IBM 5153 - Known Problems/Issues
SYMPTOM: Brightness and contrast knobs do not turn easily
A known behaviour. IBM may have intended this.
SYMPTOM: White horizontal line just below the top of the raster
A known defect (that can be corrected). See the 'Vertical Scan Issue' section (page 25) of the document at here.
Pin 7 is missing from the 9-pin D connector
That is normal. Pin 7 is not used in the CGA standard (see here).
Other
Some known electronic component failures are listed in the IBM 5153 section of the database at here.