How to.. head alignment

On the dcc170 I noticed that dropouts oftentimes reduced when the unit was upside down. After inspection I found that the head-bridge screwes were not tight. After retighten it the problem went away.
I can understand that after a lot of years using a portable device that the alignment goes bad. But what about the stationary recorders? Do they suffer? And how many faults are the cause of this? And how can you readjust this?

The stationary recorders normally do not have this problem as most alignment screws are sealed (at least all of the 1st gen.
I have only seen this problem if someone breaks the seal with stationary recorders.