Yamaha A-520 clipping and distortion - need help

[martinszathmari]

Dear forum members!

I have bought a defective Yamaha A-520 amp, and according to my measurements, it has a weird clipping problem, and it can be measured even at the RCA inputs at full volume, overloading my CD player's output. I have been trying to repair the amp, and all i could find was that if i remove the preamplifier stage PNP transistors after the input JFETs, the input is not distorted anymore. As soon as i put the transistors back, the distortion comes back again. The transistors seem to be good though, i have measured them with my multimeter.

Is there any of you who would be willing to assist me in repairing this amp?

Here is the service manual: https://www.manualslib.com/manual/1095119/Yamaha-A-520.html

[martinszathmari]

Nobody?

[Armadillo]

I think nobody want to answer because they cannot understand what you want to say.

Since you measured?
What amplitude signal is your CD-Player output to the input of the amplifier and what input did you use?
What amplitude output is your amplifier clipped at and what impedance is the amplifier load.?

Edit: When you say, "RCA Input", did you mean the phono input? Or what input did you actually use?

[oldway]

I think I understood the explanations.....CD input is low impedance and overload his cd player.....But he did not said if he has this problem on only one channel or both ?

For me, it seems to be bad input fet's TR103 and TR104.

[Armadillo]

How many ohms is the CD input impedance of the amplifier that you regarded as LOW impedance?
The CD input passes through the Loudness, Balance and volume potentiometer before it reaches the FETs.

[oldway]

How many ohms is the CD input impedance of the amplifier that you regarded as LOW impedance?
The CD input passes through the Loudness, Balance and volume potentiometer before it reaches the FETs.

940R.

In stereo position, R145 (4k7) is shorted, in min loudness position, R151 is shorted too, there is only a 470R in serie.
In mid position, balance pot is shorted, volume pot in max position is shorted, subsonic filter is also shorted in defeat position, there is only another 470R resistance in serie with gate of the fet....Then, 470R + 470R = 940R....If input fet is defective, there is a low impedance between gate and source.....

[Armadillo]

How many ohms is the CD input impedance of the amplifier that you regarded as LOW impedance?
The CD input passes through the Loudness, Balance and volume potentiometer before it reaches the FETs.

940R.

In stereo position, R145 (4k7) is shorted, in min loudness position, R151 is shorted too, there is only a 470R in serie.
In mid position, balance pot is shorted, volume pot in max position is shorted, subsonic filter is also shorted in defeat position, there is only another 470R resistance in serie with gate of the fet....Then, 470R + 470R = 940R....If input fet is defective, there is a low impedance between gate and source.....

And what is the typical output impedance of a CD player?

[oldway]

You don't know the brand and model of the cd player of martinszathmari....

it can be measured even at the RCA inputs at full volume, overloading my CD player's output.

...It seems clear to me....."at full volume", that's the lower input impedance if input fet's are defective...."overloading my CD players output" , that means that input impedance of the amplifier is too low for the CD player output.

[Armadillo]

You don't know the brand and model of the cd player of martinszathmari....

it can be measured even at the RCA inputs at full volume, overloading my CD player's output.

...It seems clear to me....."at full volume", that's the lower input impedance if input fet's are defective...."overloading my CD players output" , that means that input impedance of the amplifier is too low for the CD player output.

That's why you should state YOUR OWN basis model/brand to support your own claim! Is there any?

[oldway]

You don't know the brand and model of the cd player of martinszathmari....

it can be measured even at the RCA inputs at full volume, overloading my CD player's output.

...It seems clear to me....."at full volume", that's the lower input impedance if input fet's are defective...."overloading my CD players output" , that means that input impedance of the amplifier is too low for the CD player output.

That's why you should state YOUR OWN basis model/brand to support your own claim! Is there any?

You must be kidding... 

Do you think I will loose my time looking in the thousands of model/brand of CD players existing actually or in the past to guess what CD player martinszathmari is using ?

I only believe that what he wrote is true, even if he did not explain it very well.....

[Armadillo]

Ok, Baseless accepted.