High Current fuse blown amp fix help

[Kamzy]

Hey guys I've got a Sony subwoofer with an amp to drive the sub which has stopped working.

I'm trying to fix the amp and am a rookie in fault finding.

Some things I've noticed after having a look at the amp.

- Firstly the fuse has a high current blow and goes black
- The fuse is a time delay fuse, if that helps
- I've check the whole circuit board and havent found any blown components
- I tried turning the amp on with a new fuse and disconnected the speaker and the fuse still blows.
- When testing I set the volume to its lowest.

- Since most common amplifier faults are due to the output transistors shorting out, I found the amplifing unit which looks like it includes an op amp and some transistors. The amplifing unit or IC is called STK404-130S by Sanyo and is mounted onto a heat sink. I havent tested any voltages values while the amp is on.

This is the circuit diagram of the unit...



I checked any short circuits across any of the transistors and found that all are fine except that pins 11 and 12 are shorted, i think this is the problem but I'm not sure and i would appreciate any help

Thanks Kam

[bob91343]

Yep you have at least one blown transistor, the one that measures short. Check the drivers also. You will for sure have to replace the main power transistors but chances are there is more to do than that.

[Kamzy]

Thanks for the responce, The whole unit above is an IC with the pre drivers and power transistors in it, so I have to change the unit or IC, which will replace all components including the drivers and power transistors...

I can replace the IC but what else do I have to look out for? Do i have to check the rest of the amplfier...?

Any advice on where to start? What to look out for when a power transitor had blown and how it effects the rest of the circuit board.

I know some basics on how an amplifier works, we learnt about it in our electronic eng class... but looking at his circuit board it's much more complex.

One last thing, I heard you can check diodes and resistors and short circuit and open circuits while there is no voltage going through the system. What if I wanted to apply a voltage source small enough to check the system but not enough to harm myself... I have a power supply that can put in about 10v and the normal operating voltage of the system is about 90v or so coming of the transformer.

Kam <<<

[Kamzy]

bob91343, after replacing the pre driver and transistors, what else could i look for, where could I start any ideas?

[Tom7227]

Remove that IC and replace the fuse. Power it up. It will probably not blow the fuse. Those IC's are common failures

[Kamzy]

Thanks for the advice I will let you know how I go as soon as i get a replacment IC...

[semi932]

Power it up before you replace the missing ic.
If it is out of the circuit and doesn't blow the fuse then you have found your problem.

[Kamzy]

Hey guys, i said I would update the thread so I am with the results.

First of all thanks for the help, both Tom7227 and semi932 you guys were right about:

-The IC being a common fault in Amps and that they tend to fail. (This was the case here)

After ordering the suitable IC as stated above. I tried powering the amp with the IC out of the circuit and replaced the fuse and upon turining it on it did so without blowing the fuse =).

After this step I replaced the IC with the new one and then applied some thermal compound between the heatsink and IC after cleaning the surface of the IC.

Last step, I turned the amp on and it worked l

Thank you for all your help guys I hope this tread helps more then just me,
Kams

If anyone has some good threads on learning some other common faults please pm them to me if you have time ill much appreicate it

[AndrewT]

always use a mains bulb tester whenever you power up a modified project for the first time.

[Kamzy]

Originally Posted by AndrewT

always use a mains bulb tester whenever you power up a modified project for the first time.

I'll get one next time I'm in town, also I don't even know how they work, what they are used for, but I'm sure there is a thread on the forum somewhere. Thanks for the advice, I thought about getting one when I was reading the other threads on fixing amps, when I was looking for an answer to mine.