Columbia soundbox adjustment.

[anchorman]

I’ve been going through some Columbia viva-to Al soundboxes, and there are two small hex head screws under the needle bar on the body of the soundbox. How are these best adjusted? That is, where should they be set, how much clearance to the needle bar is best? They seem like they’re there to prevent excessive needle bar travel from damaging the diaphragm, but perhaps they have another purpose? I seem to remember reading about this somewhere, but am finding no information currently.

[Marco Gilardetti]

I actually believe these are indeed to prevent diaphragm damage, in case of sudden soundbox drop etc. Other makes also feature them (Paillard comes to mind).

The closer to the needlebar, the more they will prevent any damage of course, but in turn they may hit the needlebar causing distortion even during normal play. Personally I set them by trial and error: I progressively turn them towards the needlebar until I hear that they're hitting it during normal play, then I turn them back half turn or one turn and play-test them again.