It is in most cases misalignement of the external and internal parts of the
AFS-C-M switch. Particular with the K30/50/500 and those 3 positions this can easely happen, even if it might have worked for a short time.
Particularely because you have repaired it and had the front off, this is almost certainly the case.
But what can happen is that when the mechanism is stuck internally in AFS position but jammed, that the AF-motor can get hurt.
You find it explained in the
tutorial I wrote (step 4 etc.), here the photos of those two parts of the switch:
1. The inner part of the switch with its 3 possible positions: (note, there is a bearing-ball which can cause problems if lost, happened once to a member here repairing his K-S2)
2. The lever which switches those 3 positions and has to be aligned exactly with this inner part of pict.1:
You can see, that the screwdrive-mechanism is out, i.e. in AFS or C position!
Alignement must be done either in
AFS or
M position.
I prefer
AFS position
The rod (red arrow pict.1) has to slip into the U-shaped part (bottom arrow pict.2)