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Broken Amp - is there a dr in the house?

978 Views 7 Replies 4 Participants Last post by  operator
Okay, well i finally have done most of the testing I can do on this amp without cracking into it. I'll try and provide a clear, concise format for the problem.

Amp: Kenwood KAC7251
Problem: Cutting in and out; volume dependant

So i pulled the amp and tested in a buddies truck today, and the problem replicated itself so i am 100% sure that the problem is inside the amp somewhere. The amp will just decide not to play sometimes- it seems that at a certain volume, it will 'blip' and fail to reproduce a tone. What I mean is this: the power indicator goes off and back on in the blink of an eye but the amp will not start working until a new tone has started. This usually results in about 2-3 seconds of silence from the subs.

This particular amplifier has had the power supply replaced previously because it exhibited similar behavior (note: different VEHICLE). So i'm currently operating on the assumption that this is also power-supply related.

What will cause damage to an amplifier power supply? Is it something in my power cable / ground setup? Could the voltage regulator be bad? Would a drop in voltage (below amps operating level) cause damage?

How can I test the power supply, voltage regulator, and any other suspects? I have access to campus' electrical lab so i've got power sources, waveform generators, oscilliscopes, and multimeters to do the testing with.

If it is the power supply, where can I source on for a reasonable price?

I'm gunna bring in the amp and crack into it tomorrow or later tonight and I can post up pictures of the physical layout of the amplifier if anyone is helpful enough to want use them to point out test points.

Thanks for any help you guys can give- i'm really not looking to spend 70$ for them to diagnose and replace a power supply.
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What awg are you running? It might be an unsecure ground. Best suggestion though... get a new amp.
Its not my wiring. the problem is reproduced in another vehicle that has a known, 100% a-okay wiring setup. Its a problem within the amplifier. If i can repair it for under 30 bucks, its worth it to me instead of just replacing the amp.
It would be difficult to replace any part without a schematic.
Maybe if you open it up you might find a loose connection.
It is possible to get it fixed, but the bill would be at least $70, just for the shop time, even if you could find a shop that can do it.

Chances are one or more of the output transistors are dead, so the remaining ones have to do too much work and overheat, so the amp shuts down for a moment or two to cool them down.

The fact that it failed in the first place is a possible indication that you probably don't want to fix it. The amp just may not be able to handle the loads you were giving it. But, of course, it could just be random bad luck.

Mass production is so cheap that you can get a decent new amp for $90-$150 shipped. My 300 watt RMS Profile amp was only $90, and it has plenty of power and sounds great. The elegant look of the amplifier shaves at least $30 off the retail cost by avoiding unnecessary garish geegaws like so many more expensive amps. It should have a lot of durability due to its Class-D design, which heats the output transistors less than the usual Class AB designs.

It is capable of driving two 4 ohm voice coils in parallel, or two 2 ohm coils in parallel. I am using it to run a single 4 ohm load.
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like I said- I will be doing the work on it. I just was hoping someone could tell me what components I need to test, and whether I need to be checking their voltage under load, or a certain resistance or whatever. I'm pretty sure I remember the bill being something to the effect of 60$ diagnostic, 10$ power supply when I got it fixed last time. So If i do the work and can source an power supply, then its DEFINATELY worth me doing the repair since I dont have money to spend on an amp- but 10$ i can swing.

Geewhiz- how fast would a transistor heat up and cool down? Honestly, when this thing cuts off its faster than I can snap my fingers that the power indicator is off. According to my manual, whenever its in protect that light will stay off. Thats why Im thinking maybe its a voltage regulator problem letting a surge through instead of a heat problem.
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There may be clip detection circuitry in the amp, and when it is overdriven, it shuts down the amp for a few seconds. But with all of the fins, it probably only takes 3 seconds or so to cool down.

About the only way you can reliably determine what is wrong is by poking around with a oscilloscope while the amp is running. Even then, you would have to have a pretty good understanding of the schematic to know what to expect.

You may get lucky when you open it up and you can actually see what is fried. If the amp is about five years old, likely candidate are capacitors --- there was a bad rash of fake Nichia branded caps around that time and lots of computers and other electronics assemblers were fooled by these impostors, which tested fine before assembly, but failed very quickly due to bad electrolyte.

Unfortunately, when the caps go, they very often take other things with them as the leaking electrolyte shorts out other parts of the circuit.
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