Check the motor by rotating it with your fingers. It needs to rotate very smooth and have a good and long run-out.
If it has seized or has any friction you can feel, it will need attention in the form of cleaning/lubricating or new bearings (not available).
Relubricating sintered bearings is not easy and you will find detailed explanations if you google around on the net a bit but
basically, you need to strip the motor completely so you sit with the discrete bearings.
Warm them up in SAE30 oil to about 60-70 deg C or so and add vacuum to suck out the remains of the old grease.
Let go of the vaccum to allow fresh oil to flow in.
Repeat a couple of times.
In severe cases, the bearings will need an IPA & vacuum (no heat) treatment first to shift stubborn grease and gunk.
Clean the hard-chrome surfaces of the spindle by
rubbing hard with IPA and a cloth and don't touch them again with your fingers, even the tiniest amount of fingersmudge will
add friction and initial (start-up) torque is something that motors like this just don't have. It all needs to run smooth and clean.
Clean the reservoir felt washers in IPA, let dry and add a couple of drops of oil to prevent the felt from draining the oil from the bearings once refitted.
Put the motor together, align the bearings (a set of special tools were once available but less will do) so that the
spindle and bearings are perfectly in line, running as smooth as possible and put everything back together.
You can sometimes make the motor work briefly by just adding a few drops of fresh oil to the bearings but typically it won't last
long. A day or two and the problem is back.
If the bearings have run metal against metal with the spindle (without the thin oil film between), the fine surface of the poreus
bearing material will have been destroyed and the bearings will be worthless. You can sometimes tell from the color of the old dried
grease. If it has a green-ish color (from oxidized bronze/copper), the bearings are most likely gone.
Best thing is to relube before a problem arises.
Martin