I recently got a Tesla Stealth 70W mod. It is a great mod with excellent performance as well as great ergonomics and I overall was very happy with it. For the first couple of weeks until it broke down that is.

The breakdown was caused by a bump as the mod fell from a nightstand (less then one meter high) onto a carpet. That is not a shock that would cause things to break, but apparently nobody told that to the Tesla people. A quick search on the Net made me discover that I am by no means the only person to which this happened; there are many people out there complaining about precisely the same thing: After a however slight bump or sometimes for no reason at all the mod will start firing by itself as if somebody presses the firing button continuously.

Since I found no explanation out there I decided to investigate myself. To cut a long story short, the cause of all this trouble is a defective design.

Let's first take a look at the battery compartment and focus in particular to the negative contact (to the left in the photo):

That is a spring contact and the spring is very strong. This is a good thing since the mod can draw as many as 18A from the battery so the contact must be as firm as possible. Here is the spring (on the other side of the contact):

So far so good. Notice however the three small nuts embedded in plastic and surrounding the spring. The photo is blown up and they look pretty solid, but consider that the mod is about 18mm thick and so you will realize that the screws that go into those nuts are less than 1mm thick. This is getting kind of flimsy, but ultimately not that bad.

Now, what comes on top of the spring (fastened to those three nuts)? I would have expected a metal plate or something equally strong (to handle the tension in the spring underneath), but the Tesla designers proved me wrong. The three screws fasten a (drum roll)…

… flimsy piece of PCB! The photo shows only two screws holding the PCB in place since I dropped one and I did not bother to look for it (see below), but you get the picture. The only thing on that PCB is a firing micro-switch which is activated by the rubber firing button that presses on it from above.

This setup has several modes of failure. The PCB might fracture in time (given the continuous, strong pressure of the battery contact spring), a nut might break free from the surrounding plastic (the contact surface is pretty small and the load is pretty high), or the screws or nuts might strip (which would be the least likely possibility, but I have to mention it for completeness). The effect of any such a failure is the same: the micro-switch will get pushed by the strong battery contact spring against the firing button, and will result in the mod firing continuously. That's what happens to so many people, myself included.

In my particular case the bump mentioned at the beginning caused one nut to become dislodged from the surrounding plastic; for those who keep count that would be the second mode of failure. I believe that this will happen often when the mod is dropped and that this happens to most people experiencing the continuous firing. However, I also believe that even if you pamper the mod all the time and never drop it or shake it too much, the continuous firing will still happen to you. Indeed, the PCB is flexed back and forth every time one inserts a battery and also every time the firing button is pressed (though the force is smaller in this case). It is just a matter of time until the PCB fails.

Is there a fix for this? Maybe. If the nuts are dislodged or the screws strip then this can probably be fixed. The fix I tried was placing a strip of 1 mm thick aluminum (some hard plastic will also do) all around the PCB. This would keep the PCB down against the case. Unfortunately I placed the strip incorrectly (I should have glued it to make sure it stays in place when the mod is inserted back in the case but I did not), and when I removed the case to fix the placement the strip broke free and destroyed the firing micro-switch. That was the end of the mod for me and it promptly went to the trash. In case of PCB breakage I am sure that one can concoct a replacement but I did not get to try this.

In conclusion, I will claim that the combination of the battery contact spring and the firing micro-switch is a defective design and will always fail sooner rather than later. Tesla wanted to be cute with the placement of the firing button, but they introduced a deadly flaw while they were at it. In all the other mods I have used the battery contact spring presses against a solid piece of metal (often the case itself); that's how you do it, Tesla.

Overall, the Tesla Stealth 70W is a nice and cheap mod, but given said defective design is not worth the money.