LED flash light has a resistor between battery and LED that has come lose.
Tech , solder resistor back.
PHD in electrical engineering.
Exhaustive tests have been made of photon emitting source , carbon current restrictive device and Volta pile energy source. We feel we are on the edge of a break through so a formal request is made for additional funding.
The message I want to coney from an old fart that has been there done that is do not over think a problem. Before trouble shooting place a hand on the PC board to feel for hot spots. Also the eyes are powerful tool for discoloration , water damage , cold solder. Tap a PC board for intermittent connections. More often than not this is all that is required. Trouble shooting , Scopes and diagrams should be a last resort when all else fails.
If it does come to trouble shooting , scopes and diagrams then rule number one " do not think ". 12 years of education just to walk and chew gum in life is a strong indicator we are not the sharpest tool in the shed. With " do not think " in mind religiously measure power supplies , fuses , low value resistors and any solid state device that is on a heat sink. If the temptation to think is overwhelming then take a break to build up will power. If nothing comes from the religious measurement algorithm then you are ethically bound to phone the costumer and inform them that there is a potential danger thinking therefore the original estimate must be renegotiated.