Sometimes the internal pot gets very dirty .. that's enough to prevent the speed control working fine.
If you spray some contact cleaner through the hole, perhaps you have cleaned the internal pot rather than cleaning the brushes.
Motors are sealed, no chance of dexoit working inside unless you open it

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But with an aiwa (the only I have) one strange thing happened to me ....
The deck stop working from one day to another. The motor weas dead. It's strange, because the most comom failure mode is poor speed controling.
When I took the deck appart, I noticed that if I rotated the motor a little by hand, it started to work again.
Dependending at the point the where the motor was stoped it could start again or not.
That was caused by a bad soldering join in the rotor, which was working with 2 coils intead of three. If the motor stoped in the dead coil, it couldn't start again. It is moreless like the car engine and the cilinders.
In that case, motor is a servicable part. But you need to take apart the motor with extremely care to avoid damaging brushes.
It isn't easy.
I fixed mine without problems.
But if brushes are burned forget it. The same with the conmutator of the rotor.
Brushes most look STRAIGTH. If the are a bit rounded at the conmutator area, they are burned.
The things you can repair are the speed control board, brushcleaning if they are ok and soldering joints in the rotor.
If the motor is nearly dead but the speed control board is ok, you can replace only the motor with a similar one while keeping the speed control original (for those boxes that need strange motors other than the standard 2400 ones). At least the DC resistance would match

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