Some head units use the same output chip to drive the speaker leads and the RCA outputs, so a chip failure will stop any sound from leaving the head unit. Other brands use an internal amplifier chip that is further down the signal path than the RCA output stage, so if the output chip fails, the preamp output can still work. My bet is on the previous situation.
However, if you are able to hear sound when you connect your spare speaker to the head unit, the head unit is likely fine, and the amplifier might be at fault even though it has all the right lights lit up. Take an iPod and use a 3.5mm RCA cable to plug it directly into the amplifier. Be careful to start the iPod's volume control at zero and then raise the volume until you hear sound. If you don't hear any sound, then the amplifier needs to visit the service technician.
Q: I have a major problem. I have a '98 Cadillac Eldorado and the 100-amp inline fuse connecting to the power wire from my battery keeps melting. I have no idea why. I am running two amps, both JL Audio--one 300/4 and the other a 1000/1. My system is running at 4 ohms and my bass amp is only turned up 0.75 max. My grounds are very short and connected to a tap weld (where two pieces of the car are tapped together). Do you know why the power wire from my battery is getting hot and melting my fuse?
Thanks,
Jondel Davis
A: A melting fuse indicates a couple of key problems. Your fuse is melting due to its resistance, which can be caused by corrosion or fuse limits. I have found that a fuse assembly, or any other contact point in a high-current circuit, will generate a certain amount of heat. The higher the current, or the higher the resistance, the greater the heat generated. For example, at 100 amperes of draw, 0.1 ohms of parasite resistance will generate 10 watts of loss in the form of heat while 0.5 ohms will generate 50 watts. How hot does a 50-watt soldering iron get? If the energy was dissipating over a bigger chunk of metal like a good length of wire, you would not have a problem, but the metal parts of a fuse are quite small, concentrating the heat to an extreme.
My guess is that you are using an AGU-style fuse in good condition (not corroded). The AGU fuse is essentially a jumbo version of the common AGC glass fuse. The friction fit contact points are too small to be carrying 100 amps of current. In contrast, the heavy-duty ANL wafer-style fuse has its connections bolted in place--ideal for high current.
My rule of thumb is to use a fuse that is at the lower end of its rating range than the high end. For instance, if an AGU-style fuse is normally found in 40 to 100 ampere sizes, I would use them only in applications from 40 to 70 amperes. For anything over that, I would use an ANL-style "wafer fuse," which starts at 80 amperes and goes up to 400 amperes. By using an AGU fuse close to the top end of its rating means that all of the fuse parts like its endcaps, fuse holder and connections are already adding to the loss. This results in heat even in a perfectly new assembly.
The ANL fuses are more expensive, but you have learned that you can keep paying for AGU assemblies until you have well passed the cost of the only ANL fuse assembly you will ever need.