So I just received a 1984 Panasonic RX-5050 that I ordered off of eBay, and on the cassette deck, audio can only be faintly heard when play is pushed. On top of this audio is very loud noise / buzzing (when plugged into the AC). All other functions work correctly, including record, which I tested using another boombox.

The heads and roller are clean, and I have cleaned the controls and selectors. I have a feeling it's in the tape head ground wire, but I'm not sure. Watching the level meter on the front of the boombox clearly indicates that the head isn't picking up much.