Intonation Problems! Suggestions?
 

Hello! A friend of mine has a custom bass that has serious intonation problems from 0 to 7th fret, then the rest of them has perfect tuning, the question is simple, which can be the factors that may be cause this? he already has tried different strings, and checked the intonation at the bridge! Thankyou (sorry for bad english)

Custom bass? My hunch is whoever made the bass put the first seven frets on the wrong positions.

+1 That's really weird, I've never heard of that before

If the intonation is correct at the 12 fret, the problem is mis-cut frets. There's just no way around it. Chances are the intonation isn't set correctly at 12, and has been compensated via tuning.

re set the intonation with new strings, and see if the problem is still there

A "putting the cart before the horse" solution is to just pull out all the frets and play fretless. That might just be the most economic solution.

Sounds like the nut slots may not be deep enough. If notes play sharper, the closer you get to the nut, I'd say that's it.

^^^This would be the cheapest and easiest thing to check. If that's the problem, you should be very happy.

How about pressing down on the strings behind the nut and at the saddle to set the witness point.

+1 Had a bass with a similar problem. Excessive string height at nut means excessive pressure to fret notes for first few frets, then slack in string makes fretting easier further along the neck. Check your string height at the first fret, if it is too high, nut needs adjustment.