Rebuilding soundboxes in Europe recommendations

[snallast]

Hello

I need to get at least two exhibition soundboxes rebuilt, I tried on my own but am not happy with the sound! Can anyone recommend someone in Britain - or the rest of the EU for that matter - who does this well? The part with changing the gascets isn't too hard, I think it might be the tuning?

[Orchorsol]

http://www.soundbox.co.uk/

Ian Maxted is a grand master of the subtler arts involved! Top recommendation.

[snallast]

Thanks!!! I'll get in touch with him right away, grand this with being able to ask / and getting good advice!

[Steve]

I've rebuilt over 30 Exhibitions and about a dozen EMG/Expert sound-boxes. What specifically do you need to know?

[snallast]

Well - I rebuilt a couple of exhibitions, changed the gascets and in one also the mica, replaced the backpart on the one that wasn't tight / still they didn't sound any better, possibly worse, and what's more they lost volume.

I'm thinking I must be doing something wrong, people talk about "tuning" them - maybe that's where I took the wrong turn?

I remember following a video step/by/step on the process on youtube. To no avail. I just concluded that I'm probably just no good at this!

Where did I go wrong?

[Henry]

If you want to have another go at it, this may help:

http://www.nipperhead.com/old/rep_exhib.htm

In my experience, it is crucial that the new front gaskets be very soft and pliable, the back flange seal airtight, and the needle bar just touching the face of the diaphragm, before applying the hot beeswax. As to the latter, I use a leaf of cigarette paper* as a spacer between the needle bar and diaphragm face while adjusting the spring nuts (these affect distance between bar and face), so that there is just ever so slight drag felt at that critical point, which is the "sweet spot" you are seeking. HTH
___________
* I don't smoke, but my wife was an oboe player and she used the paper to wick moisture from underneath the tone hole pads, so I had a supply on hand!

[Steve]

"Tuning" is a bit of nonsense to be honest - it implies something far more sophisticated than what we really need to do. We are not calibrating the soundbox to hit certain notes, but simply adjusting it to "balance" it between low bass frequency response (resonance) and the high frequencies. Ideally we want as much of both as possible.

The key to success is to ensure that the gaskets grip the diaphragm tightly all the way around both the front and back. Also it is essential to set it up so that when the springs are tensioned the stylus bar rests gently on the diaphragm dead in the centre but only just touching it. Too much spring compression one side will push the diaphragm inwards whilst too much the opposite way pulls it outwards in the centre. The best way to ensure that you have it spot on is to view the diaphragm under a good light and examine the reflection: is it "bent" or distorted? Have you replaced the tiny springs? Sometimes these get less malleable with age and end up too stiff, thus not doing their job properly. To test the Exhibition simply touch very gently the top of the stylus bar with the s/box lying on its back and the diaphragm facing upwards under a good light. You should see the mica flex AND very importantly, spring back to its original position when your finger is removed. To test the reverse action simply apply a little finger pressure to the needle screw over the pivot so that the piston action of the stylus bar pulls the diaphragm centre outwards. Watch it drop back quickly when you lift your finger off the screw.

There are many other checks but start with the obvious and see how you get on.

[snallast]

Have you replaced the tiny springs? Sometimes these get less malleable with age and end up too stiff, thus not doing their job properly.

Thanks for all the good advice!! This is what I've done: I had another go at one of them... but the springs, what springs? Maybe they're missing from both soundboxes or are they the part I've pointed the arrow on? The little screws make the mica move backwards and forwards I've kept it in the middle exactly... but the tiny nuts I don't understand.

It does sound better this time, so far.

[OrthoSean]

The arrows point to the springs, yes. The two nuts should be tightened down even with the reproducer body once you have the adjustments done, these keep everything "in place" once you've got everything correct.

Sean

[Henry]

From its appearance in your photo, I'd say that your back flange is definitely shot, and needs replacing. This alone would probably make a big difference. To ensure a proper air-tight seal, apply Vaseline (petroleum jelly) or lanolin between the flange and the sound box. Over here there's a product called A&D ointment, which is a combination of petroleum jelly and lanolin, which is ideal for the purpose. Look for it at the drug store (pharmacy, apothecary, however you say it.

As OrthoSean says, the function of the nuts is to lock in the screws that position the tip of the needle bar in relation to the diaphragm. The actual adjustment is made by turning the screws. Turning one screw (I forget which, but it's obvious when you do it) moves the needle bar tip toward the diaphragm face, while turning the other screw moves the tip away from the face (it's the lever-and-fulcrum principle). During a rebuild, you perform this operation in conjunction with the cigarette paper spacer between tip and mica, such that there is but the slightest drag felt between them. Once positioned, the setting is retained by tightening the nuts down, as the left one in your photo shows. Then you replace the tiny screw (that you previously removed from the needle bar tip!) and apply the drop of melted beeswax as a seal. Obviously, you must start the whole procedure described above with a clean mica diaphragm, either a new one, or your old one cleaned up; at least, that's how I did mine. I don't believe that you can properly adjust the sound box otherwise.