You have taken turntable off and you hear a buzzing when you turn that switch on. These BSR's are mostly idler drive. This means that the problem is one of things. The motor could be frozen, or the idler drive could be frozen or not making contact. If you can turn the turntable by hand, then the changer mechanism is probably not frozen. Buzzing indicates that motor is drawing power.
With turntable off, you should see motor shaft sticking up next to idler wheel. It will be a shaft with 4 different diameters. With turntable unplugged, turn the shaft by hand. It should turn freely, and keep turning after you stop spinning it by hand.
Unplug turntable and look underneath and be sure that there are no bare wires and no loose wires. All connections and wires should be clean and good. Insulation should be flexible. If there is any doubt, fix the problem or have a pro look at it. Plug the unit in and turn it on. Motor shaft should spin by itself and you should be able to see it spin. If it doesn't your problem is motor or wiring or switch. If motor spins, then spin rubber idler wheel carefully and see if it spins easily. Then move idler through its sideways travel and see if idler can engage with edge of turntable and be sure that the spring that pulls idler against edge of TT is pulling idler in the right direction, toward edge of turntable. This info should give a start toward diagnosis. Hope it helps! Let us know progress and we will continue to help you sort it. I am sure that others will step in and assist also.
And good luck from the old turntable inspector,