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How to make a Leak Light - by John Kilpatrick 

This describes a simple and inexpensive single-bulb leak-light for finding badly seated pads on clarinets, saxophones, or similar. The simplest versions described below can be made for under 1 - and the others if one happens to have the right bits and bobs knocking about such as 3mm rod/tube and 6/4mm flexible tubing.

Leak lights 

Starting point:

Acquire some white "LED Balloon Lights" (for example off eBay). These work on 3 x LR41 batteries (included), and are switched on by removing the insulating tab and tightening the end screw. First they flash, then with more tightening they go onto constant. Don't lose the tab - they can't be relied upon to stay off when turned off without that as a safeguard. The lights are about 12mm diameter, so won't go down the top end of a Bb clarinet past the register hole tube - but that is no drawback. Their design includes a convenient fastening loop. Leak light bulb

Solutions: - Three Ideas 

1. Fasten light to rod.

Heat and flatten the end of a length of 3.2 mm rod, drill a hole, and connect it to a light, e.g. with a 6BA screw & nut, or a small split-pin, or a bent nail of the size useful for ad-hoc repairs to umbrellas.
Optionally slide onto this a small tap washer, or something with a hole of about 3mm; not essential, but quite good for locating the device in the instrument, or for limiting the amount of back-lighting.

Leak light on stick
2. Connect to tubing and use a rod as a stiffener. 

Push-fit a length of 4mm i/d tubing onto the light - it should hold well enough. Push a slightly longer length of rod into the tube to hold it straight - or withdraw it slightly to achieve a flexible end section.
Optionally slide onto this a large tap washer, or something with a hole of about 6mm; ... as above.

Flexible leak light

3. Use string. A really simple method, and perhaps all that is needed!

Use a length of narrow cord, e.g. 1.5mm picture/curtain cord, somewhat longer than the instrument. Fasten one end to the light - it might be easiest to push the cord through the hole and fasten it to itself with a piece of sticky tape, as knots tend to leave a loose end sticking out that gets in the way.
At the other end of the cord contrive and fix a pull-through weight. For a clarinet this needs to be quite narrow, for example a short length of brass or aluminium tubing, or perhpas a bundle of small paper-clips. Whatever is used must be just heavy enough to drop the cord down through the instrument, and narrow enough to go past any obstacle such as the internal register-hole tube on a Bb clarinet.
Having to hand a clip able to hold the cord in place - i.e. act as a stop - is useful; say a paper-clip or clothes-peg.

How to use: 

It is necessary to work in the dark (so not easy in mid-summer!). Have a torch handy to see what you are doing between inspections: better than fumbling for the room's light switch.
In all cases the idea is to switch on the LED light and position inside the instrument and near to the hole(s) you want to check. In the dark, what you see depends on the nature of the pad, as some types are translucent, but where there is a leak it should always be possible to see a thin crescent of light between the pad and the tone hole, and in some cases (e.g. a metal-bodied instrument) some tell-tale reflected light from the gap

Example for Bb clarinet using the string solution

Switch on the LED light (see 2nd paragraph). Assemble the clarinet without the mouthpiece or bell. Hold it bell-end upwards and drop the pull-through down it. Turn the clarinet the other way up, pulling the light up to near the top.
Position the LED light near the hole(s) you want to check. Turn off any room lighting etc. and take a good look at the seating of the pad on the tone-hole.

Example for Alto/Tenor sax using the rod or tube solution

Remove the crook and lower the LED down the instrument to an area you want to check. For the lowest notes work from the bell end.

Ah - what about Baritone Sax?

A method here is to use about 5m length of cord (1mm or 1.5mm) and tie the LED to the middle of it. Fasten a steel weight - something like an M4 screw with a couple of nuts attached - to one end. Somehow thread the cord through the instrument, probably starting at the narrow end and turning the instrument upside-down to get the weight to go through the bend. It's hard enough to get the weight as far as the bottom of the bell, and probably easiest once that is achieved to use a magnet on a string to pull it far enough up the bell to grab it and pull it the rest of the way.
You then have two free ends, and it should be possible to pull the LED (after switching it on) to any position inside the instrument.

Baritone Sax leak light