Look at the PC board, directly above the RCAs (either side of the stereo/mono switch, behind/under the two, yellow, metalized Mylar caps). Examine the solder through-holes (solder points) for the RCAs (there should be three each) and look for cracks, in the board. If everything still looks secure, the jack itself may be broken. If not: re-solder, or repair the board, as necessary (ie: solder/jumper the trace and Superglue or epoxy any crack). If the RCA needs replacement: the board has to come loose. That will require detaching the output transistors from the heat sinks, a few mount screws and unplugging that connector between the power supply caps. Remember to refresh the heat sink compound, during reassembly. I've replaced the RCAs in a few Haflers and other manufacturers' equipment, with these: (https://www.wbt.de/english/products/a/Detailansicht/Artikel/topline-cinchbuchse-3.html), though: I can't swear they will fit the 9130.