Welcome to Beoworld !
I am not sure exactly which part(s) you refer to, there are many
and they differ according to the exact version of 4002.
You will need to be more specific.
There are several threads on this forum about speed issues and several troubleshooting clues, do a search.
If you have the DC version, there's a good chance that the
fault lies within the motor itself, I've seen this quite a few times lately.
Take it apart, clean everything and relubricate the bearings.
Check and/or replace PCB trimmers, speed selector relay and the fine adjustments on the front panel.
It's rare to see the C1003 speed regulator IC fail but it can happen (out of production but I have a few used ones in stock).
I'll be happy to service it if in - or near Denmark.
Martin