I am in the process of restoring a Quadra 840AV and one of the problem it has is that the 3.5mm headphone connector is loose, which causes the connector's internal switch to not trigger, making both internal speaker and plugged in headphones be audible at the same time.
I'm wondering if you know if there's a replacement that might be compatible, or perhaps some other permanent fix (i.e. not jamming paper etc).
I attached two pictures showing the connector.
Hum. Is it actually visibly and physically loose? Or do you say that because of the symptom?
It's an odd symptom because on the 475 and other LCs that I've studied, the headphone jack physically disconnects the feed to the internal speaker when you plug in. It pushes contacts apart. The usual failure mode is that the contacts get tarnished and the speaker doesn't turn /on/ rather than won't turn off.
Are you getting both left and right through the headphones? Or just one?
Truth is it sounds like something is shorted. I wonder if there is something stuck inside the jack?
Now, major warning - those jacks are /extremely/ difficult to desolder. The housing is connected straight to the ground plain and there is so much thermal mass and conduction I really had difficulty removing one. I would be very unlikely to try to do so again, so if there is any way at all you can repair it without desoldering, I suggest you consider it.