|
Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc. |
|
Thread Tools |
17th Oct 2019, 10:07 am | #1 |
Hexode
Join Date: Jun 2014
Location: Chelmsford, Essex, UK
Posts: 386
|
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?
__________________
Small Scotch, New Aids and Happy Ears |
17th Oct 2019, 11:01 am | #2 |
Dekatron
Join Date: Apr 2012
Location: Worksop, Nottinghamshire, UK.
Posts: 5,549
|
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. |
17th Oct 2019, 11:02 am | #3 |
Dekatron
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,941
|
Re: A question of magnets
Blu tack. The getting screws into inaccessible place's friend.
Craig |
17th Oct 2019, 11:19 am | #4 | |
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,061
|
Re: A question of magnets
Quote:
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. |
|
17th Oct 2019, 11:30 am | #5 |
Dekatron
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 5,190
|
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. |
17th Oct 2019, 1:35 pm | #6 |
Nonode
Join Date: Aug 2015
Location: Cambridge, Cambs. UK.
Posts: 2,196
|
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.
Martin
__________________
BVWS Member |
17th Oct 2019, 2:31 pm | #7 |
Octode
Join Date: Jul 2006
Location: Perth, Western Australia
Posts: 1,965
|
Re: A question of magnets
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.
|
17th Oct 2019, 2:46 pm | #8 |
Hexode
Join Date: Jun 2014
Location: Chelmsford, Essex, UK
Posts: 386
|
Re: A question of magnets
Thanks guys, looks as though it’ll have to be spit n grease then. Thanks for the advice.
__________________
Small Scotch, New Aids and Happy Ears |
17th Oct 2019, 3:22 pm | #9 |
Pentode
Join Date: Jan 2010
Location: Lichfield, Staffs, UK.
Posts: 150
|
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.
P.P.
__________________
"Ohm's law rules here" - Oxygen free speaker cable not required! (Quote: Quad Service lab) |
18th Oct 2019, 2:01 am | #10 |
Octode
Join Date: Jul 2006
Location: Perth, Western Australia
Posts: 1,965
|
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. |