All,
Before all this misinformation gets out of hand, let me state that I was the one who designed and tested the switch mechanism, the electronics and the software for the Acr4. Therefore, I am the person most likely to understand the various failure modes. After reading this, you may do whatever you want, but these are the recommendations from the designer and tester of the Arc4 - excluding the modifications made to the Rev2 - which I never tested or approved. Still, most of what follows will also apply to the Rev2 Arc4.
The switch uses a high impedance circuit to detect the closing of the ED Dome Switch (part number EDSSC1). During manufacture, a liquid flux was used that had an affinity for moisture. If that flux was not completely removed, it would cause a relatively low resistance path across the printed circuit board that would - in effect - short out the switch - specifically when exposed to high humidity. I have personally seen several cases of this. The only permanent fix for this manufacturing defect is to desolder the Positive Contact Spring, desolder the switch, clean the circuit board and switch and then reassemble - using no flux or a non-conductive RMA flux. The fix for this problem takes a skilled person with proper soldering equipment about 15 to 20 minutes. The primary symptoms of this problem are the light turning on by itself, the light refusing to turn off or the light powering up without RFS (when enabled) at a medium brightness. All of these are symptomatic of a switch that is "pressed" - or shorted.
There are two other causes of these same symptoms. The most common of those two is an improperly adjusted Positive Contact Spring (or a damaged Positive Contact Spring) which does not allow space between the switch and the positive contact or a damaged switch. Another thread was referenced earlier in this thread that talks about how to properly adjust the Positive Contact Spring. This adjustment is a mechanical issue. If the spring tension is adjusted too low, the switch will be too sensitive and potentially remain in the "pressed" position after you release the button. If the spring tension is adjusted too high, the button pressure required to "press" the button is higher that necessary. There is a second spring in the button itself that is used to "bias" the Positive Contact Spring and the Negative Contact Gold Disk Spring. It is fixed in pressure (about 3.5 pounds) and should not be changed. The only adjustment to be made should be to the Positive Contact Spring - as described in the aforementioned thread. A damaged switch will not "click" and must be replaced.
Note that after a bezel-down drop from head height or a tail-down drop from head height, the Positive Contact Spring can become "dished". This is considered physical damage to the Positive Contact Spring. Often, this will also damage the switch itself. With careful application of force, the Positive Contact Spring can have the "dish" removed and is thus repairable. A new switch (part number EDSSC1) can be ordered from DigiKey.com.
If there is not enough total pressure applied via the springs - due to improper spring adjustment, manufacturing defects or unauthorized modifications, you may see the symptom of the light resetting (going through RFS if enabled) when turned on or immediately following a button press. This represents a bad battery connection - most likely through lack of total spring pressure on the battery. It can also be caused by dirty battery contacts, but this is not very likely due to the battery contact design.
Note that your Arc4 will always turn on dimly for 5 seconds after applying power (i.e., as part of a reset or changing a battery) - that is a normal part of the reset procedure. If the light then does an RFS (if enabled) and then turns off or just turns off (if RFS is not enabled), the switch was "off" during the reset sequence - i.e, all is well. If, on the other hand, the light remains on at a medium setting, the switch was "on" during the reset and is assumed to be damaged - per the documentation.
The third and only other cause of these same symptoms is if there is a defect on the circuit boards due to improper assembly procedures. If the defect is here, the fix would be way too difficult for the average (or even very experienced) person to fix because the circuit boards are encapsulated in epoxy and thus this problem is considered to be non-fixable.
As you can see, the mechanical mechanism is only one of the several possible causes of these symptoms. If you can clearly hear a click when you press the button and another click when you release the button - faint as it may be, the mechanical mechanism is not the likely culprit. However, since adjusting the Positive Contact Spring is the easiest procedure for the average user to perform, that is the simplest place to start. Cleaning the circuit board and switch (under the Positive Contact Spring) is the second thing to try because it is more difficult - and generally the last resort for the reasons noted above.
When the approved components are properly manufactured, assembled and adjusted, the switch can accommodate the varying battery lengths commonly found. However, when incorrectly manufactured parts are used, when parts are not properly assembled or when parts are not properly adjusted, the switch can exhibit sensitivities to battery length.
For the full "official" documentation from the creator of the Acr4, please read
this.
Henry.
