Was this EMG's answer to the Credenza & HMV 163?

[Inigo]

Thanks for your advice and story. I will try that... Silicone caulk.

[CarlosV]

Instead of using the conventional white tube gaskets positioned in front and in back of the diaphragm, Bob used GE silicone caulk--which stays semi-soft--for the back gasket, and a white tube gasket for the front. This totally eliminates the mid-range peaks, and allows the tone of the sound box to be adjusted by tightening or loosening the pivot joint screws.

The caulk seems to be a good solution worth a try, but it requires a mold to make a ring, right? Do you know how he made them?

[OrthoFan]

Instead of using the conventional white tube gaskets positioned in front and in back of the diaphragm, Bob used GE silicone caulk--which stays semi-soft--for the back gasket, and a white tube gasket for the front. This totally eliminates the mid-range peaks, and allows the tone of the sound box to be adjusted by tightening or loosening the pivot joint screws.

The caulk seems to be a good solution worth a try, but it requires a mold to make a ring, right? Do you know how he made them?

Based on what I was told, for the Exhibition sound box Bob rebuilt for me, he smeared a tiny amount of the silicone caulk around the edge of the inner rim and positioned the diaphragm over that so that diaphragm's edge did not touch the rim. After letting it set for a few hours, he then positioned the top (conventional) gasket in place and screwed the back plate on, firmly. He then carefully adjusted the needle bar pivot so that the needle bar just touched the diaphragm, and then secured the needle bar's foot in place with the tiny screw. (Bob used a tiny drop of clear shellac to seal the joint, but a conventional method should work.)

I once tried rebuilding an Exhibition sound box using silicone caulk for both the front and back gaskets. I didn't use a mold but drew out long beads of silicone--approximating the width and thickness of a conventional gasket--onto a sheet of wax paper. I let them set completely before peeling them off the paper, and then cut them to the proper fit, with the "flat" side touching the diaphragm's surface. The result was quite good, eliminating the "ringing" mica tone, though the sound box sounded more mellow than I expected. It did an excellent job playing electrical recordings even with a loud tone needle.

As to what I mentioned in my earlier post, when adjusting the tension of the needle bar, to make sure that more pressure is exerted by the left hand pivot, Bob positioned a tiny piece of paper between the knife edge fulcrum and plate on the right side.

After adjusting the two screws so that the needle bar just touched the surface of the diaphragm, Bob gently pulled out the paper from the right hand pivot. Hope this makes sense.

OrthoFan

[Inigo]

he smeared a tiny amount of the silicone caulk around the edge of the inner rim

Sorry... the edge of which inner rim?
And ...

he then positioned the top (conventional) gasket in place and screwed the back plate on, firmly

Which is the top gasket? It's the one at the front ring, or it is another one at the back on top of the silicone? I don't understand all this when compared with the former idea:

Bob used GE silicone caulk--which stays semi-soft--for the back gasket, and a white tube gasket for the front

Sorry if I seem a bit silly, but it's a bit difficult for me tho imagine all that together...

[OrthoFan]

he smeared a tiny amount of the silicone caulk around the edge of the inner rim

Sorry... the edge of which inner rim?
And ...

he then positioned the top (conventional) gasket in place and screwed the back plate on, firmly

Which is the top gasket? It's the one at the front ring, or it is another one at the back on top of the silicone? I don't understand all this when compared with the former idea:

Bob used GE silicone caulk--which stays semi-soft--for the back gasket, and a white tube gasket for the front

Sorry if I seem a bit silly, but it's a bit difficult for me tho imagine all that together...

Sorry, I got it "bass-ackwards"! By "front" gasket, I meant the one that faces outward or is visible to you when the record plays.

1. Holding the front plate of the sound box in your hand, position a conventional gasket in place, as shown. Again, that will be the one that is visible.

Screenshot 2024-06-29 150320.jpg (28.58 KiB) Viewed 2351 times

2. Position the diaphragm on top of the gasket and apply a coat of silicone caulk to the edge of the diaphragm, as crudely shown by the yellow circle here:

Screenshot 2024-06-29 150709.jpg (29.54 KiB) Viewed 2351 times

Not too much, just enough around the edge of the diaphragm to create an air tight seal--slightly less then the thickness of the "front" gasket. I think Bob told me that he used a flat toothpick as a mini spatula to apply the silicone.

3. CORRECTION TO MY ABOVE POST:
BEFORE the silicone completely sets, carefully adjust the needle bar pivot and attach the needle bar's foot as I described before. (In that way, you can align the screw with the hole more easily.) After this is done, allow the silicone to set completely--an hour or so.

4. Attach the sound box's back plate. If you wish you may apply a very thin bead of silicone around the edge of the front or back plate before you put the two halves together, to ensure a completely air-tight fit.

Hope this helps.

[Inigo]

Perfect! Thank you very much, now it's completely clear to me. The photos and additional information help a lot, many thanks.
As soon as I have the materials and things to put my hands on it, I will make this to one of my Exhibitions, and let's hope I'll do well and it gives a good result. I'm tempted to try this with one glass diaphragm that Norman Bruderhofer sold to me a few years ago.

[CarlosV]

Based on what I was told, for the Exhibition sound box Bob rebuilt for me, he smeared a tiny amount of the silicone caulk around the edge of the inner rim and positioned the diaphragm over that so that diaphragm's edge did not touch the rim. After letting it set for a few hours, he then positioned the top (conventional) gasket in place and screwed the back plate on, firmly. He then carefully adjusted the needle bar pivot so that the needle bar just touched the diaphragm, and then secured the needle bar's foot in place with the tiny screw. (Bob used a tiny drop of clear shellac to seal the joint, but a conventional method should work.)

I once tried rebuilding an Exhibition sound box using silicone caulk for both the front and back gaskets. I didn't use a mold but drew out long beads of silicone--approximating the width and thickness of a conventional gasket--onto a sheet of wax paper. I let them set completely before peeling them off the paper, and then cut them to the proper fit, with the "flat" side touching the diaphragm's surface. The result was quite good, eliminating the "ringing" mica tone, though the sound box sounded more mellow than I expected. It did an excellent job playing electrical recordings even with a loud tone needle.

As to what I mentioned in my earlier post, when adjusting the tension of the needle bar, to make sure that more pressure is exerted by the left hand pivot, Bob positioned a tiny piece of paper between the knife edge fulcrum and plate on the right side.

After adjusting the two screws so that the needle bar just touched the surface of the diaphragm, Bob gently pulled out the paper from the right hand pivot. Hope this makes sense.

OrthoFan

Thanks, it seems easy enough, I will give it a try.