Questions on Exhibition reproducer rebuild

[Inigo]

I'm not sure of what you're meaning... But the balance springs can be set at any tension from zero to a high value, just with the small run the screws allow, maybe 2mm when you give them several turns. I feel these springs are pretty stiff. I'll explain myself.
When starting the adjustment with the screws fully out, you give them several turns until the head starts barely touching the springs. This first position is what I'd call 'no pressure' or 'zero'. If you balance the needlebar in this position, the groove vibrations actually bounce at the diaphragm connection, and come back to the weak point in the chain, which is at the loose knife supports, causing the needlebar to rattle at this point. Indeed it will work as an inverted lever, the attachment at the diaphragm being the pin point, the needle point being the action, and a reduced movement caused at the knife supports, which are loose and will rattle.
You need a certain pressure at the springs to maintain the lever supported on the knife supports. So you must give some pressure further than that zero point.
The problem is these balance springs do a double function: to support the needlebar on the knife edges, and to provide a certain stiffness at the fulcrum. Other soundboxes as the 4/5a or the orthophonic solve this issue using no springs, but a fixed bushing, which allows for a secure needlebar attachment with infinite compliance, so all the energy goes up to the diaphragm (or the spider case of orthophonic). But the exhibition uses springs, so we must live with a certain stiffness at the balance point which actually steals part of the groove energy. But this is allowable, and well adjusted in conjunction with the diaphragm stiffness, forms part of the design.
If the springs are a bit more tight but not too much, the fulcrum is secured on the knife edges, but a secondary parasite rattle may appear, at the balance springs screws heads. The springs are not perfect, and the contact with the screw head may not be full. When the needlebar vibrates, the springs ends also vibrate up and down, rattling against the screws. This is also inherent to this design, and can be corrected on two ways:
1) the factory setting; you give more tension to the springs until this secondary rattle disappears.
2) a modification; if you add two small thin felt or rubber washers between the screws heads and the springs, this eliminates the rattle at these points, and allow a better contact of the screws heads with the springs. This has an added effect of reducing the overall apparent stiffness of the springs, so more vibration energy goes up to the diaphragm. This sounds better, but has again the effect of bouncing we talked before, and in turn you can experiment again the rattling at the knife edges, which must be corrected again applying a bit more tension on the springs.
Anyway, we always arrive to the same point (washers or not) which is that a certain tension further away from the zero point is needed for the system to work properly.
The other problem is when you apply excessive tension. Then the springs will steal the vibration energy, preventing it to arrive properly to the diaphragm attachment, and modifying terribly the transmitted wave spectrum. Not only they steal volume, but they modify the sound quality, muffling it.
So one must search the sweet point, starting from the zero, increasing the tension gradually until there is no rattling or buzzing with most records.
This can be a very long discussion. I'm preparing an article about what I've found when analysing the exhibition...

[Inigo]

audioengr,
Reading again your message, I believe I understand you, and I find a shorter answer.
If you're meaning... elongating the needlebar by means of straightening a bit the square angle at the diaphragm end, this indeed would apply less stress on the balance springs, but I find two reasons for not doing that;
1)it would severely alter the needlebar, which would be dangerously weakened at the angle, and possibly break at this point, rendering it unusable. It would lose stiffness, be no more original, etc. I found dangerous the non-reversible modifications.
2) I cannot see why you might need to do that. If the needlebar and springs are in good shape, the screws at the balance springs must be inserted and given several turns into their holes without exerting any pressure on the springs, until you reach that point where they begin to slightly press on the springs.
But you asked this, so you’re having a problem with the needlebar which I can imagine. You may have a needlebar WITH THE SPRINGS MOUNTED IN REVERSE ! That’s the only possible way with which you may need to exert a high pressure on the springs to insert the balance screws. This, or your springs are badly bent out of shape.
The springs are both equal. Each spring has one end with a neat round hole in the flat end. The other end of the spring is curved, and has an oblong hole.
Each spring comes with one short screw and one long screw, plus a locking nut.
The short screw is the fixing screw. It must go in the round hole at the flat end, and the spring must be attached by this end to the needlebar plate, by the face opposite to the needle chuck. the side of the long needlebar. So when the needlebar is put in place, you can’t see the screw head as it will stay by the inner side of the plate. The spring must be placed so that the curved end has a convex (inverted cup) shape facing you, where you insert the long screw. Then goes the difficult part: you must insert carefully the locking nut below the concave end of the spring, and the straight plate protruding from the front ring of the soundbox, where the screw end will go. Then you rotate the screw until you catch the nut, then insert the nut on the screw several turns, to clear out the screw end, and then that end is screwed into the front ring plate.
If the springs are mounted by the outer side, the long screws will result too short to be inserted in their places, and you’d need a great pressure to insert them; impossible to do without bending something out of shape.
A drawing would be better.

1.jpg

Why don't you post a photo of your soundbox, so we can see what happens...?

[audioengr]

Thanks for all of the information.

I was thinking that I need to bend the lever to elongate it in order to loosen the springs to prevent the attachment point to the mica diaphragm from moving. It is a small hole in the diaphragm and I don't want to move the diaphragm off-center. It should not touch the housing.

Is it possible to adapt the arm to accept the Orthophonic reproducer? What generation of player did this first appear on?

[phonojim]

The method I use to be sure the diaphragm is centered is as follows: First, assemble the needle bar to the reproducer body and tighten the adjustment screws so the needle bar is snug on the knife edges. The spring tension is not important at this point. Next install the tubing in the front housing, lay the diaphragm in place and adjust the needle bar so that the foot is touching the diaphragm. Insert the screw through the diaphragm into the needle bar foot and tighten it, but leave it slightly loose so that the diaphragm will rotate. Rotate the diaphragm until you get it centered in the front housing and not touching the inside surface of the housing at any point then tighten the screw to hold the diaphragm snugly in that position. Install the rear tubing and the back of the reproducer. At this point, remove the diaphragm center screw and proceed with the spring adjustments then reinstall the diaphragm center screw and test the reproducer. Once satisfied with the results wax the needle bar foot and screw (both sides of the diaphragm). This is more easily done without the rubber back flange installed. I lay a sliver of beeswax on each side of the diaphragm then heat the needle bar with a soldering iron held against the bar near the diaphragm. The wax will flow into the joints. Use as little wax as possible for a neater looking joint; the wax will flow on its own and it takes very little to make a good seal.
Hope this helps.

Jim