Bad vibrator: This effects the high voltage DC.  Typically, these can be replaced with a modern solid state unit which is unquestionably more reliable and quieter.  These solid state vibrators cost about $30 and are readily available.  It should be noted, that many older mechanical vibrators will work fine.  But, it is hard to predict when the contacts become so bad that the unit will cease to operate. 

Noisy Volume Potentiometer:  After age, they do get noisy (like in most radios).  Dirt and dust is the major culprit along with wear of the carbon element inside.  It is very hard to fine NOS replacements.  About the best that can be done is to clean and spray the noisy potentiometer with appropriate cleaner.

Leaky and Bad Capacitors:  As noted in the pictures, there are paper audio bypass and power supply filter capacitors that almost always should be replaced.  The power supply filer capacitors should also be changed with new comparable ones.  In these capacitors, the electrolyte dries up with age and the elements inside short out.  If not addressed, the short in the capacitor can be catastrophic as damage will occur to the power transformer.

Replace Vibrator Timing Capacitor:  Always replace this with a modern "Orange Drop" or equivalent type 0.006 ufd rated at 1600 VDC.