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Yamaha A-520 clipping and distortion
Yamaha A-520 clipping and distortion
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Old 6th February 2018, 09:47 PM   #1
neptunes4you is offline neptunes4you  Hungary
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Join Date: Oct 2012
Location: Debrecen
Yamaha A-520 clipping and distortion
Default Yamaha A-520 clipping and distortion

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: YAMAHA A-520 SERVICE MANUAL Pdf Download.

Last edited by neptunes4you; 6th February 2018 at 11:41 PM.
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Old 7th February 2018, 06:29 PM   #2
neptunes4you is offline neptunes4you  Hungary
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Yamaha A-520 clipping and distortion
Nobody?
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Old 7th February 2018, 08:55 PM   #3
mjf is offline mjf  Austria
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your description is not easy to understand........;
both channels or only one ?
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Old 7th February 2018, 09:04 PM   #4
neptunes4you is offline neptunes4you  Hungary
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Join Date: Oct 2012
Location: Debrecen
Yamaha A-520 clipping and distortion
Quote:
Originally Posted by mjf View Post
your description is not easy to understand........;
both channels or only one ?
Sorry if it is hard to understand, i will make oscilloscope measurements tomorrow and will post the pictures here.
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Old 8th February 2018, 06:56 AM   #5
mjf is offline mjf  Austria
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there are some questions......
are both channels distorted (or only one)
signal distorted at the output of the preamp (or at the power amp)
what distortion (do you use a testsignal .....sinewave....)
have you measured voltages at some testpoints and compared these with
the schematic
and so on........
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Old 8th February 2018, 01:34 PM   #6
neptunes4you is offline neptunes4you  Hungary
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Join Date: Oct 2012
Location: Debrecen
Yamaha A-520 clipping and distortion
Quote:
Originally Posted by mjf View Post
there are some questions......
are both channels distorted (or only one)
signal distorted at the output of the preamp (or at the power amp)
what distortion (do you use a testsignal .....sinewave....)
have you measured voltages at some testpoints and compared these with
the schematic
and so on........
Hi!

I am using a sinewave, it is a 1kHz signal played with a CD player. The left channel is working properly, this is the max signal i can get out from it to 8 Ohms without clipping, but it's more than enough.

Click the image to open in full size.

The right channel looks like this (the volume pot is set to 20%).

Click the image to open in full size.
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Old 8th February 2018, 04:13 PM   #7
mjf is offline mjf  Austria
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.........upper halfwave lost in the dark.........
good to hear that one channel is working.
i would begin to measure the dc-voltages (perhaps with a dmm, no input
signal)
in both channels and compare them - there must be somewhere
a mistake.
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Old 8th February 2018, 06:37 PM   #8
neptunes4you is offline neptunes4you  Hungary
diyAudio Member
 
Join Date: Oct 2012
Location: Debrecen
Yamaha A-520 clipping and distortion
Quote:
Originally Posted by mjf View Post
.........upper halfwave lost in the dark.........
good to hear that one channel is working.
i would begin to measure the dc-voltages (perhaps with a dmm, no input
signal)
in both channels and compare them - there must be somewhere
a mistake.
It seems like the JFET input stage is faulty, because the DC voltages are not OK. Also, the properly working channel uses the 2SK117-GR types, the faulty channel uses the 2SK117-BL types.
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Old 8th February 2018, 08:47 PM   #9
neptunes4you is offline neptunes4you  Hungary
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Join Date: Oct 2012
Location: Debrecen
Yamaha A-520 clipping and distortion
So, the JFETs are OK, i have swapped them and nothing changes.
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Old 9th February 2018, 05:16 AM   #10
Steve52 is offline Steve52  Netherlands
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Join Date: Aug 2008
manual: ftp://77.172.185.158
gebr : schematic
ww : schematic
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