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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 17th Oct 2019, 10:07 am   #1
Linnovice
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Default A question of magnets

Morning all.
I’m not sure where to post this question but, here goes.
Like the majority of members on this forum I enjoy servicing and repairing my tape decks. I do find though, that as I’m getting older, the dexterity of my hands is becoming a problem. Especially when it comes to inserting screws/Allen bolts in difficult spaces and near PCB’s.
With this in mind I’ve bought myself a long-reach screwdriver that takes various bits at the business end to cover all manner of screws, etc. The actual holder is magnetic so I’m wondering if I will cause myself more problems than it solves by using it. Especially around static sensitive components. I will obviously avoid using it around the heads.
What would be the consensus of opinion here? Good or bad idea?
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Old 17th Oct 2019, 11:01 am   #2
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Default Re: A question of magnets

If you use a magnetic holder near heads all you need to do is to set it to record with an unwanted recording on the tape for a couple of minutes.
Sets with DC bias will need a de-magnetiser.
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Old 17th Oct 2019, 11:02 am   #3
Craig Sawyers
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Default Re: A question of magnets

Blu tack. The getting screws into inaccessible place's friend.

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Old 17th Oct 2019, 11:19 am   #4
kalee20
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Default Re: A question of magnets

Quote:
Originally Posted by Linnovice View Post
The actual holder is magnetic so I’m wondering if I will cause myself more problems than it solves by using it. Especially around static sensitive components.
Theoretically you can damage static-sensitive components with magnetic tools - if they move. But it would have to be extremely fast movement.

Since voltage is proportional to rate of change of flux, it's possible to calculate roughly the voltage that would appear on a component. For a typical permanent magnet, with flux density 0.3 teslas and size 6mm dia, that's a total flux of 8.5 microwebers.

If you pull it away from a loop of conductor in 0.05seconds - which is a pretty fast snatch-away - the induced voltage associated with a change of 8.5μWb in 0.05sec will be 170μV. Since static sensitive components tend to suffer damage at around 10V, and base-emitter breakdown is a concern above at 5V, you still have a safety factor of 30,000. You can forget it!

However, the danger of magnetising heads or any ironwork is real, and magnets are bad news in tape recorders as they can cause partial (or complete!) erasure of your tapes. Others have already commented and given good advice.
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Old 17th Oct 2019, 11:30 am   #5
TonyDuell
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Default Re: A question of magnets

I find magnetised tools to be a curse, and in fact paid a fair amount of money for non-magnetic, non-magnetisable screwdrivers and tweezers.

The problem is that when you are assembling small steel components (the classic example is the blades of a camera leaf shutter or iris diaphragm, but it applies to things like washers, leaf springs, etc), a magnetised tool will pull the bits out of position. Ones that you have spent time aligning.
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Old 17th Oct 2019, 1:35 pm   #6
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Default Re: A question of magnets

I find that a bit of sticky wax from an old sticky wax capacitor will usually hold a screw to a screwdriver.

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Old 17th Oct 2019, 2:31 pm   #7
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Default Re: A question of magnets

Quote:
Originally Posted by Refugee View Post
If you use a magnetic holder near heads all you need to do is to set it to record with an unwanted recording on the tape for a couple of minutes.
Sets with DC bias will need a de-magnetiser.
That applies to the erase and record heads to some degree but it depends on how badly magnetised those heads have become. Bias signal decay alone may not be enough for a strongly magnetised head. A magnetised play head in a 3 head machine needs demagnetising. Setting to record will have no effect.
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Old 17th Oct 2019, 2:46 pm   #8
Linnovice
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Default Re: A question of magnets

Thanks guys, looks as though it’ll have to be spit n grease then. Thanks for the advice.
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Old 17th Oct 2019, 3:22 pm   #9
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Default Re: A question of magnets

Many years ago, when walking down the Tottenham Court Road, I spotted a "demagnetiser" in a shop window. This consisted of a soft iron core with an end that stuck out of a 250volt coil. On plugging the latter in to the 50Hz mains the alternating magnetic field at the end of the core did an excellent job of demagnetising record and playback heads. In fact, before starting a recording job it became standard practice to demagnetise my tape heads with it as this made a noticeable reduction to the background noise level.

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Old 18th Oct 2019, 2:01 am   #10
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Default Re: A question of magnets

Demagnetising tape paths only has a benefit if the tape path needs demagnetising. Often it doesn't need it.

But knowing that one play of an irreplaceable recording on a magnetised tape path can ruin that recording... it makes sense to demagnetise before playing that tape.

Demagnetising is like insurance. We never know when we'll need it. When we need it but dont have it, it's already too late. We cant pay the premium after the accident.

That's not quite true. There are sensitive gauges which can measure whether parts of the tape path are magnetised.

But demagnetisers have to be more powerful than the magnetism they're designed to neutralise. Inexpert use can can accidentally magnetise metal parts so badly that it becomes very difficult to demagnetise them. Used incorrectly, a powerful demag tool can make a right mess. It only takes one small slip up.
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