I got in the habit of asking the customer how big the fuse was when he got the wires reversed.
I would get a puzzled look, and explain that the higher the fuse rating the higher the price of the repair.
Biggest risk is that the protection diode will get a big enough surge of current to blow it apart.
This diode is connected to the DC power input. When the polarity is correct, no current flows through it, as if it were not there at all. But when the polarity is reversed, it becomes a near dead short. The common name for this sort of protection is "crowbar".
The diode will be rated for 1 Amp, typically. This means that the 2 or 3 or 4 Amps it takes to trip the correct-size fuse will decisively overload the diode, overheat it and cause it to fail, becoming a dead short.
For either the right OR the wrong polarity. If it gets enough current to rupture the body of the diode, the thing disintegrates and leaves an empty space between the two lead wires.
When this happens, you no longer have any protection.
If the reverse polarity voltage is now permitted to reach the circuits inside the radio, a series of dominoes begins to fall.
Electrolytic capacitors are polarity sensitive. Reversed polarity will make the water content of the fluid inside turn to steam and rupture the aluminum housing of the part. Might push the rubber plug out the bottom, or split the can. Might just explode and hurl fragments of aluminum foil in all directions.
Next casualties will be the large semiconductor devices, the audio-power chip, voltage regulators, final and driver transistors. Next will come the PLL chip and smaller transistors in the receiver circuits.
General rule is that the more current a device draws in normal use, the more likely it is to fail from reverse polarity.
We adopted the same precaution that Galaxy did around 20 years ago. We solder a 6-Amp rated diode directly across the pins on the inside of the power socket. The body of this diode has enough metal in it to prevent the "pop apart" risk, even with a large fuse. Besides, if you hook up the radio backwards with a 3 Amp fuse, you probably won't damage anything more than just the fuse.
Won't send a repaired mobile radio home without that particular upgrade.
Cheap insurance.
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