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So my friend drives an old, second-hand car in which the original radio has been replaced by the previous owner for an aftermarket radio.

His issue with the radio is that the volume slowly decreases while driving and after about five minutes driving there's no sound coming from the speakers at all. The radio works perfectly fine and when you turn the volumeknob you can see the volumenumber on the display changing, but there's no sound. Turning the radio off and on also doesn't change anything.

I'm guessing it's a wiring issue, but than I'd expect the sound to start/stop immediately while driving when the wires (dis)connect for a moment. Any idea what might be the cause?


Update:

(sorry for the late response, I almost forgot about this question :S )

So a couple of weeks ago we swapped the "broken" radio unit with an old one I still had lying around. He has been driving with it since than and hasn't noticed any issues yet.

The slowly decreasing volume is probably caused by an overheating amplifier, as HandyHowie already mentioned. That's all I actually wanted to know as it seemed odd to me and I couldn't find any information about it.

It's either a dying unit or a wiring issue, but for now it works. I'll update this post if the problem occurs again.

Thanks for the help.

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    Next time he uses the car, does it work for 5 minutes again? – HandyHowie Oct 22 '15 at 11:50
  • Yes, every cold start the radio works for about five minutes. And now that I think about it, I'm not sure what happens when he starts the engine again after driving for a while. I'll go and ask him that. – Spike Oct 22 '15 at 12:40
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It sounds to me like the speakers are not wired correctly, which is causing the amplifiers to overheat. As they heat up, they are probably lowering their output to try to stop themselves burning out. Hence after cooling down they start working again.

Check that the speaker wiring diagram of the car matches that expected by the radio.

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    I took a couple of radio removal tools with me today and checked the wiring. The previous owner bought a factory wiring harness to connect the aftermarket unit to the original connector, so that seems just fine. I didn't know an overheating amplifier could cause this and there isn't really much info about it on the web. I guess the easiest thing to do now is to hook up a different radio and check if the issue still occurs. – Spike Oct 23 '15 at 11:08
  • While you have it removed, power it up and check if the heatsink on the back gets hot as the volume starts to decrease. Also check that you are able to adjust the balance and fader to select each speaker individually. After that, I would disconnect the speaker connector and check the impedance of each speaker and also check the resistance to ground of each speaker wire. – HandyHowie Oct 23 '15 at 11:10
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It doesn't sound like a overheated amplifier to me. It sounds more like internal radio power supply issue (inside the radio itself) maybe a cold solder joint on the regulator or a defective filter capacitor. As the cold solder joint opens up the radio cuts out. A lot of amplifiers are designed to power down if the temperature is excessive. Reversing the speaker would cause a weird stereo image as one speaker is in phase and the other one is out of phase. It would also potentially cause damage to the speaker. In Phase the voice coil pushes outward, Out of Phase the reverse is true and the voice coil is damaged.

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