The Fine Things are Always Hand Made
There is a switch on the back of the tuner chassis for external/internal antenna. The switch looks like a short potentiometer shaft with a screwdriver slot. Make sure it is in the external antenna position.
I restored an 800B some years ago for someone, and the FM sounded good.
FYI - with a little rewiring, you can change any AM button to FM, and vice versa. But it takes a little forethought so you get the rear housing adjustments in a workable order.
Check that the AM/FM relay in the tuner chassis is really functioning. Also read the instructions about certain controls being in the correct position to allow the AM/FM relay to operate.
Try different positions of the bandwidth switch.