Background Noise (Hiss) -- How to Trace

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I recently bought 2 receivers to play with (fix or break – whatever the fates have in store): a Yamaha CR-240 and a Sansui 6060.

I have cleaned up the controls so they work quietly.

They both have some noticeable hiss – the Sansui is much more noticeable. At first I had naively thought I could replace some capacitors to take care of this. Then I searched this forum and found that resolving hiss may be more complicated.

The hiss in the Sansui starts at ¼ turn of the volume knob and remains constant at higher volumes. The hiss in the Yamaha is proportional to volume.

I have an oscilloscope and a VOM, but I am a novice at this (1 college general electronics course long ago).

At any rate, I have some questions:

1. As I trace the circuits (live), what should I be looking for on the oscilloscope, as far as hiss goes?

2. As a variation of #1, when I probe a capacitor in the circuit, what should I be looking for to differentiate good capacitor from bad?

3. I have looked for visible evidence of blown capacitors. The larger ones sit in puddles of hard goop, which I originally thought were leaked electrolyte. After more study, I suspect this is in fact adhesive. Were larger electrolytic capacitors commonly glued to the boards?

Prepare yourself for a journey - tracing hiss is not particularly easy.

Hiss is essentially a form of noise, and has many sources. A possible reason you notice more noise with one amp is that it has a higher gain, thereby making the hiss louder. Capacitors are very far down the list of possible noise sources, so look elsewhere first.

What kind of scope do you have - and what is is finest volts/division setting?

Your questions:

1. this is noise:
it can be somewhat more spikey, depending on the noise content and your scopes time setting.

2. Mm, caps can go bad in a number of ways, most of which you might not be able to see on a scope. Luckily, they don't happen often, and if the amp is working, there probably isn't a reason to worry.

3. Yes, they are sometimes glued down. A burst cap probably wouldn't adhere to anything.

Measuring noise is tricky business, because you're working with something small and slippery. One thing to keep in mind is that your noise could be coming from somewhere before the amp, so the amps might not be the problem - and more so, noise is cumulative, and a complete system has many parts making small amounts of noise. The trick is finding the biggest, and doing something about it.

Another question is, how loud is the hiss when listening at normal levels, at the listening position? Barely audible hiss at the listening position is not unusual.

I guess the first thing I would try is shorting the inputs on the amps, and measuring the noise on the outputs with your scope - also look if there is any regularity in it - like a 60Hz sine wave.
Vintage of the receivers:
Sansui 6060: about 1975 (finish in 1978) -- has discrete power transistors
Yamaha Cr-240: about 1980 -- has power amp IC's

The oscilloscope is a Tektronix 212:
50V/div down to 1mV/div (1 div=5mm on screen)
0.5second/div down to 5 microsecond/div

The his described in my original post is with the inputs shorted.

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I think it's going to be "normal" , is it about the same on each channel ?. Failure of a cap won't give hiss (white noise) and neither will a PSU fault. Do you have instruction manual that might quote the signal to noise ratio.
Regards Karl
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