Problems with Pioneer SX535 and SA900

[bmpn]

Good afternoon to all!

I have two Pioneer "oldies", namely two amplifiers, the SX535 (also a tuner and I have the service manual) and the SA900 (no service manual).

One of the channels in both of them has some problems, since the speaker (it is not a speaker problem, at least I know that) connected to
the mentioned channel makes an awful and loud noise, instead of sweet music. This noise is also louder than it should, if you consider a balanced volume between both speakers.

Since I'm quite new in these matters, I need all the help I can get from you all, so please could you give some tips on the subject and how to try to fix it?
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Thanks in advance for your kind attention to my post,

Best regards,

Bruno Nogueira