Double Bass Detached bass bar repair question

[Heifetzbass]

Yeah... there's a neverending supply of broken school basses, but that is where I got most of my repair chops!

Keep on fightin' the good fight!
BG

 

[layrepairs]

So, the next one they gave me had a sinking bridge again.
This time I knew as soon as it was handed to me.
I can't identify the maker but it is a cheap ply teaching 1/2 size.
Urethane glued back together at one point.

The bass bar was detached from the f-holes up and looked very crude from the start. This time I reshaped it and clamped it as suggested before. The sound post was at an odd angle and cracking the back due to taking all the force. So, the back and both sides under the bridge were failing. The bridge was adjusted way 'up' to compensate for the sinking front rather than getting it fix it in a timely fashion.

It also had half of a volute missing and some relatively minor edge problems. Scroll looks good now - I can't see the repair from a few feet away.

Maybe I should have asked first again but this is basically what I did and it seems to be holding together (4 days now with the strings).

- Took off the front
- Clamped the bass bar to the front, light spray of water a few times, waited about a week
- Glued up the crack on the back and added a 3'' disk, 1/16'' thick birch ply (it was what I had) over the crack for the sound post.
- carved the missing volute section
- Worked on the bass bar and then clamped it again
- glued up the bass bar
- partially failing end block, glued it up
- edge repairs and various cleanup
- put it back together and adjusted the post
- took both the nut and bridge down some, the strings were about 2mm above the FB at the nut.

The bass bar side f-holes look pretty aligned but the sound post side still looks distorted. It was badly cracked here too.
I don't know what I could do to that short of intentionally pulling the laminate apart.

Appropriate?

 

[Matthew Tucker]

You can sometimes restore the arching between the ffs with heat and weights and moisture, and before re-gluing the bassbar.

That bassbar is way too thick at the ends; it needs to be thinner at the extremities so it flexes when the top moves. This will help stop the ends from ungluing over time. Next time, leave it the way it is in the middle and graduate in a smooth curve taking it down to about 9-10mm at the tips.

 

[layrepairs]

Wow. I did take it down some too. I'm not very aggressive about changing the original.

It had those thicker bumps near the ends that I reduced some but left on it. I'll look for some example pictures.

For the front, would I put it upside down over say some blocks set to the desired position and sand bag it? The problem with this one was the bass bar side came back up easily but the sound post side was badly cracked with lamination problems on the outer layers (oddly the inner layers had not broken).

I really do appreciate the comments.

BTW: The next one has a broken neck near the block with a failed older repair; seemingly with a dowel or screw right through the finger board. A new adventure for me.

 

[Matthew Tucker]

You need to make a cradle for working on the the top; I made mine from a 3/4" sheet of MDF with a bass-shaped hole cut out of the middle (but about 1" smaller than the average bass top all round) and the edges chamfered nicely. A few wood blocks screwed on underneath to raise the cradle 2" off the bench. You can have all sorts of fun with the bass top clamped in the cradle. Keeps the rim flat.

When the belly is badly misshapen on a cheap ply bass I have found that a belly patch works wonders to maintain the shape after sandbagging, but it's a fair bit of work for a crap bass, and so if you were charging by the hour ... :-(

 

[layrepairs]

Well, I do just happen to have a sheet of 3/4 MDF in the garage from a shelf mistake last summer.

The next one is a 1960 Kay. It has a snapped in half bass bar, urethane glue all over, a funny twist in the front, two steel screws near the neck pocket (cut now, with care) and a 1/2'' hole in the finger board that did nothing, not even a dowel.
It looks better than the recent neck pocket post but what I would have to call a big dovetail was never flush with the neck block.

This one is close to the fire pit. Yet, the ribs are like new...
I will probably need help on this one too.