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Another AU-9500 with static sound on the left channel!

I received yesterday from the technician my Sansui AU-9500 with all the noisy transistors exchanged as well as the rectifying bridges and 2 jfets. I had asked him about the offset settings and bias, and he had told me that everything was okay, according to specs. I turned the amplifier on and at first it was good, no noise, no hissing, but the sound could be better, so earlier today I decided to check the settings with a multimeter, and to my not so great surprise the offset was a bit off and the bias was at 3.2 mv on one channel and 6.1 on the other, I set the offset as well as the bias to 15mv (measured through white rectangular resistors) and the sound changed from water to wine, a marvel! Thanks for the help of all, because without you it would be much harder to perform this repair, even my technician thanks!
 
The only difference I noticed was that the amplifier is turning on faster, the delay of the protection circuit takes about 2 to 3 seconds, before it took about 5 to 6 seconds to set up the relay. I do not know if it could have been due to replacement of the grinding bridges or another component that is about to fail. What do you think?
 
Ronald.C said:
It's normal?
Click to expand...
It could be that the time delay capacitor had drifted in capacitance (usually up) so was taking longer to charge up and trigger the protection into operation mode. So when the new cap was put in with the correct value, it will now be charging up a bit quicker.
Its nothing to worry about.
 
Could someone explain me better the mathematics of bias tuning measured through the 0.47kohm resistor? I did some accounts and got the following result: 30mA = 14.1mV; 40mA = 18.8 Are my calculations correct or am I missing out on the way?
 
The Service Manual (page #22) states "to adjust for between 30mA and 40mA" (by removing a fuse and placing a current meter across the fuse holder).

So, 30mA is approximately 15mV (by measuring the voltage dropped by one emitter resistor).

Good enough?
 
Hyperion said:
The Service Manual (page #22) states "to adjust for between 30mA and 40mA" (by removing a fuse and placing a current meter across the fuse holder).

So, 30mA is approximately 15mV (by measuring the voltage dropped by one emitter resistor).

Good enough?
Click to expand...

This I understood, I would like to delve a little more!
 
Ronald.C said:
The last question: -What would be the best fit sonically speaking 30mA or 40mA?
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I would say the optimum bias current has a fairly wide corridor, I normally go for the mid point, so 35mA on this spec.
The way to do it is to look at the pre driver stage where you can actually see the crossover distortion, and adjust the bias until you null it out.
But with the negative feedback in these amplifier designs, by the time the signal gets to the output, the crossover distortion is all but cancelled out.
You'll never hear any distortion on an amplifier design like this when the bias isn't quite right, so as long as its between 30 and 40mA its perfect.
 
I understand! So 15mV would be something around 32mA. And to get to 35mA I would have to raise the bias to about 16.5. Should I change from 15mv to 16.5mv?
 
Ronald.C said:
I understand! So 15mV would be something around 32mA. And to get to 35mA I would have to raise the bias to about 16.5. Should I change from 15mv to 16.5mv?
Click to expand...
You can if you want, but I doubt it would make little if any difference to the sound, if thats what you're concerned about...
 
kevzep said:
It could be that the time delay capacitor had drifted in capacitance (usually up) so was taking longer to charge up and trigger the protection into operation mode. So when the new cap was put in with the correct value, it will now be charging up a bit quicker.
Its nothing to worry about.
Click to expand...

So ... sansui is going back to the bench! He's turning on too fast! And this is causing a click on the speakers, in fact it's the noise of the relay setting!
 
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