Re: sound box membrane
Posted: Mon Jul 06, 2020 1:33 pm
Where did you get the wooden diaphragm stamping parts?
First, the soda can tin is too thick : you'd better try an aluminium sheet from a much thinner material. The aluminium from the food pans used for roast chicken or these prepared foods found in the supermarkets is much thinner (and delicate) and works much better.
Second, the shape of that diaphragm is like the Columbia or Thorens diaphragms. But the back of your soundbox has the shape of an hmv 5 type soundbox. Your diaphragm should have the same concentric corrugations as the back. The principle of these orthophonic soundboxes is having an air chamber of constant thickness, between the diaphragm and the back. You'd better take a mould of the back, filling those three triangular air passages in the center. You'd better look in the web for this, and try to copy it. The most difficult part are the radial or tangential corrugations at the edge, which give the flexibility to the diaphragm to work as a piston. Also you should add a central spider attached to the first big ridge from the center. The spider is who has the central dome with the hole for attac hing the needlebar little fork. This spider can be done from a thin soda can. Observe these images
- images.jpeg (8.05 KiB) Viewed 794 times
Here you have a video of a repair of a 5b soundbox, where he shows the diaphragm
https://youtu.be/sLIOO-34zxU