This is the schematic of Vieta-225, and so it is not AT-230 but I think this should be almost the same.
At first I would check audio cable connection at the input of the amp bacause the connection contact error in the ground side of the audio connector can cause buzzing.
This is the Vieta 225 circuit diagram:
http://www.arrakis.es/~igapop/vieta225.htmGood luck!
gylacz
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