A customer brought in the nice Gibson L-48 because the top is sinking under the bridge.  This repair poses some real challenges.  Access to the interior is limited to the two f-holes which don’t give much room for visibility, clamping, or maneuvering.  In order to help with all of the above I ordered a tiny web camera on a gooseneck which can be slipped through an f-hole.  With a little light it produces amazing pictures and videos of the interior.  By watching the screen I see exactly where my tools are.

Here you can clearly see that both tone bars are loose from about the center of the top plate back.

In this short video clip one can clearly see the movement of the bars as the top is flexed and my cleaning the old glue from the underside of the top and the bars.  I have 400 grit sandpaper stuck to my palette knife with double sided tape.  After I did the underside of the guitar I switched it to the other side to clean the tops of the tone bars.  I was careful to only give them a light cleaning so I wouldn’t round over the edges.  I really like this new web camera!

I needed to make a clamping caul to pull the top into the proper arch.  The top in the back look to be about the same except where the top has sunken due to the loose tone bars so I decided to use the back as a template.  I cut a suitable piece of mdf and added two small blocks at either, resting just on the edges, to raise it above the arch.  Using a compass I scribed the contour of the arch on the mdf and bandsawed the caul.  In the last two photos you can see the how it matches the back and how much more of a gap is on the top.

Here you can see a significant change when clamped.

Here is the glue syringe I used. It has a long brass extension tube added which is crimped at the tip to slip under the tone bars and restrict the amount of glue dispensed.

Here is a little video of me injecting the glue.  Using the camera I was able to guide my extended syringe needle under the bars. There were some glue drips but it was successful.  This was no easy operation.

It took a while to figure out the clamping orientation.  The clamps would on fit at certain point in the f-holes and only a certain angles.  With a bit of trial and error I figured out an arrangement that allowed 4 clamps are regular intervals.

With the tone bars reglued the top is now ridgid and does not give under the pressure of the strings.