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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 16th May 2010, 2:23 pm   #1
Scottart
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Default A 'scratchy' loudspeaker

Hi everyone,

I've got a 'scratchy' PM loudspeaker. . I can see through a magnifying glass a few very small pieces of metal ( iron/steel ) trapped between the inside of the voice coil and the central pole piece. I've tried using a vacuum cleaner and brush , but to no avail. Has anyone got any ideas or tips to solve this ?
There are 4 screws holding the magnet assembly to the main yoke, but I'm reluctant to remove it, for fear of being unable to re-centre afterwards.
The coil resistance seems fine.

TIA Stephen
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Old 16th May 2010, 2:34 pm   #2
Boom
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Default Re: A 'scratchy' loudspeaker

A half inch wide of thin stiffish paper pushed into and wiped between the pole piece and coil.

Dave
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Old 16th May 2010, 4:02 pm   #3
Scottart
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Default Re: A 'scratchy' loudspeaker

Thanks Dave, works a treat.

As a matter of interest, has anyone had success dismantling and rebuiding PM speakers? If I did decide to at a later date, what's a good method.
Below is the speaker in question, the only identification is the number 1432 in red. It's 5" in diameter

Cheers, Stephen
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Old 16th May 2010, 4:22 pm   #4
Boom
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Default Re: A 'scratchy' loudspeaker

I'd be happy removing the magnet so long as it was done with extreme care.

Dave
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Old 17th May 2010, 8:46 am   #5
swordholder
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Default Re: A 'scratchy' loudspeaker

Stephen,
Don't be afraid, as long as you do things carefully, you should be OK.
I have several times re-centred my Lowther loudspeakers as they have very light suspension which causes the cone to sag over a period of years.
You will need suitable spanners/screwdiver depending how the magnet is mounted, a rubber headed mallet and sone stiff thin card.
Remove the magnet, and with a thin slice of stiff card, dig out as much "crud" as you can from the gap.
Reassemble the magnet to the frame, and if the cone doesn't have a dust cover over the coil, fit 3 pieces of thin card equally spaced aound the coil.
If there is dust cap carefully remove with a scalpel and reglue on later.
Adjust the magnet by moving it until the friction on all the cards is equal, then semi tighten the fixing screws. Remove the cards, and see if the cone moves freely by applying equal pressure on opposite sides and listening.
Connect the speaker to an amplifier (or the set) and inject a 400hZ signal, listen for a pure note. If needed GENTLY tap the magnet with the mallet until you get the purest sound. I found that this method will move the magnet by much smaller amounts than you can do by hand alone.
Fully tighten the fixing bolts and recheck.
Job's a good un!!
Good luck

Mike
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Old 17th May 2010, 9:21 pm   #6
PJL
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Default Re: A 'scratchy' loudspeaker

Thin plastic sheet worked around the gap should fix it.
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Old 18th May 2010, 7:46 am   #7
Scottart
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Default Re: A 'scratchy' loudspeaker

Many thanks everyone. I now feel more confident about rebuilding a speaker
Stephen
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