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 Post subject: Exhibition Soundbox Question
PostPosted: Tue Jul 09, 2019 3:41 pm 
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Victor II
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Joined: Tue Sep 21, 2010 5:52 am
Posts: 201
Location: St. Albans, UK
Hi all,

I have here 2 Exhibition Reproducers which I have carefully taken apart, cleaned and rebuilt each keeping its original mica diaphragm as they both look to be in good condition. I used the same gasket tubing in each. One soundbox sounds great and the other bloody awful.

Could this be due to poor mica?... despite its good appearance. Maybe tired or too restrictive balance springs? I wonder if anyone else has encountered this problem. I am thinking of trying one of the glass diaphragms from Germany...

Thanks, Jamie


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 Post subject: Re: Exhibition Soundbox Question
PostPosted: Tue Jul 09, 2019 7:10 pm 
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Victor III
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Keep'em well oiled
Joined: Mon Dec 18, 2017 2:51 am
Posts: 567
Location: Madrid, Spain
Jamie, I'm not in a position to be of great help, just now owning and in the shop for adjusting my very first Exhibition in 40 years collecting... A bit late, isn't it? :D
But two things i can say.
About mica, I would suggest you to take the mica of the bad soundbox and test it. Holding the mica between your index and thumb fingers, vertically, just ½" over a hard table, release it so it drops that small height over the table, bounces up and there you catch it again, do it several times, and listen to the sound. A Cristal clear klinnk! must be heard, and the mica is good. If the sound is a thump, then the mica has an issue (delamination?). The higher the tone, the better the mica is. If you happen to have several examples, compare them visually and audibly through that Kling sound. You will see the differences with a bit of careful examination.
Also examine the central hole for signs of cracking or delamination. Although the central hole is not good, of the rest of the mica is pretty solid and tight you will hear the klink sound as well, but a bad center will make it work bad.
The mica must be clear transparent all through, without strange reflections. Mica delaminates in very thin transparent leafs. You must not see any of that all across it. Ideally the cut at the edge must be neat as well cut brown glass all along, same colour as the mica, but darker. Whitening is a signal of border delamination. But if you have this defect, the sound would not klink cleanly, most probably.

About the exhibition springs... I've found they are pretty hard, so this needlebar is not very compliant. But no problem, as this seems to be the original design. What is true is this: with such hard springs, the needlebar must be very carefully adjusted so it does not press the diaphragm at either side when relaxed. The needlebar must be adjusted before mounting the central screw. First, the two fulcrum screws must be pressed slightly, so there's no loose sensation when you move a bit the needlebar with your fingers, just like the record groove vibration would do. But the movement must be same amount as if a record groove were moving the needle. If this gives a loose sensation, the soundbox will surely buzz. But they must not be either too hardly pressed, for this would limit the compliance too much.
I've also noticed that sometimes it's a bit tricky to make the fulcrum rest on its due location: with both knife points in their grooves on the fulcrum plate. The first tweakings with the screws can misadjust that.
Adjustment as usual: the needlebar foot must touch the diaphragm very slightly. Then the central screw can be fixed.

And once assembled, it must be tested and tuned somehow. I've learned in this forum all these things, and studying the exhibition closely, all that has its own logic. When testing the soundbox, a certain sweet spot of tuning must be found by tweaking a small bit the spring screws. Move one at a time, and notice the sound changes. Movements must be very small, and the sweet spot could be found in either direction. If one tweaking doesn't improve the sound, then return to the original adjustment and try the other screw.
That's all I can say. I have ended thinking this is a trial and error process, and then you eventually get it.
Inigo


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 Post subject: Re: Exhibition Soundbox Question
PostPosted: Wed Jul 10, 2019 6:40 am 
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Victor I
Joined: Sun Mar 10, 2019 4:02 pm
Posts: 132
Location: Central PA
Good explanation Inigo!


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 Post subject: Re: Exhibition Soundbox Question
PostPosted: Wed Jul 10, 2019 3:17 pm 
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Victor II
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Joined: Tue Sep 21, 2010 5:52 am
Posts: 201
Location: St. Albans, UK
Thanks Inigo,

It is a good explanation! The fine adjustments I have already done as I've rebuilt quite a few Exhibitions over the years so I think this is probably due to the mica. Your suggestion of a short drop to give a 'ping' test is a good one. Eric Reiss recommends this test but I had completely forgotten it until you mentioned it. It might save some time in future.

I have ordered some glass diaphragms from Norman Bruderhofer in Germany. He says these are more resistant against distortion, especially on vocal recordings, than mica. This particularly is a problem with this soundbox.

I was going to order some new balance springs if I can find them in Europe. The postage on the US ones is double the cost of the springs. Crazy really when they could surely go letter post for a few bucks, they couldn't get much smaller.

I was interested in the balance arrangement that 'micamonster' has come up with using 'rifle wire'. This product doesn't seem to exist in Europe but maybe safety pin wire would do. Inigo, Are you tempted to give this a try?


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 Post subject: Re: Exhibition Soundbox Question
PostPosted: Wed Jul 10, 2019 4:57 pm 
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Victor III
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Keep'em well oiled
Joined: Mon Dec 18, 2017 2:51 am
Posts: 567
Location: Madrid, Spain
Yes, yes, Jamie.
The springs, surely Mike Child (soundgen) which has a stall in Portobello Road on Saturdays, and a shop in Bedford, and carries mail orders too, has new springs for the exhibition. PM him.
Other UK dealers may also have them. The shipping can be as a letter, so cost is low. In case you can't find in the UK (not probable), Mr. Notenboom in the Netherlands also has them. He is zwartjeschift on eBay and I believe he must be in forum too. PM Mike Child first, if he cannot supply these, then ask other UK dealers (Howard Hope, Ken Priestley, John Sleep, etc).
About the ingenious system developed by that man, the spring wire I believe could be substituted by any high quality spring tempered wire. First thing that comes to mind is piano strings. Maybe steel guitar strings would also work. But to make the small whirls at both ends to catch the screws... you may need to untemper the wire without affecting the central part, which must remain springy. Maybe s could also do.
I've also read that thread and I'm tempted to try it. But I go slowly.... First thing I need is the back rubber isolator (I'm waiting for it to arrive home) to try the soundbox in its standard configuration. Then I'll start with the experiments... But the first ones I did were not successful, I believe I was running too fast... :oops:
First I must learn how to adjust that soundbox as usually.
Actually I'm looking for another exhibition for experiments. You need at least two, and maintain one as original for being able to compare.
In 41 years collecting, the one I have had been the very first one!
BTW, I've also purchased glass diaphragms from Bruderhofer. I'm impatient to try them, but first things first!
Inigo


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