"Shallow Light" waterproof LED torch/flashlight tear down and diagnosis

I have four of these torches (flashlights for Americans!) on my boat Tui. Two yellow ones in pretty much constant use and two more of the same model in blue stored in a waterproof "grab bag" which contains stuff that is need if we have to abandon ship. In my annual inspection of the ditch bag I found that one of the torches was permanently (but dimly)  on irrespective of the switch position. This made me wonder how they work and how to fix them.

I bought these blue ones on Amazon for £3.59 each including postage but not batteries, not there now but for example on ebay fro £1 + postage from Hong Kong.  You will find the same model in different colours for a wide range of prices. For example here it is for £32.97. On one side it says Shallow Light and the other side is a picture of a diver. Of course there might be some that are imitations of others so perhaps more expensive ones are better but I doubt it! I think they are all cheap copies of a LENSER D14 which is rated to 60m in a nice case with a 5 year guarantee for $59.85 on one site. I expect the real thing has better seals and is actually suitable for diving. The imitations are pretty bright and waterproof in shallow water at least. Torches on a boat take quite a hammering and often get dropped as well as wet. So far the ones in use have held up well.

Underside of switch showing magnets

One think to be careful of is how the batteries go in - as this could go wrong in an emergency I have a sticker on the side saying which way they go.  Also the lens and LED unit has grooves that have to locate in ridges in the casing so the magnetic switch is lined up.

There is a youtube video someone has made showing putting in the batteries.

To see how it works see the dismantled photo. Like a lot of diving torches it uses magnets to activate the switch so that there is no moving part penetrating the waterproof case. Of course that also means you need to be careful not to put them near compasses. The switch is retained by a 2mm Allen screw and underneath has three magnets. If you gently prize off the circuit board from the lens you will see that inside it has an LED a resistor and a magnetic reed switch. In one of mine it is this magnetic reed switch that has failed and is now permanently off whatever magnet is applied.

Dissembled

Allen key inserted to remove switch