noisy channel

[shinnen]

Hi,
I know this is not the usual post for this forum but I'm wondering if someone can give me some advice on what my options are. I expect many of you are familar with amplifiers.
I bought an old Sansui 771 amplifier, for next to nothing at a yard sale, and upon gettig it home realized why - it has a noisy channel; at least that's what I've concluded after spraying and cleaning as many internals as I can. The noise is only in one channel. I'm wondering if there is an easy way of telling if this thing is a boat anchor in waiting, or can be fixed, by someone with 'little' expertise. It has beautiful sound, so I'm a little reluctant to just junk it. Any help would be appreciated.
Thanks,
...... john

[kozikowski]

http://www.classicsansui.net/images/Lit ... 1/771a.jpg

It's a full-on integrated receiver. No simple amplifier this.

<<<spraying and cleaning as many internals as I can.>>>

Spraying as in FreezeMist®, or spraying as in Windex®?

Can you tell the physical diving line between the Left and Right amplifier and can you tell if one side is getting hotter than the other? This thing uses FETs for switching. Does it make any difference to the noise what buttons you push or knobs you turn?

This is one way to do it...

http://www.itcelectronics.com/store.php?cPath=1167_1222

Slowly cool individual components off one at a time, wait, and listen for the noise to change. Do you have a DC voltmeter? If there is any DC voltage on the output lines, you may damage your speakers if you leave them connected to the damaged side too long

Koz

[shinnen]

Hi Koz,
Thanks for the suggestions. I knew the Windex was not the right thing.
Actually I used a contract cleaner, that contains lubricant. No, I can't tell where the divider is
between the right and left amplifier. It doesn't seem to matter what knobs I turn the result is the same.
And........ the loudness of the static is the same regardless of the volume setting, if that means anything.
I don't think I'm going to go the component checking route. If that's the problem I'll just junk it or give it
to someone. It's a shame. It has a beautiful sound.
Thanks again,
...... john

[steve]

What sort of noise?

[shinnen]

Hi Steve,
Static really. The sort of sound you might get from a scratched record.
Here's a sample <http://www.shinnen.ca/Noise.wav>
It's not the speakers. When I switch the speakers, the noise switches too.
.... john

[Trebor]

Could be thermal noise. It's a long shot, but if any components have a heatsink attached (metal fins), you could try thermal paste to make sure heat is being conducted from the component to the heatsink. (NB: thermal nose would get worse as the amp warms up: it would be less when switching on after it has been allowed to cool).

http://www.maplin.co.uk/Module.aspx?moduleno=33695

[shinnen]

Hi Trebor,
Well then it's not thermal noise. It changes very little from the time it's turned on 'til the time it's turned off.
..... john

[steve]

There's two different noises there.
One is 60Hz "mains hum" (US frequency, not European), and the other is the crackle.
There's also a bit of DC offset.

Is the hum present on both channels?

The noise that's difficult to identify is, and I guess this is what you are complaining about, is the crackle.
Since it does not change with the volume settings, it must be in the power amp section.

If the noise gets worse as the amp heats up then it is most likely to be a transistor, which can then be located with the aid of a can of freezer spray.
However I suspect that it is a leaking capacitor, in which case the noise will start almost as soon as you switch on, and may sometimes (but usually doesn't) improve as the amplifier warms up. You can sometimes spot a capacitor that is on its way out by looking at them - the cylindrical type will often start to bulge at the flat end.

In any case, it sounds like a component is breaking down, which is either a repair job or get rid.

[Trebor]

One other thought, a worn potentiometer, used in the rotary or sliding control knobs, (e.g. volume control), can cause sizzling noise.
If so the noise would change when the affected control knob was waggled a little.

[kozikowski]

They said it didn't matter what the settings were or where the controls were set.

After the switching and volume controls, etc, etc, is the final output amplifier. There are no controls on it and it comes on the instant the amplifier is powered up. If one of the power transistors or any component in that group is failing, you get exactly this symptom.

I can't read from the picture, does this thing have a speaker switch? A, B, Both. Did you try exercising that switch?

Koz