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#300502 - 12/07/2007 16:43 How to stop magnetic interference ?
Cris
pooh-bah

Registered: 06/02/2002
Posts: 1896
Loc: Leeds, UK
Problem - My house is probably too small to have a pinball and arcade machine in, but I got 'em and they have to go right next to each other. This is causing the newly arrived arcade cab a few problems.

I am getting magnetic interference on the screen when pushed back against the wall next to the pinball table. I think the old 28" tube does have degauss but I guess it doesn't work.

I am worried about this becoming a permanent feature, so I have the following questions...

1 - How long will it take to make them permanent ? It's not too bad, I've tried to take a picture but failed.

2 - Is there anything I can do to stop it? i.e. protect the tube, or shield the pinball table?

I don't think there are too many magnetic components in the pinball, just the speakers and a couple of motors.

Please empeg gods, give me the answer.....

Cheers

Cris.

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#300503 - 12/07/2007 17:06 Re: How to stop magnetic interference ? [Re: Cris]
tanstaafl.
carpal tunnel

Registered: 08/07/1999
Posts: 5402
Loc: Ajijic, Mexico
Quote:
I've tried to take a picture but failed.


Make whatever adjustments you need to ISO and aperture in order to get your shutter speed down to 1/30 second or slower and your picture should then come out.

tanstaafl.
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"There Ain't No Such Thing As A Free Lunch"

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#300504 - 12/07/2007 17:11 Re: How to stop magnetic interference ? [Re: Cris]
wfaulk
carpal tunnel

Registered: 25/12/2000
Posts: 16706
Loc: Raleigh, NC US
Magnetic shields are metal sheets that have high magnetic permeability, but are not likely to become magnetic themselves in the presence of a long-lasting field, that is, they have low coercivity. The only material I'm aware of is Mumetal, and it's extraordinarily expensive. (Actually, a little research also turns up supermalloy.) That said, you can't just put a sheet of it between the two items. Basically, you have to encase one or the other object, as magnetic fields are toroid, which means that the object you want shielded is being affected from all directions but the farmost. So, conceivably, you could get some Mumetal, shield the back of the CRT, and face its back towards the pinball machine, and not see any shielding at all. I don't know if Mumetal is available commerically or not. Oh, here are some sample kits.
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#300505 - 12/07/2007 17:47 Re: How to stop magnetic interference ? [Re: wfaulk]
Cris
pooh-bah

Registered: 06/02/2002
Posts: 1896
Loc: Leeds, UK
Quote:
Oh, here are some sample kits.


Wow! That is expensive

I did think about shielded speakers in the pinball machine, but I guess there are a hell of a lot of electro magnets when the thing is turned on!

I guess they shouldn't really live together, I wonder how they do it in the arcades?

Cheers

Cris.

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#300506 - 12/07/2007 18:18 Re: How to stop magnetic interference ? [Re: wfaulk]
Robotic
pooh-bah

Registered: 06/04/2005
Posts: 2026
Loc: Seattle transplant
Quote:
Magnetic shields are metal sheets that have high magnetic permeability, but are not likely to become magnetic themselves in the presence of a long-lasting field, that is, they have low coercivity.

So... since the fields are only on when the arcade games are on a simple piece of sheet steel would do?
I always assumed that the 'shielded' computer speakers just had a couple sheets of steel inside the case. It wouldn't need to be thick at all.
Cris- can you slide a sheet of steel between the machines for a quick test?
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#300507 - 12/07/2007 18:34 Re: How to stop magnetic interference ? [Re: Cris]
wfaulk
carpal tunnel

Registered: 25/12/2000
Posts: 16706
Loc: Raleigh, NC US
Quote:
I wonder how they do it in the arcades?

Every arcade I've ever seen segregates the pinball machines from the video games. It had never occurred to me that magnetic fields might be the reason, but it makes sense.
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Bitt Faulk

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#300508 - 12/07/2007 18:37 Re: How to stop magnetic interference ? [Re: Cris]
tfabris
carpal tunnel

Registered: 20/12/1999
Posts: 31286
Loc: Seattle, WA
I've seen the magnetism from unshielded speakers permanently damage a CRT, so I would recommend not letting those two things live together until you've sorted that problem.
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Tony Fabris

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#300509 - 12/07/2007 19:29 Re: How to stop magnetic interference ? [Re: tfabris]
Cris
pooh-bah

Registered: 06/02/2002
Posts: 1896
Loc: Leeds, UK
Quote:
I've seen the magnetism from unshielded speakers permanently damage a CRT, so I would recommend not letting those two things live together until you've sorted that problem.


Bugger

Not an easy problem to solve, they are both in the only place they can go, I can't even get them further apart.

I will try the steel idea, I have a couple of space PC case sides around the place....

Cheers

Cris.

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#300510 - 12/07/2007 19:55 Re: How to stop magnetic interference ? [Re: Cris]
Cris
pooh-bah

Registered: 06/02/2002
Posts: 1896
Loc: Leeds, UK
OK, 2 major discoveries....

1 - It's not the pinball that is causing the main problem, but it is part of it

2 - The 2 sheets of steel seems to work, but I don't know for how long of course.

The pinball is causing the problem to the left of the screen (pinball is on the left) but something else is causing the bigger problem at the top right. If I use the 2 sheets of PC case Steel the problem goes away, so the monitor itself is fine. I have removed the surround sound speakers to the right of the arcade cab but it's still there, in fact I have now removed all electrical items from near the cab and it's still there. I can't think of what else would be causing the problem.

For the time being it doesn't seem to be causing a major problem for the monitor, but now I have noticed it I find it hard to look at the screen without getting annoyed.

And of course there is the problem of mounting 2 sheets of steel....

Cheers

Cris.

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#300511 - 12/07/2007 20:32 Re: How to stop magnetic interference ? [Re: Cris]
wfaulk
carpal tunnel

Registered: 25/12/2000
Posts: 16706
Loc: Raleigh, NC US
The problem with steel is that it will eventually become magnetic on its own in the face of those fields. This may be less of a problem if the field goes away with the pinball machine off, but I'm not even sure of that. I don't know if a rest period will reduce any magnetism that the steel has built up, or if it just delays the effects.

How is it that you know that the top-right distortion is not also the pinball's fault? However, assuming it's not, and you can't find any other source, it may be your electrical wiring. Does your wiring run through a wall near the CRT? This can be a problem in the US, and I can only think that twice the voltage is bound to make it far worse.
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#300512 - 12/07/2007 20:56 Re: How to stop magnetic interference ? [Re: Cris]
AndrewT
old hand

Registered: 16/02/2002
Posts: 867
Loc: Oxford, UK
Quote:
I am getting magnetic interference on the screen when pushed back against the wall next to the pinball table.

Can you describe the interference -- is it a shimmering (moving) distortion of an otherwise OK display or is it a discolouration in certain areas of the CRT (perhaps the corners)? If it's moving, are you seeing a 'band of movement' progress up or down the screen or is it a 'jaggedy' left-right movement affecting the whole display? Or is it something else entirely?

A picture, as they say.....

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#300513 - 12/07/2007 20:59 Re: How to stop magnetic interference ? [Re: wfaulk]
Cris
pooh-bah

Registered: 06/02/2002
Posts: 1896
Loc: Leeds, UK
Quote:
Does your wiring run through a wall near the CRT? This can be a problem in the US, and I can only think that twice the voltage is bound to make it far worse.


Interesting thought, there is a point that is fed from above just to the right of the cab, but if I pull it say about a metre away from that it stays the same. Unfortunately with the way my house is wired I can't easily knock off the ring main and still power the cab, it is possible just not right now.

I know the stuff on the left is the pinball as it only happens when I move the cab back towards it. Now I check the stuff at the top right is pretty much there wherever I put it within the space (maybe a 3m square of floor space).

The more I think about this the more annoying it gets!

Cris.

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#300514 - 12/07/2007 21:23 Re: How to stop magnetic interference ? [Re: Cris]
Cris
pooh-bah

Registered: 06/02/2002
Posts: 1896
Loc: Leeds, UK
Please excuse the state of the cab, I am still at the "lets check out everything works before you make it look nice" stage...



As you can see the image is stable, it is a magnetic problem.

The brightness down the left of the screen is the effect thie pinball is having, that goes away if I pull the machine forward by a metre.

Cris.

PS - The PC speakers you can see on the cab are there as I thought it may have been the cheapo speakers in the cab, but it wasn't, and it sounds much better with the creative set any way


Attachments
301492-cab2.jpg (392 downloads)


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#300515 - 12/07/2007 21:26 Re: How to stop magnetic interference ? [Re: Cris]
Cris
pooh-bah

Registered: 06/02/2002
Posts: 1896
Loc: Leeds, UK
Here is a close up of the top right when holding up the 2 steel sheets tight to the RHS of the cab.



It's a nice screen really, it's well over 10 years old, it does have a little bit of screen burn, but you have to iew a solid block of colour and really look hard to see it. Overall not bad for 80. Lots of work to do as you can see, but it will make a nice toy when I'm done.

Bitt - I think you may have hit the nail on the head with the wiring. If I cover the wall above the socket with the steel the problem spreads but is weaker. I have no idea how I can prove this, I assume I will have the same problem where ever I move the cab, it needs to be near a mains point and all mine feed from the top

Cheers

Cris.


Attachments
301494-cab3.jpg (1057 downloads)


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#300516 - 12/07/2007 21:33 Re: How to stop magnetic interference ? [Re: Cris]
tfabris
carpal tunnel

Registered: 20/12/1999
Posts: 31286
Loc: Seattle, WA
Yeah, Cris, that is a classic portrait of a powerful magnet near the CRT. I had a friend who had placed a home speaker near a monitor and it produced exactly that pattern.

At first, we verified that the speaker was the culprit by temporarily removing it from the area. Knowing that the cause of the problem was the speaker, but not having a better place to put the speaker, he put it back. A year later he discovered that the "stain" was there for good even after he removed the speaker.

Earlier in the thread, you asked how arcade owners handle this problem. Well, haven't you ever noticed, in most arcades, how the pinball machines are in their own "row", set aside (more or less) from the video games?

Of course, I'm sure there are such things as a "well made" pinball machines that are magnetically shielded. As well as arcade video machines that are similarly shielded to protect them from outside sources of magnetism. It's your unfortunate luck that you've got two machines, neither of which is shielded.
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Tony Fabris

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#300517 - 12/07/2007 21:33 Re: How to stop magnetic interference ? [Re: Cris]
wfaulk
carpal tunnel

Registered: 25/12/2000
Posts: 16706
Loc: Raleigh, NC US
I'm sure you've thought of this, but why not swap the video cabinet and the pinball machine? Hopefully that would move the CRT further from the electrical wiring.
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Bitt Faulk

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#300518 - 12/07/2007 21:49 Re: How to stop magnetic interference ? [Re: wfaulk]
Cris
pooh-bah

Registered: 06/02/2002
Posts: 1896
Loc: Leeds, UK
Quote:
I'm sure you've thought of this, but why not swap the video cabinet and the pinball machine? Hopefully that would move the CRT further from the electrical wiring.


Yes, idea covered. I checked the wall to the left of the pinball machine and there is a electric point right on the other side, cardboard walls too so I think I will have the same problem. Also while it's fun to have these in your house they are quite big and having them the other way around would look a right pig.

We don't really have pinball machines in arcades here in the UK, hence the need to have one at home And after owning one for 3 years I can see why, it breaks all the time, balls get stuck and it needs cleaning all the time. But I love it, I would sell my car before it if I had to

I am worried about damage to the CRT, but for now I'll have to risk it.

If anyone knows where I can get some cheap shielding from please let me know, ta

Cris.

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#300519 - 12/07/2007 22:15 Re: How to stop magnetic interference ? [Re: Cris]
AndrewT
old hand

Registered: 16/02/2002
Posts: 867
Loc: Oxford, UK
That's a colour purity problem where the CRT's Red, Green and Blue electron beams are being distorted by a magnetic force and not hitting the correct phosphor dots for that colour.

If you really can't eliminate the source of the magnetism causing this, it might be possible to have the purity ring magnets on the neck of the CRT adjusted to correct this, but it's probably got to be your last resort at fixing this. It's perfectly OK to mark their positions with a felt tip pen and adjust them (and only them) to see if you can fix this yourself - purity faults show best on a red background BTW).

I'm sure that I don't need to warn you about the 20kV feeding the CRT - don't be earthed while doing any of this and keep your left hand in your pocket while working with your right.

Adjusting these magnets may cause convergence errors that will also need to be corrected - convergence is where all 3 electron guns are calibrated to hit the same 'spot' on the CRT at once to produce white (white being a mixture of RGB all at once, for example). If the convergence is out then you will see red, green or blue borders for white objects where some of the colour guns are misaligned. If you get this far and are bothered by convergence errors then call a TV engineer and see if they're prepared to come and adjust it.

As has been pointed out already in this thread, it is possible to "ruin" a CRT with residual magnetism. The "ruined" CRT will have a magnetised shadow mask (the 'gauze' behind the glass front) or a magnetised rim band (the steel mounting/reinforcing band around the extremities) that the degauss coil is unable to demagnetise. A TV engineer can use a handheld degaussing wand to manually demagnetise the screen and that device is an order of magnitude more powerful than the inbuilt degauss circuit but there's still situations where it isn't possible to degauss sufficiently without removing the CRT from the cabinet.

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#300520 - 12/07/2007 22:17 Re: How to stop magnetic interference ? [Re: AndrewT]
tfabris
carpal tunnel

Registered: 20/12/1999
Posts: 31286
Loc: Seattle, WA
Oh yeah, I forgot to say, my friend tried messing with those purity rings on that CRT after that, and he just made it worse, completely screwing up his convergence and being unable to fix it.
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Tony Fabris

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#300521 - 12/07/2007 22:20 Re: How to stop magnetic interference ? [Re: AndrewT]
tfabris
carpal tunnel

Registered: 20/12/1999
Posts: 31286
Loc: Seattle, WA
Quote:
it is possible to "ruin" a CRT with residual magnetism. The "ruined" CRT will have a magnetised shadow mask (the 'gauze' behind the glass front)

Based on the positioning of the speaker, I'll bet that's exactly what happened to my friend's CRT.

Quote:
A TV engineer can use a handheld degaussing wand to manually demagnetise the screen

Wish we'd known that back then.
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Tony Fabris

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#300522 - 12/07/2007 22:24 Re: How to stop magnetic interference ? [Re: tfabris]
wfaulk
carpal tunnel

Registered: 25/12/2000
Posts: 16706
Loc: Raleigh, NC US
I used to work monitor support. While degaussing coils are powerful, they usually do not solve that sort of problem, at least in my experience. And, for the sake of completeness, will do nothing to resolve Cris' problem.


Edited by wfaulk (12/07/2007 22:27)
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#300523 - 12/07/2007 22:31 Re: How to stop magnetic interference ? [Re: wfaulk]
AndrewT
old hand

Registered: 16/02/2002
Posts: 867
Loc: Oxford, UK
Quote:
While those things are powerful, they usually do not solve that sort of problem, at least in my experience.

Generally speaking, there are two scenarios where a degaussing wand doesn't fix things:

1. The wand can't get within close enough proximity to the rim band - requires the CRT to be removed from the cabinet (the labour time involved here makes most sets BER at this stage).

2. The CRT has been dropped and the (fragile) shadowmask is distorted - trashed CRT in other words.

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#300524 - 13/07/2007 10:49 Re: How to stop magnetic interference ? [Re: Cris]
pca
old hand

Registered: 20/07/1999
Posts: 1094
Loc: UK
Your problem may well be a magnetised bolt.

What happens is like this: The machine is installed in it's original home (presumably an arcade) and left there for a long time. Turned on and off for years, the degauss coil, or even just normal current flow in the deflection yoke, will slowly allow the steel fixtures in the machine to become magnetised, usually in line with the geomagnetic field at that particular location and orientation. It essentially 'sets' a local magnetic field which aligns with the geomagnetic field. This normally doesn't show any ill effects as everything is lined up correctly as long as the thing is left where it is. If you physically change the orientation or location of the machine, THEN you get all sorts of pretty results as the field lines now don't line up properly.

This will effect any crt based system, but normally things which are designed to be moved around such as oscilloscopes are both shielded correctly and have compensation controls to allow you to adjust this sort of effect. Scopes, in particular, show it as a tilt of the trace from horizontal, since it tries to line up with the local inclination of the geomagnetic field.

TVs and similar colour CRT based equipment are much more sensitive to magnetic effects, as you can immediately see even a small field change as colour fringing. A good demo of this is to turn on a TV or monitor, wait for the degausser to finish doing it's stuff (about 15 seconds) then turn the thing through 90 degrees onto it's side. Trippy The risk is eventual long-term and very difficult to remove magnetisation of the shadow mask. The degauss coil will normally remove this local field, or actually realign it, when the thing is moved. In your case, I would imagine that your degauss coil isn't working at all. The local field causing the problem is probably coming from the CRT corner mounting bolt. You either need to get the internal degauss circuit working, or use an external degausser on the tube.

pca
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#300525 - 13/07/2007 13:39 Re: How to stop magnetic interference ? [Re: pca]
Cris
pooh-bah

Registered: 06/02/2002
Posts: 1896
Loc: Leeds, UK
Quote:
Your problem may well be a magnetised bolt


Would holding up the shielding on the outside of the cab cure this problem? What ever the problem is it goes away when I do this, so I had assumed that it was something external that was the cause.

Cheers

Cris.

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#300526 - 13/07/2007 13:59 Re: How to stop magnetic interference ? [Re: Cris]
Robotic
pooh-bah

Registered: 06/04/2005
Posts: 2026
Loc: Seattle transplant
Is an upgrade to LCD out of the question?
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#300527 - 13/07/2007 14:22 Re: How to stop magnetic interference ? [Re: Robotic]
Cris
pooh-bah

Registered: 06/02/2002
Posts: 1896
Loc: Leeds, UK
Quote:
Is an upgrade to LCD out of the question?


Yea, I want the full arcade experience, I think LCD's look too small in these old cabs.

I did think about building from scratch, but this way is much cheaper (and more fun I think)

Cheers

Cris.

PS - I'm off to Abid's again tonight, been starving myself all day, gunna try a family nann to myself

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#300528 - 13/07/2007 14:54 Re: How to stop magnetic interference ? [Re: Cris]
Robotic
pooh-bah

Registered: 06/04/2005
Posts: 2026
Loc: Seattle transplant
Quote:
PS - I'm off to Abid's again tonight, been starving myself all day, gunna try a family nann to myself

Excellent! Afraid I can't make it this time.
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#300529 - 14/07/2007 02:52 Re: How to stop magnetic interference ? [Re: Robotic]
Ross Wellington
enthusiast

Registered: 21/02/2006
Posts: 325
Hi,

The link provided below is a company that provides Mu Metal at reasonable prices and smaller quantites. Their product includes different thicknesses of Mu Metal and High Permeability materials for magnet shielding. You would be surprised what some of the thin materials will do.

I have used them for past projects at work and they work well.

If you make a Mu Metal cage for the CRT, it will take a lot of material. It may affect the deflection yoke or the mask behind the CRT screen. Watch out for high voltage and don't get it too near the yoke. I recommend source suppression at the speakers.

Sometimes it is easier to shield the source of the magnetic interference rather than the victim.

The idea as has been correctly stated is similar to "shorting out" the magnetic flux path. This provides a preferred return path for the flux lines to follow (around the inside of the Mu Metal material instead of allowing it to extend out to other metal objects to return to the source). This provides the field containment that you are looking for. The high permeability provides the ability to contain the field.

This company will provide you sheets of the material which you could wrap the material around the speaker magnet assembly. Be aware that the speaker frame may also be part of the problem. Don't cover the rear cone area though if you want any low frequency performance (grin).

In working with Mu Metal, it is not a good idea to hammer on it, just form it to the shape you want and test it out. If you hammer it, it will probably need to be re-anealed (heat treating process that re-polarizes the material).

http://www.lessemf.com/mag-shld.html

I am not affiliated with the company, I do Signal Integrity & EMC design.

Ross Wellington
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