Sony STR-DB795 crackle noise in front L channel

That's an odd one. What might be a clue is that the next point that crackles is always 7.5dB up on the previous point, which would suggest that it is something 'digital', such as the volume control IC. I seem to have the service manual for just about every other STR series amp from around that time, but not the DB795, so unfortunately, I can't look any closer. However, I would say that often, the volume control IC also does input selecting, so it would be worth having a look to see if that is the case with this amp. I have had these ICs go faulty, and this is the sorts of problems that they can cause. I had one recently that kicked up a lot of noise on one channel, above a certain volume level. I would think that you would be able to see this crackle on a 'scope, and then know for sure where it was getting into the chain, but failing that, dabbing a 0.47uF cap to ground from various key points, such as the output of the volume control IC, might prove revealing.

Arfa

crackle is most certainly caused by loose connections. either via solder, or input plugs to the power amp board.

On what do you base that ? Crackle is *not* most certainly caused by 'loose connections'. It *may* be in some instances, where it is random and intermittent, but if you actually read and understood the OP's symptoms, you would see that what he has is neither random nor intermittent, but a well defined set of conditions, producing a well defined and repeatable problem.

Arfa

I agree.

I put couple block diagrams in here:

If I understand right, there is separate ICs for volume control(each speaker pair) and input selector.

I don't have oscilloscope, only basic multimeter. And I don't fully understand what you mean this dabbing.

Looking at the block diagram, without doing any 'scoping, I think that my first favourite would definitely be the volume control IC on the offending channel. If you have the necessary desoldering equipment and skills to get the IC out without damaging either it, or the print that it's soldered to, perhaps you could swap it for the one on the other front channel ? That would at least prove the point one way or the other.

As far as using a cap to help with diagnosing where noise is getting into a channel, it works best wher you have a low-level crackle or hiss. Crackles and hisses contain significant high frequency components. At these frequencies, a cap of value 0.47uF, has a low reactance - around 300 ohms or so at 10kHz. So, if you connect one side of it to ground, and touch the other end to various places in the amplifier chain, such as inputs and outputs of ICs, transistor collectors and so on, then wherever the noise is present, it will be significantly attenuated when you touch or 'dab' the capacitor onto such a point. Working backwards, when you find the first place that dabbing the cap on has no effect on the noise, then you know that the point that the noise is originating is the next stage forward. This can be a very effective way of finding a noisy opamp or transistor or resistor, when the noise signal is just too small to see effectively on a 'scope. I would normally use only a low-leakage polyester or similar cap for this, and the value is not critical. 1uF or higher is fine, but not in an electrolytic format, as the leakage from one of these might in some cases, be enough to upset operating conditions in a DC coupled amp.

Arfa

Thanks for your reply. I changed these volume control ICs place and found out that two of these are faylty. The one which controls front speakers and the one which controls center speaker and subwoofer output(I didn't tested swf-output before).

Now I have to find new volume control ICs. What is good place to buy these?

Part No: 6-702-387-01 Description: IC CXD9725M

Don't know whereabouts you are, but a quick Google turned up many Sony parts suppliers. First one I tried located in the U.S. had the part in stock. See

Arfa