Rebuilding the generators is very costly, usually more than the value of the organ. I have successfully extended the life of screeching organs for several more years using Teflon-based chemicals. I still have some used tonewheel generator units available, too. Each tablet controls several switch contacts. The contacts gradually oxidize , creating an insulating barrier to the electrical current trying to pass through them.
Most technicians clean the contacts, but never lubricate the tablet mechanisms .
Each key has nine horizontal contact wires in a vertical column below it. Nine buss bars extend the length of the keyboard, each one aligned with a particular row of key contacts. Nine drawbars control the attenuation of the pitches applied to the nine buss bars. This is a big job, but it will make the organ sound much better than it has in years. Most people don't notice pitches gradually dropping out from the total ensemble in a spotty manner.