Why audio amplifier is blowing fuse?

I have an audio amplifier (FPL 400) which constantly blow its fuse when started. I reproduce it even without speaker connected or audio input source.

The fuse is located just after the power supply cable.

I did tests by isolating some parts. When I disconnect the cable A and B, the fuse blow but when I disconnect the cable C, it didn't blow (see photo). So the cause must be in the main card.

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No component of this card seems visually to be degraded, so I don't know which one is defective.

With a multimeter, how can I identify the defective component?

I did my tests with slow fuses, same as provided by the manufacturer (T1AL250V). I also tested the diode bridge after having unsoldered it and it was ok.

Update (07/04/2018)

2 of the transistors looks to be ok (between 0.5 to 0.6 with the diode test). The 2 others ones are both at 0.0v.

Отвечено! View the answer У меня та же проблема

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Выбранное решение

With cable C disconnected, there is no power draw at all since there is no power being sent to the board. The problem is likely to be a shorted transistor on the output side. There are 4 in this unit. They are the 3-legged parts connected to the heat sink. If you have a meter that has a diode check mode, test the 3 leads of each transistor. They should read around 0.6 with a diode checker. If they read shorted , then they are the problem. Note: There are likely to be other parts that require replacement as well as the transistors.

Dan

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The two transistors that read 0.0 are defective and need to be replaced. General practice is to replace all 4 since the two that failed likely placed stress on the remaining 2. You can either replace all 4 now or 2 now and 2 more later when the now-good ones fail. Note: there are likely other components that will require replacement as well. There should be 2 resistors of low value (0.22-0.47 ohms) that may be open. Check those with a resistance meter.

Keep us posted.

Dan

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Наиболее полезный ответ

@akh01

my only concern would be the power IC’s on your power board. Post a close up of that board with your question as well. The part where your cable C goes to is a toroid transformer. It could be shorted out but unlikely. My money is on the power board. The reason why you do not blow the fuse when you disconnect cable C is that your board does not get power. We have a model number but not a maker for your amp. Let us know who that might be.

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Thanks for your answer, the amp is a SPL-400.

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@akh01 Skytek SPL400?

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Yes, it is a Skytek SPL400.

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@akh01 see if you can find some identifier on those transistors and at this point I would change all of the. No sure about the diode testing on those components.

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I finally succeeded to fix it !

The two transistors where broken, but also the 4 resistors close to the transistors.

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I changed all the components with an arrow.

Those with a red arrow were broken. Those with a yellow arrow were functional but were changed as a precaution.

Now, everything is working, thanks !

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Good news!!

For future reference, can you post the markings on the transistors? From the picture, it looks like the resistors you changed (near the transistors) were 0.47 ohms. The others look like a 220 ohms.

Most likely a single transistor failed which put stress on the other one for that side. With the voltage imbalance on the power supply, I'm guessing that the emitter resistors ( the large 0.47 ohm ones) failed on the other channel. It is good that you changed all 4 as the 2 that appear to be OK would have likely failed soon.

Dan

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