Ok, So you just bought an old Ukulele at a thrift store /music store/ pawn shop or flea Market. You take it home, tune it up, strum, and it sounds beautiful. Then you make a chord and, eeewww!.
So, you go pick up the Uke from where you inadvertently threw it in horror, and check your tuning, Hmmm, it's still in tune. You strum again, sounds good. But every time you fret or make a chord, something is just not right.
Just a bad Uke?
OR it might just be set up wrong. Now, Feeling brave? Ok. lets start with a little physics lesson in regards to the mechanics of a stringed instrument. There are many, many things which directly and indirectly contribute to the sound of a Ukulele (or any instrument). Resonance of the material it was constructed from, size and shape of the body and sound hole, length, tension and type of the strings, material and placement of the frets, nut, fingerboard, bridge, saddle...
I can't go over it all, but I can give you some main points.
Where does the sound come from?
When you strum or pluck a string, The vibration of that string is passed on to the material of the Sound board through the bridge, This vibration is amplified by the hollow space of the body, and the result is the sound you hear. The length and thickness of the string determines that strings sound range, while the tension of the string determines the pitch within that range. A long loose thick string will produce a slow vibration, and hence a low note. A short tight thin string will produce a fast vibration, and hence a high note.
When a string is tuned, you are changing the tension.
When a string is fretted, for the most part, you are changing the length.
So a string of a certain thickness and length can be tuned (tensioned) to a certain starting range, and then shortened (fretted) to achieve the desired pitch or note. The proper amount of shortening is achieved by the placement of the frets in relation to the over all length (or scale) of the string, from nut to bridge. This is why the relationship of all these parts in regards to their distances from each other is crucial. If the bridge, nut, or frets are not the proper heights or distances from each other, the result is an unplayable Uke. Fortunately, some of these problems can be fixed.
The first thing I do when I get a "new" old Uke is check the strings. If the strings that are on it already sound good, and play fine, they probably are. If not, take the old cracked fishing line, guitar strings, or picture hanging wire off, and put on a nice set of Aquila Nylgut strings, and then play with it for about a week. The reason for this is that bad strings sound bad, and new strings can take about a week to "settle in". This is a good idea before you mess with the set up. If you did that and it still sounds bad, well, read on.
First, lets get our terms in sync. Here is a picture to help.
So, you are going to need some tools. First of all is a good electronic tuner. (I like the LED type. the "Snark" is what I use) So, first, find a quiet place and tune up. Once you are in tune, strum for a bit and check your tuning again, and make adjustments. Now lets check your harmonics. First, turn on the electronic tuner, and then rest the tip of your finger on the C string around the 12th fret. Don't push down, just touch it. then pluck the string lightly while sliding your finger (just slightly) up and down the string. There should be a point where it stops going thuk, thuk, thuk, and makes a pleasant "chime" like sound. Once you find that spot look at your tuner, it should read C. (same as the open string) Now push down (fret) at that same spot. It should be the same note. If not, is it sharp or flat? if it is flat, then the bridge is to far away. If it is sharp, the bridge is too close, or the "action" is too high. Do this with each string, (G should read G, E should read E, Etc.) and you should find that each string is pretty much the same amount sharp or flat (except the third string, which is usually a tiny bit sharp). (If the first one is sharp and the fourth one is flat, or vise-versa, the bridge is crooked.
2. checking the bridge.
First, get your ruler and measure the distance from the nut, or if your Uke has a "zero" fret, then the very first place the strings are free after the nut (see illustration)
to the middle of the 12th (harmonic) fret. (the raised part. not the space on the fret board) Write this measurement down.
Now measure the distance between the 12th fret and the center of the raised part on the bridge. Are the two measurements the same? They should be about the same, but with the 12th fret to bridge being slightly longer ( like 1/8 inch or so) on some Ukes, depending on the action. This is to compensate for the distance the string is stretched in fretting (the action) at the 12th fret. If this measures up, then the problem is somewhere else. If this distance is way off, then take it to your Luthier for adjustment.
There are some other common problems with older and/or less expensive Uke's. First is bad tuners. If a tuner is "loose" you can try tightening the little screws on the end of each peg. (but don't over tighten them!) If that works, you are set, if not then your tuners are bad, or the holes are oval. When tuners go bad, they will not hold the string properly even after you tighten the little screws, and the string will go out of tune the first time you fret. Bad tuners is an easy one, just get new ones. (some new ones will require drilling the holes a little bigger, or beveling them. Another common problem is the nut or saddle being too tall, so that it actually changes the tension, rather than just the length, of the string when fretted. The strings should have to be moved only slightly to contact the fret, but enough to keep from fretting accidentally, or from contacting the strings during a strum. The amount of movement is known as "action". Optimally, you want the action to be positive, but minimal, and at the same time you don't want the strings touching any other frets as you strum a note or chord, or you will get a buzzing sound.
If the action is too high at the nut, it will cause the first few frets to be sharp. if the action is to high at the bridge it will go sharp as you move down the fretboard. If you measure from the top (or crown) of the 12th fret and the bottom of the string, it should be around 1/8th to 3/16ths of an inch, (or 3 to 5mm for our metric friends) This is of course assuming that all the other measurements, such as scale length and fret placement are correct. Lowering the action is done in a couple of ways. At the nut it is usually a matter of making the notches in the nut deeper. You can do this with a file. Just don't make them too deep, or you will get buzzing on the upper frets. Also make sure that you use a file that is the approx. same size as the string. If the slot is too wide you will also get buzzing, too narrow, and the string will not slide properly while tuning. Lowering the action at the bridge is a matter of shaving a bit off the saddle.(or if it the removable type, just getting or making a shorter one)
Again, your best bet for any of these things is to take it to a Uke friendly Luthier.