Amp non working

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Hello!
It is an car amplifier 50W+50W but is not working.
When i connect both channels everything is ok untill i turn high the volume speakers start buzzing. There is a small "protect" led and turn on when this happens.
Some kind of protection.
When i connect only "left channel" it works well at high volumes so i think the dc-dc convert stage is working perfectly, i have also measured dc-dc stage voltage and is +-30V.
Also i have connected another amplifier and works without problems.
I think the right channel is oscillating but i do not why.
I have replaced all output transistors MJE2955/3055 but the phenomenom is always there.

Could you help?

Thank you!
 

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