Quick update,
I'm pretty sure now, that the motor magnets degraded over time. The shaft turns without any drag. The bearings run smooth. Unfortunately there is no easy way to open it without the risk of damaging it completely, so this is the end of this road.
I couldn't find any original replacement or any datasheet. I'll try to find a new motor that might fit but I have no idea what rpm (without load) would be ok. The range goes from 2.000rpm to 30.000rpm.
I think, even with degraded magnets, the rpm without load should be fairly the same as it was when new. What dou you think?
Here is the data I found so far:
Max. supply voltage: 12V
Current without load: 145mA
Current at stall: 1,26A (I think this originally was at ~2 maybe 2.5 amps as the transistor in the power circuit is rated for up to 3 amps) (I could check at which current the limiter circuit kicks in..)
Housing diameter: 39mm
Housing lenght: 40mm
Shaft lenght: 11mm
Shaft diameter: 3mm
Mounting holes: 4x M3 (circular at 90°, dia. 29mm)
some pics:
Part number is: 432201075112
Datecode is: 230 (1982, week 30)
Don't know what the NM01 stands for. Maybe "NIHON Micro"??
I found a comment in a vietnamese forum that this is a Philips 15V/2A motor but no evidence for that.
Any ideas or sugestions for a replacement?
[EDIT]
The replacement motor shouldn't exceed 45mm in diameter but it could be twice as long as the original. There is plenty of space behind the motor.
Thanks