Buy the adapter if the only headphones you own are 3.5 mm and you need to hear this stereo. Wait if a 1/4 inch pair is already in the drawer. On this Sony HP-510A one SS-510 speaker plug was broken, so the whole diagnosis ran on a single speaker until the jack could be used as a second ear.
The jack is the big 1970s size. A 3.5 mm plug from a phone or a modern pair stops at the rim. The adapter that works is 1/4 inch male into the stereo, 3.5 mm female for the headphones. The reverse adapter, the one that puts a 1/4 inch hole on a phone, does not fit this jack.
What the headphone test tells you
Plug in, set the speaker selector so the headphones are the path, and listen to FM at low volume.
- The bad side is clean in the headphones. The amplifier can make that channel. The fault is the speaker, the plug, or the wire. On this unit the plug was already broken, so that result would have ended the hunt at the cabinet.
- The bad side crackles in the headphones too. The speaker path is innocent. Continue with the one-channel tests.
Many of these receivers mute the speakers when a plug is in the jack, through a switch inside the jack. A dirty contact there can mute the speakers even after you unplug. If the speakers go silent and stay silent, that switch wants the same DeoxIT pass as the tape monitor.
While the broken plug is off the speaker, keep its two wires from touching. An unloaded channel is fine. A shorted one is how output transistors get burned.