Just as a point of interest, since Chony is going to have the 8000 professionally repaired -- the headphone circuit Gary posted shows use of only 5 of the 7 available jack connections. Three of them are for the headphones (left, right, common ground). Two more are likely used to mute the speakers (I assume the 3000 shuts them off when phones are plugged in, as is usual), but the jack apparently isn't doing the work directly (otherwise the unused two jack switch contacts would be connected). It appears therefore that any actual speaker switching is done elsewhere, perhaps by a relay or other device(s), and the jack just switches that circuit.

That's why, since I don't have a schematic, I asked for a picture of the 8000's actual headphone wiring. If the 8000's circuit is like the 3000's, the speaker problem may not at all be related to the headphone jack.
In fact, some keyboards have a separate amp to drive phones, and if that's the case the speaker amp could be out on one channel and the phones would still work on both.

Enough conjecture; Chony, when you get this fixed, would you let us know what the actual problem was? BTW, if you're in upstate NY (not NYC), I might be able to take a look, if you'd like.

--Barry