Hi!.The problem is solved.The cause was dirty switches on the board.Mainly the Phono/Line-in switch and DIN socket (sulfides and oxides).Fixing cold solder joints.Also a poor quality 10 meter Jack-RCA cable. But also it turns out that different models of boomboxes need different power of signal delivered to Line-In.In my case I was confused by the comparison to my Hitachi TRK-8800E.
Thanks and Greetings