For the John Deere 6506 in the 6000/6000SE series the range is wide: more than 500 parts, including lighting, trailer couplings and electrics.
Keeping a John Deere 6506 running often comes down to the front axle. The plates in the ZF front axle, the belts at the front of the engine and the output of the alternator are where this tractor tends to ask for attention after years of work. This page lists more than 500 parts that fit the 6506, from GRANIT and from brands such as HELLA, ROCKINGER, Wabco and ZF.
For the ZF front axle you will find both outer plates and inner plates. When you rebuild a plate pack, count the plates as you strip it and note the order in which inner and outer plates sit. Replace the pack as a set rather than single plates, and check the clearance once it is back together. Fill the axle with the oil specified for it, because the wrong oil makes plates slip or chatter.
A multi-rib belt that squeals on a cold start or when the air conditioning kicks in is usually glazed or slack. When you fit a new belt, check the tensioner and pulleys too. A rough tensioner or a damaged pulley groove will chew through a fresh belt in no time.
If the alternator is no longer charging properly, measure the battery voltage with the engine running and inspect the earth connection and the plug at the back of the alternator first. A corroded contact produces the same symptoms as a worn regulator. When a function in the cab stops working while the fuse is fine, a micro relay is often the cause. Swap the suspect relay with an identical one from a circuit that works, and you know straight away.
For the cab there are door gas struts and handles. At the rear you will find ROCKINGER drawbars and bushes for pivot points. If the door no longer stays open in the wind, the gas strut is usually the reason.
Check the type plate and keep the old part to hand. The reference number on it, or the ZF number on axle plates, is a far safer match than the outside diameter alone. The 6506 is part of the John Deere 6000 range, as are the John Deere 6100 and the John Deere 6900. Both have their own page with the parts that fit them, which helps if you look after a contractor fleet or a mixed yard. Other models from the brand are under John Deere parts.