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 Post subject: Need help restoring a Meltrope III soundbox
PostPosted: Tue Jul 03, 2018 11:21 am 
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Victor O
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Joined: Mon Dec 18, 2017 2:51 am
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Location: Madrid, Spain
I've recently acquired a Meltrope III from a friend in the UK. One day I decided to overhaul it, as I compared it with my restored HMV 5a/5b soundboxes and found the sound of the Meltrope rather dull in comparison. This was never touched before (original) and when disassembling it I noticed a hardened front gasket. So I decided to renew the gaskets. I took everything apart and put new soft rubber gaskets behind the diaphragm and in front of it. I noticed a better sound, but still boxy, not the open sound I expected. Usually the Meltrope III is compared with the EMGs, etc. as a very good soundbox.

The first odd thing I noticed was in the front gasket, the one just behind the cover. I noticed when reassembling that this gasket is pressed against the needlebar oval plate, the one which acts as a fulcrum. So it prevents the needlebar free movement. Also, the gasket is pressed into place by the soundbox cover in a somewhat strange fashion.

Thinking later about it, and disassembling it again, I discovered a third rubber gasket hidden in a deep groove at the back; a deep groove between the soundbox body and the center portion at the back, which is raised. That gasket was also very hard.

The question is: the original arrangement was this one? a double back gasket? (1: the one into the deep groove, and 2: another one on top of it)? Then the diaphragm, then the front gasket (3), pressed against the cover and also under the needlebar fulcrum plate? It is a bit strange.

I decided to put only the gasket in the groove (1) then the diaphragm, then the front gasket (3). This arrangement avoided touching the fulcrum, and also the cover was less jammed and forced onto the front gasket. It seemed reasonable, although the diaphragm is almost touching the raised center portion of the backplate, resulting in a very thin air chamber behind it... It looks better, but I suspect this is not right either... The sound continues being very boxy, and not as true and clear as with the 5a/5b.

Can any kind soul explain to me how to assemble this soundbox properly? How many rubber gaskets it should have?

Thanks,

Inigo, from Spain.
Inigo


Last edited by Inigo on Mon Jul 09, 2018 3:23 am, edited 1 time in total.

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 Post subject: Re: Need help restoring a Meltrope III soundbox
PostPosted: Tue Jul 03, 2018 11:47 am 
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Victor IV
Joined: Mon May 13, 2013 2:04 pm
Posts: 1343
soundbox has two gaskets one each side of the diaphragm , Meltrope gaskets were quite soft large internal diameter gaskets , are the ball bearings all in place ? and is the diaphragm centre fixing firm ?, some pictures of the disassembled soundbox would help , is the gasket retaining ring present ?


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 Post subject: Re: Need help restoring a Meltrope III soundbox
PostPosted: Tue Jul 03, 2018 1:31 pm 
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Victor O
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Keep'em well oiled
Joined: Mon Dec 18, 2017 2:51 am
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Location: Madrid, Spain
Dr Mike thanks for your interest. The answer to your questions is yes to all. maybe my rubber gaskets are slightly smaller than the black gaskets that it carried originally. I don't know if this makes such importance, but the sound continúes to be somewhat boxy.
I've been this afternoon tinkering with it once more, and readjusting the pressure of the cover, etc, but the sound of the 5B continues to be much better.
Maybe it is so simple that the sound of a perfectly restored 5B soundbox is really very very good, and the Meltrope simply cannot compete with that... the shape of the diaphragm, its size, the spacing to the back, the phase plug, and all the characteristics, are so different... In the small 102 the 5B also sounds great, more open and clean that with the Meltrope...
Inigo


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 Post subject: Re: Need help restoring a Meltrope III soundbox
PostPosted: Wed Jul 04, 2018 2:00 am 
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Victor III
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Joined: Wed Feb 22, 2012 10:03 am
Posts: 680
Location: near Utopia, UK
With high quality soundboxes it's also a question of how they match with various horn systems and what they were designed for. Try it on another machine!


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 Post subject: Re: Need help restoring a Meltrope III soundbox
PostPosted: Wed Jul 04, 2018 7:00 pm 
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Victor III
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Joined: Mon Jul 02, 2012 9:57 am
Posts: 536
Location: Hampshire, England.
Not strictly relevant to rebuilding a Meltrope soundbox but hopefully of interest and showing contemporary thoughts--The Gramophone April 1930. Expert Committee comment on page 533.

A Meltrope III is certainly capable of performing every bit as well as a 5B. Are your gaskets completely airtight ?


Attachments:
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 Post subject: Re: Need help restoring a Meltrope III soundbox
PostPosted: Thu Jul 05, 2018 3:02 am 
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Victor III
Joined: Sun Mar 15, 2009 6:18 am
Posts: 836
Inigo wrote:
I've been this afternoon tinkering with it once more, and readjusting the pressure of the cover, etc, but the sound of the 5B continues to be much better.


One problem I had with a Meltrope was that its cover lid was not perfectly flat. The cover lid holds the bearings, and as it was slightly warped the bearings did not sit properly in their slots and the soundbox distorted. Such warping was not immediately discernible, I had to lay it on a flat surface to detect it. After some tapping it improved, but at the end this soundbox never really performed as the other Meltropes I have. You may want to check that point in yours.


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 Post subject: Re: Need help restoring a Meltrope III soundbox
PostPosted: Thu Jul 05, 2018 10:47 am 
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Victor O
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Joined: Mon Dec 18, 2017 2:51 am
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Location: Madrid, Spain
The cover... In the Meltrope, the cover never rests flat, for the front part must raise to hold the fulcrum arrangement, which raises frankly over the soundbox body... with a flat cover, when you adjust the cover screws tightly, the cover raises over the fulcrum and never rests flat. Its this strange arrangement that makes the cover act as the spring for the fulcrum.
With tight screws, the pressure on the upper tiny balls is enormous, which seems not right. If you release the pressure unscrewing the cover, then it seems better, but the cover separates from the body. It never rests flat.
There its also the retaining ring for the front gasket. It exerts almost no pressure on the gasket.
I will post some photos later to illustrate these difficulties, or the things I don't understand about this soundbox design.
Inigo


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 Post subject: Re: Need help restoring a Meltrope III soundbox
PostPosted: Thu Jul 05, 2018 11:24 am 
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Victor O
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Location: Madrid, Spain
Attachment:
File comment: The Meltrope with the cover screws completely tight. Note the cover raising from the nearest screw up to the upper small ball. The pressure exerted on the tiny balls, and on the fulcrum plate is large.
1.jpg
1.jpg [ 2.87 MiB | Viewed 577 times ]
Inigo


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 Post subject: Re: Need help restoring a Meltrope III soundbox
PostPosted: Thu Jul 05, 2018 11:27 am 
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Victor O
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Keep'em well oiled
Joined: Mon Dec 18, 2017 2:51 am
Posts: 63
Location: Madrid, Spain
Attachment:
File comment: With the nearest screws released, the pressure on the fulcrum is lower, but the cover raises sloping from still further, up to the fulcrum.
2.jpg
2.jpg [ 3.51 MiB | Viewed 577 times ]
Inigo


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 Post subject: Re: Need help restoring a Meltrope III soundbox
PostPosted: Thu Jul 05, 2018 11:33 am 
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Victor O
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Keep'em well oiled
Joined: Mon Dec 18, 2017 2:51 am
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Location: Madrid, Spain
Attachment:
File comment: The cover resting upside down on the table. Note the tiny ball bearings.
3.jpg
3.jpg [ 3.78 MiB | Viewed 575 times ]
Inigo


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