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Nogoton 9/14 stereo receiver with vacuum tubes

Hello ,

I own a Nogoton 9/14 (or 914)stereoreceiver using vacuum tubes . German maufacturing.
This receiver is still working .
Just after a time due to temperature drift (i guess) this unit doesn't stay tuned on his frequency.
So , my questions : does smobody has experience with this unit?
Has somebody the shematics ?
I don't have no knowledge with tube-shematics.

Best Regards