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Hitachi audio receiver and amplifier model SR 504L service manual needed

I just bought yesterday this audio receiver model SR 504 which looks great.
But while trying it, it seemed the sound level was very low.
Could anyone on this forum give me a help with providing the service manual?
I saw on that web site that this request exists already but the link is dead.

I would really appreciate the help of this community of vintage hifi amateurs, and I hope to help others in return.
Thank you very much.

Best regards.