Originally Posted by HighVoltage
A couple of things to throw into the conversation:
First, a battery is a VERY consistant power source unlike say a generator. Batteries provide an extremely steady voltage source and the voltage dropoff is generally quite predictable. However if there is a voltage loss at the battery (say you left the lights on for extended period), the voltage regulator from the alternator will begin increasing the voltage output so that current will flow from the alternator to the battery and begin recharging. However when you turn the key an event known as inrush current occurs. Every electrical devices has a nominal current draw, but it also has a maximum. The maximum usually occurs as a spike during the first few uS (microseconds) of applying power. Usually large capacitors are placed in parrallel on the system to help handle events like this, especially with extreme audio systems.
EDIT: Frankly, fuses generally are in place to prevent the accessory (RD) from damaging the voltage system and not vice-versa. In the event the system is supplying an excessive voltage (it cannot supply an excessive current) the fuse can act as a circuit protector for the accessory. However, the reaction times of most fuses are usually in terms of ms (milliseconds), so they do not generally protect from any kind of "spiking", but rather are a protection in the event of a fairly steady state condition.
The max/min input voltages of the RX is strictly determined by the power circuit. If it is a simple linear regulator then it is probably quite wide (~5v to 40v). If it is a switching supply, it is generally much narrower as the wider the input range on these kind of supplies, the more $$ the cost (somewhat exponetially). In any case, an engineer worth his/her salt would/should simply install some kind of voltage clamping to prevent an constant over voltage from destroying the circuit. Granted this doesnt cover fast voltage transients (FVTs), which may be the source of the issue.
Second, when your crank the engine over, you are consuming the largest amount of power from the battery to supply power to starter. Its possible some vehicle starters are producing a significant back EMF that results in a FVT spike onto the system that the vehicles system isnt prepared/designed to handle appropiately.
Third, even though the switch of the RD is in the off position, the power input circuit could still be connected, but no load is being applied. Hard to tell without a schematic. Even then, if the RD isnt switched on it is still possible for FVT to "creep" across the board and damaged other portions of the circuit.
Take all of it with a grain of salt though. I cant imagine a manufacture of $$ equipment not covering themselves on this sort of problem. In fact if the RX65 has been CE tested it likely isnt the source of the problem, as FVT testing (both radiated and coupled) is covered. Usually any electronics sold into Europe are required to go through CE testing. My experience with field failures on circuits tends to have alot more to do with quality control issues of components and assembly then the design itself. Although there is something to be said of designing the product to predict and handle these sort of things as well.
Enough of my rambling. Hope I didnt bore you all to death.
