Amplifier noise - wind blowing sound on one channel only (sample inside)

[espresso]

Hello,

I bought an old Sony MHC stereo system from 1997. It needed a little repair and I was happy to get it working. But I'm left with this problem on right channel. I can hear a constant hum - sometimes very quiet so I can barely hear it, and sometimes audible in a quiet room. It doesn't get louder when turning up the volume control. It sounds exactly like wind blowing into a microphone.
The only thing I tried was reattaching flat cables. The amplifier has STK circuit.
I uploaded a sample so you can hear for yourself. It's very quiet so you might want to increase the volume to hear it. No funny surprises (sudden loud sounds, crackling, etc).
If you heard anything like this before, maybe you can tell me what to look for.

 

[Mooly]

It could literally be anything from a leaky cap (like a ceramic disc type) to a noisy semiconductor junction, or even a dry of some sort.

Those kind of faults are really hard to pin down. You either need a scope or do some shorting of the signal at various points to see where the noise enters the signal chain.

 

[espresso]

Thanks. I did a little research after you mentioned leaking capacitor. And it seems like they should be the first thing to check. They could cause static noise which is, I guess, similar to what I'm hearing here.

 

[JonSnell Electronic]

If you disconnect the input coupling capacitors to the STK that will tell you if it is the problem. If it all goes quiet, move back in the circuit a bit at a time.
You will soon find the problem component.

 

[espresso]

I will definitely try to trace it when I find some more time. The noise actually disappears when I turn the volume completely down. But even going up 1 level (out of 30) triggers the noise which stays constant throughout the entire volume scale. This leads me to believe that there's something wrong in the pre amp stage.

 

[Mooly]

That suggests that the volume control is electronic (a rotary encoder rather than a simple pot ?) and that it perhaps mutes the signal line on the lowest setting.

 

[espresso]

That's right, it's a rotary encoder.

 

[mr4fox]

Sorry to dig up an old thread but did you ever figure out the cause of this problem? I have a very similar problem with a Peavey 6505 guitar amp (tube amp)...I was kinda hoping it’s just a bad tube but still waiting for tubes to arrive.

Cheers
J

 

[MAAC0]

I own a MHC 5900 and it does not have a rotary encoder.
Does it have a mic input. Could it be the mic or phono preamp perhaps ?

 

[mr4fox]

I own a MHC 5900 and it does not have a rotary encoder.
Does it have a mic input. Could it be the mic or phono preamp perhaps ?

Was that question for me MAAC0?
My guitar amp doesn’t have a mic or phono preamp. Just the a preamp and power amp like a normal guitar amp.

 

[Mooly]

You need to be asking about the Peavey over in the 'Instruments and Amps' section of the forum.

Good luck

 

[mr4fox]

My question was to the OP asking if he figured out what the problem was. It’s not about a peavey 6505

 

[Mooly]

Fair enough, its just that your post reads as though you have a similar symptom with a Peavey.

The OP hasn't logged on since the end of 2013......

 

[mr4fox]

Fair point. It is similar symptoms with a peavey but it’s nit a specific peavey problem. I’ve read of similar cases in several forums but no o e ever comes back and posts what the problem was in the end and there is no amp tech anywhere near me (I’m in Norway) that I can find. So I’m trying to figure out what causes this symptom in an amp (any amp). If it’s capacitors, tubes, pots etc.

 

[Mooly]

I replied back in post #2 after having listened to the file provided and the same advice holds (such as it is).

Its very non specific and could be caused by any number of things. You could have the same type of noise caused by a leaky cap in one unit and a noisy semiconductor in another.