transformer just to find out.

I've no doubt that you guys could design a much better device, so anybody wanna share?